Crossover (episode)
An accident inside the wormhole sends Kira and Bashir into the "mirror universe", where Klingons, Cardassians, and Bajorans rule over their Human slaves.

Summary

Teaser 

"Medical log, supplemental. Major Kira and I are returning home after opening the hospital on New Bajor, the first Bajoran colony in the Gamma Quadrant."

As Major Kira and Doctor Bashir make their way back to Deep Space 9, Kira asks Bashir to take over the helm control of the runabout , as she has a headache and would like to meditate. Bashir mentions that he once had the opportunity to study with a rhythmic breathing expert named Isam Helewa, but Kira seems rather less than interested. When he mentions that he would like to learn any Bajoran techniques, Kira tells him that Bajorans usually just sit quietly. There is a brief pause as Kira meditates before Bashir begins breathing spastically. She looks up, irritated, and asks him what he is doing. He explains that he is using a technique called Bellows Breath meant to increase one's energy flow.

In an attempt to change the subject, Kira suggests that they listen to some music. An undaunted Bashir asks what type the Major prefers, but unfortunately her musical knowledge is limited to Bajoran composers. He immediately tells the computer to play a selection by Tor Jolan, one of Bajor's greatest composers, much to Kira's surprise. She wonders how he knows of Tor and he explains that he has made a point to listen to Bajoran music ever since he was assigned to DS9. They seem on the verge of finding a common ground when he starts comparing Tor to the great Boldaric masters of the previous century which seems to further annoy Kira. He remains as gung ho as ever, attempting to get to know Kira on a first-name basis.

Bashir goes on to say that Nerys is one of the most interesting women he has ever met and he is glad they are putting their past conflicts behind them. Kira sarcastically adds that they are making room for new ones. Laughing, Bashir relates that he and Chief O'Brien were once at odds with one another, but now he claims they are "like this" as he crosses his fingers. He presses on, trying too hard as usual, and tries to get Kira to call him Julian instead of Bashir. However, when he attempts to hit on her, she tells him to stick with Dax instead. This finally silences the doctor.

As their runabout drops out of warp, the warp field does not collapse properly and they are engulfed in a white flash upon entering the wormhole. In the wormhole, both Kira and Bashir feel dizzy, as Kirk and crew did in the original transporter incident. When they exit the Bajoran wormhole in the Alpha Quadrant, they are shocked to see the station is gone, instead orbiting Bajor much as it did during the occupation. Before either of them can react, the runabout's sensors detect a Klingon  attack cruiser moving towards them. Two Klingon officers beam aboard the runabout and are shocked to see Kira. Apologizing profusely, they offer to escort her back to the station; she simply nods, obviously confused.

Upon arriving at , the two Klingons confer with a Cardassian officer named . Bashir and Kira board the station, where they are confused to see Garak and even more confused to see another ... dressed entirely in black.

Act One 
Bashir politely asks where they are and Garak informs him that they are aboard , the center of authority in the Bajoran sector for something simply referred to as "the Alliance." Kira observes that she and Bashir took a wrong turn at the wormhole, suggesting they return to their runabout. However, the other Kira, whom the Klingons and Garak address as "Intendant," seems to have other plans. Bashir attempts to intervene, only to be vociferously chastised as a "" for his tone of voice. The Intendant believes she has an idea of who their visitors are, and if she is right, she tells Kira she cannot let them go. Dismissively, she orders one of the Klingons to put Bashir to work and reminds them to tell the supervisor to keep an eye on him, as the doctor is unaware of their rules.

Garak and the remaining Klingon escort Kira as they follow the Intendant out the airlock area and onto an identical version of the Promenade. Large banners display what appears to be a winged version of the Cardassian emblem enveloping the Klingon emblem, and below the flags, exhausted Humans carry large pieces of machinery in service to the Klingons and Cardassians. A Klingon officer named Telok informs the Intendant that a Terran has been discovered attempting to stow away aboard a freighter and brings the man to her. Approaching the Terran, the Intendant takes on a compassionate tone, asking his designation; she is dismayed to hear he is a  and observes how long and hard he had worked to earn this designation. Her tone changes to business as she tells Garak to send the Terran to the mines. Garak wants to execute the Terran to make an example of the man, but the Intendant claims Garak has made a career out of setting examples. He presses the issue, pointing out that two other Terrans have been caught trying to escape and asking that he be allowed to conduct an interrogation at least. While the Intendant permits this, she warns Garak not to kill his prisoner.

Elsewhere on the station, an ore processing section identical to the one on DS9 houses dozens of Humans, who perform the same slave-like role as their Bajoran counterparts once did in the "normal" universe. As Bashir is escorted to the processing area,  warns  that they need to upgrade one of the machines' thorium containment cells or there will be an accident. O'Brien stresses to Odo that he will not be held responsible for the resulting consequences. The Klingon escorting Bashir brings him to Odo and informs Odo that Bashir does not know the rules. Odo asks Bashir for his designation. Bashir can only think of his name, which Odo interprets as a joke; as a result, Odo brutally slaps him. Again, Odo asks for a designation, so Bashir informs Odo he does not have one; Odo corrects him, adding "sir" to the end of the statement. A rhetorical response garners another slap. Bashir becomes visibly enraged as they talk, but it is apparent that Odo takes perverse pleasure in provoking him. Upon hearing that Bashir is a doctor, Odo sarcastically reminds him to scrub before he operates. Bashir is put to work but notices that O'Brien has observed the exchange with great interest.

Leading Kira and the guard escorting her to Ops, the Intendant passes several Alliance emblems before entering the office normally occupied by Benjamin Sisko. She sits and urges Kira to do the same, dismissing the guard, her eyes feasting on Kira like an excited child on Christmas. The Intendant reveals to Kira that almost a century ago, a Human named James T. Kirk accidentally traded places with his mirror universe Terran  and drastically altered the course of history on this side.

At the time of the switch, the Terran Empire was barbaric but strong. While in the mirror universe, Kirk met a Vulcan named  and had what the Intendant calls a "profound influence" on him. Spock preached peaceful reforms after "our" Kirk left, rising to commander in chief of the Empire in the process of doing so; however, once he had completed his reforms, the Empire was too weak to defend itself from the newly-formed Klingon-Cardassian Alliance. Kira wonders how  fits into the picture. The Intendant explains that the planet had been occupied by the Terrans for decades when the Empire fell. Upon becoming free, it petitioned for entry into the Alliance and became "quite an influential member" in later years. Her story finished, the Intendant wonders if the other side is similar at all.

Unfortunately, Kira informs the Intendant that her Bajor has not been so fortunate. The Intendant is eager to hear about life in Kira's universe, but Kira insists that she cannot stay. However, the Intendant does not know how to send her back, nor could she if she wanted to do so. The Alliance, upon seizing power, declared that any visitors from other universes must be killed immediately to prevent future incursions after the first crossover, but unlike Garak, she says she does not like using violence. Kira knows exactly what she means, and that she is looking for an excuse not to kill "our" Kira.

Kira solicits the Intendant's help, as she feels a leader like the Intendant could teach her what she needs to know to bring about a stronger Bajor in her universe. "My side once changed the course of your history," she says. "Well, maybe your side can change mine." While the idea appeals to the Intendant, she says she will still have to kill Bashir. When Kira asks her not to, she says letting him live is too dangerous and the Klingons and Cardassians would not allow it; however, Kira knows herself too well and points out that the Intendant does not give a damn what the Klingons or Cardassians think. "Oh!" the Intendant croons. "You know how to manipulate me!" She agrees to go along with Kira's idea and calls for a guard to find her some quarters.

Act Two 

The next day, Kira, who has essentially been given free rein, visits the ore processing center to find a now-filthy Bashir pushing a cart full of ore. Mirror Odo greets her at the door. Like his counterpart on the other side, he prides himself on his efficiency. He offers assistance should she need him to explain anything, and when she demurs, citing that she has worked in ore processing before, he laughs out loud. When Kira mentions the name Kirk to Bashir, he immediately realizes where they are, having read about the transporter accident at the Academy. He plans to befriend the mirror O'Brien in the hope that this O'Brien knows as much about transporters as theirs does; meanwhile, Kira tries to find other help.

 is no different in the mirror universe aside from the clientele, with a mixed group of Bajorans and Cardassians standing around a dabo table as Kira enters.  has heard of the crossover and remarks that Kira is indeed the spitting image of the Intendant. However, he is surprised to find that Kira knows him. She claims that the two of them are close friends on her side and that her Quark does her a lot of favors. As her Klingon escort still observes them from the doorway, Kira nonchalantly mentions that her Quark gets things done for her that no one else can and inquires about a transporter. He is aware of how the first crossover occurred and agrees to help her if she can show him how to send other people across. Although Kira begins to talk about revolving doors, Garak enters followed by two Klingons and yells Quark's name angrily.

Greeting Garak amicably, Quark attempts to act as if nothing is wrong. However, Garak informs him that he is under arrest for helping Terrans escape from Terok Nor. Quark claims he is a coward who does not stick his neck out for anyone, but the Terran whom Garak interrogated has said otherwise. Knowing what is about to happen, Quark pulls out a Cardassian phaser rifle and attempts to escape, only to be dragged away by the Klingons. "I shall miss him," Garak observes. "He always let me win at dabo." Shortly thereafter,  and a group of Terrans enter, helping themselves to free drinks in Quark's absence, and Kira sees another potential window of opportunity.

Meanwhile, the exhausted Terrans are allowed a lunch break. Bashir seizes this opportunity to introduce himself to the mirror O'Brien. Handing what little food he has been given to one of the other workers, he makes his way over to where O'Brien is fixing a panel of some sort. Hearing that the two of them are best friends in the other universe piques this O'Brien's interest; he wonders what he does on the other side. Bashir informs him that he is chief of operations of the station, giving him reason to pause his work and consider the possibilities. When Bashir describes the other details of his O'Brien's life, mirror O'Brien says his other half must have gotten the lucky draw of the two of them.

Bashir switches subjects, asking how much O'Brien knows about transporters. While he claims to be no expert, he adds that he knows as much as any Terran. "I hope that will be enough," Bashir remarks, explaining about the transporter plan. O'Brien finds the plan untenable and believes Bashir is lying to solicit his help. While Bashir insists that he is sincere, the spell that came over O'Brien when he heard of his other half seems broken, as he goes back to work and tells Bashir he does not know him. Odo interrupts to announce that break time is over and that Sisko has requested O'Brien's presence. O'Brien protests that he must finish his repair work, but as usual, no one listens to him.

Back in Quark's, Sisko and the other Terrans are enjoying themselves at the bar and he remarks that the Intendant called him all the way from the Fowla system just to see Kira. She observes that he and his friends do not act like typical Terrans. As he explains, the Intendant provides him with a ship in exchange for "duties... of a sort" that he collects from other vessels. While most Terrans are enslaved, he and his crew are allowed relative freedom because he amuses the Intendant. He observes that Kira must amuse her as well.

O'Brien enters the bar to ask what Sisko wanted, and seeing him seems to brighten Sisko's day. Referring to O'Brien as "Smiley" despite O'Brien's objection to the nickname, Sisko asks how he is being treated in an attempt to make small talk. He observes that Smiley does not like him, which O'Brien reluctantly confirms, evoking a laugh from Sisko. He states that the fact that Smiley is not afraid to show this dislike is why he amuses Sisko. Getting to the point, Sisko tells O'Brien that his impulse driver coil needs to be repaired. O'Brien notes the problems in ore processing which he needs to fix first, but Sisko cannot be bothered with such things and orders him to fix the ship first. As the Intendant calls for Sisko and he leaves the bar, Kira realizes that she may not have any hope at all.

Act Three 
Kira enters the Intendant's quarters to find the Intendant in a bathtub with several Vulcan servants massaging and bathing her as Sisko sits on a couch facing the door. The Intendant wants to know if there is another Sisko on the other side, and on hearing there is, the idea of two of Sisko pleases her. As she speaks, Sisko gets up to check on his ship, obviously not amused, but the Intendant stops him to ask if she has hurt his feelings. Obediently, he replies, "I never had any to hurt," and exits.

Once Sisko is gone, the Intendant talks to Kira as the Vulcans help her out of the tub and dress her. She mentions that she has heard Kira is looking for a transporter, and Kira reminds the Intendant that she is looking for a way back home. However, had Kira come to the Intendant with her request, she would know that transporters were re-designed after the first crossover to prevent future accidents. That leaves the Intendant to wonder why Kira did not do so. As she approaches Kira, her voice betrays a combination of hurt and distrust. Kira reluctantly admits that she is afraid of the Intendant. "I don't want your fear," the Intendant confides as she stands face-to-face with Kira, "I want your love. If you can't love me, who can?"

The door chime rings. Telok informs the Intendant that Garak has arrived with a prisoner. She tells them to come inside; they drag Quark, who has visibly been beaten, with them. As Quark apologizes for what he has done, the Intendant kneels beside the Ferengi and comforts him, explaining that he helped the Terrans because he felt sorry for them. However, she reminds him that the Alliance needs the labor to process ore, for without the Terrans, nobody would perform such tasks. Rising to her feet, she tells Garak to give Quark a quick death and seems to all but forget about the incident as Quark is dragged away begging for mercy. Kira is aghast, but the Intendant, now in a happy mood, announces she plans to throw a party. "What shall we wear?" she asks her pet.

Kira enters her quarters carrying a lavender dress to find Garak waiting for her. "Lovely," says the Cardassian of the garment. "I do admire a well-tailored gown." He proceeds to explain that the Intendant will never let Kira return to "our" universe as she is clearly enamored with Kira. Describing how the Intendant talks about Kira constantly, Garak compares Kira to a Drathan puppy lig left on the Intendant's doorstep and hints at his true intentions as he makes a point of mentioning that the Intendant trusts Kira with her life. Kira points at the guard outside her door as evidence of how much the Intendant trusts her, but Garak reveals that he arranged the guard, not the Intendant. "Oh, she trusts you, as much as she trusts anyone. So who better to betray her?" he suggests. Garak's plan is that the Intendant will be "gone" in the morning and Kira will take her place. Shortly thereafter, he explains that Kira will retire to Bajor and he will take her place, at which point he will become the Intendant and Kira and Bashir will be sent home. He leaves Kira to ponder the idea and adds that, should she refuse, he has arranged for Bashir rather than the Intendant to be gone.

Act Four 
Hurriedly entering the ore processing center, Kira warns Bashir that the two of them must escape tonight. She does not have time to explain the web of lies and treachery, but she tells him to be careful because he might be in danger. Regardless of where they go from there, she insists that they must find a way to the runabout. She tells him that there will not be much warning if an opportunity presents itself so he should be prepared.

On the Promenade, Kira finds Sisko half asleep, his eyes closed, leaning against a pillar. She claims to have valuable information that she is willing to trade, and he finds amusement in the idea that Kira has already "gone into business for herself." When she tells him she wants her runabout or a way off the station, he laughs. "She'll have my head... or something else," he says of the Intendant's reaction to such a plan. Kira dispenses with the negotiations and announces that Garak is planning to kill the Intendant. However, Garak has been doing so since he first arrived on the station. She reveals the plot in hope that it will convince him she is serious, but he laughs and observes that the plan is more creative than Garak normally gets. Frustrated, Kira wants to know why Sisko does not seem to care about the fact that he and his fellow Terrans are slaves; he counters by arguing that he has made the best of a bad life for himself. Disgusted, Kira accuses Sisko of being no less a victim than anyone else noting that out of everyone she has met in this universe the only person who had a shred of decency was, of all people, Quark; as she leaves, her words seem to weigh heavily on his conscience.

Kira attends the Intendant's party in Quark's that night wearing the purple dress and Garak greets her charmingly. Telok brags to his fellow Klingons about his time in service to the , commenting on the unpredictable nature of  and , when one of Sisko's crewmembers accidentally bumps into him. He punches the Terran dismissively, adding, "Stupid Terran pig!" The Terran pulls out a knife and rises to his feet, but does so with restrained anger as he knows he is not an equal. At seeing that the Terran is willing to challenge him, Telok nods approvingly, ready for a fight. However, when Sisko shakes his head, the Terran puts the knife away. "What's wrong, Terran?" Telok taunts. "Lost your nerve?" Telok spits in the Terran's face and the man almost loses control, but another shake of the head from Sisko prompts him to restrain himself. Visibly disgusted, Telok orders the Terran to get out of his sight, and as the man does so, Sisko takes his place threateningly. However, the room shifts its attention as the Intendant enters.

The guests cheer as the Intendant enters followed by her Vulcan servants and the musician asks what her pleasure is. She tells him to play something bright in a happy mood and joins the festivities as Kira observes her, deep in thought.

Meanwhile, Bashir sits exhausted in ore processing in front of a pile of rocks. Odo approaches him from behind and kicks him, noting that Bashir is not accustomed to the workload and, as such, has much to learn. "It's a shame this is going to be your last night on the job," he adds, with a hint of sadistic pleasure. His taunting is interrupted when, just as O'Brien warned, the thorium containment field on one of the machines short-circuits. The Terran workers scatter as Odo contacts engineering, and Bashir uses this opportunity to steal a phaser from one of the Bajoran deputies. Still facing the deputy, he begins to back out the doorway following the panicked workers. Odo spots Bashir and reaches for his own phaser, but Bashir shoots the Changeling, causing him to explode, and splattering goo everywhere. Not taking the time to gawk, Bashir leaves immediately.

Act Five 
Telok and some other soldiers can be heard in the distance as Bashir runs through one of the corridors and enters a conduit. A pair of Klingons runs by, oblivious to his location. He crawls a short way before he finds O'Brien attending a panel on the wall. However, when he solicits O'Brien's help, O'Brien claims not to see him and goes about sealing the thorium leak. Bashir realizes it is futile and remarks that he thought O'Brien was a decent man, which O'Brien insists he is. O'Brien is worried that the Alliance will find and kill him if he helps Bashir, but Bashir points out that O'Brien, as well as every other Terran, is already dead. He begins to enter another conduit, but O'Brien points out that the runabout pads are through another conduit, asking if there is room on the ship for him. The two of them emerge in another corridor only to find the Klingons have cut them off.

Interrupting the party, Telok brings O'Brien and Bashir and informs the Intendant of what has transpired; she immediately looks at Kira furiously. She declares that Odo's death is her fault for keeping Bashir alive in the first place and laments Odo's death. Garak and the others observe closely as she proclaims that Garak will use Bashir as his "example," and kill him slowly in public view for all Terrans to see. When Kira attempts to speak, the Intendant threatens to have Kira join him. She then turns her attention back to O'Brien. As he has been a perfect worker for years, she wonders what got into him, and he tells her that hearing about life on the other side and himself being chief of operations made him realize that, whatever the other side is like, there has to be something better than life as a Terran. "Not for you, Mr. O'Brien," the Intendant informs him. "Oh, he's going to be taking you with him – just not exactly where you thought he would be taking you."

With a nod from the Intendant, Garak begins to lead Bashir and O'Brien out of Quark's, but taking Garak's gun from its holster, Sisko intervenes, inspired by O'Brien's words. The Intendant does not believe what she sees and asks Sisko if he has lost his mind. He quietly replies, "No, I didn't lose it. I just... changed it." By this time, "our" Kira and Sisko's crewmen have taken the cue and are all armed. They slowly exit the bar before Sisko shoots the panel above the door, locking it shut. As he does so, the Intendant is near tears; for all the disparity between them, she had true feelings for Sisko.

Kira warns Sisko as he escorts her and Bashir to the runabout pad that the Intendant will track him down wherever he goes. Bashir suggests that he and his crew come with them, but Sisko assures them he can stay ahead of the law. "Maybe we'll stir up some things on this side," he adds with a grin. On hearing this, O'Brien changes his mind about leaving and joins Sisko's crew, as Sisko says he can always use a good tinkerer and putterer. Kira thanks Sisko before she and Bashir board the runabout.

Disengaging the docking clamps, Bashir and Kira take the runabout straight to impulse and depart Terok Nor as quickly as possible. As they head for the wormhole, Bashir reports that the runabout still has a plasma injector leak from the warp core, which Kira is counting on to return them home. They mimic the conditions of their original entry into the wormhole that brought them to the mirror universe, but a Klingon cruiser heads straight toward them, trying to keep them from leaving and nearly destroying the runabout. However, as the Klingons on this side know nothing of the wormhole, Bashir and Kira are able to maneuver toward it and enter. They are inside the wormhole when one of the runabout's systems short-circuits and there is another brilliant flash of light.

In Ops aboard Deep Space 9, Commander Benjamin Sisko emerges from his office and asks if the crew has found anything. Odo stands next to Dax as she reports that a search vessel picked up traces of the runabout's warp signature and indications of a plasma leak, but no ship. There is an alert from the computer, and O'Brien announces it is them, although their deflector grid is damaged. Sisko tells Dax to hail the runabout and with a mixture of annoyance and concern, asks where they have been. "Through the looking glass," Kira responds.

Memorable quotes 
"Bartender!"
"He's just been arrested."
"Arrested? What a damn shame. Who wants a drink on the house?"
 - Mirror Sisko and Mirror Garak

"I'm Kira Nerys."
"That makes two of us."
 - Kira Nerys and Intendant Kira

"Wouldn't that be something, Benjamin? Two of you at my side!"
"I need to check my ship."
"Benjamin. Did I hurt your feelings?"
"I never had any to hurt, Intendant."
 - Intendant Kira and Mirror Sisko

"Smiley!" 
"Miles, Mr. Sisko."
"I don't like the name Miles. That's why I named you 'Smiley'!"
 - Mirror Sisko and Mirror O'Brien

"What do you care about Terrans' freedom?"
"I care about freedom! What I don't understand is why you don't care. Why the only one I have met on this station who seemed to give a damn was a Ferengi toad named Quark!"
 - Mirror Sisko and Kira

"You're not accustomed to this workload, are you, Doctor? You have much to learn. It's a shame this is going to be your last night on the job."
 - Mirror Odo, to Doctor Bashir

"On this side, Terran workers do not speak to their superiors as you do. They certainly do not murder them! He was the only one of his kind, the man you killed. Do you realize that? He is irreplaceable! No one ran that operation as efficiently as he did. No one kept order among the workers as he did. This is my reward for treating you Terrans with the least bit of respect! Very well. I can learn from my mistakes. You want to set an example, Garak? Use him! Set an example for all Terrans! Let him die slowly in public view on the Promenade! Let his pleads for mercy echo through the corridors for all Terrans to hear!!"
"Intendant!"
"Another word from you, and you will die right beside him."
 - Intendant Kira and Major Kira – 

"This man is a doctor where he comes from. And there's an O'Brien there, just like me... Except he's some kind of high-up chief of operations. And they're Terrans. Can you believe that? Maybe it's a fairy tale he made up, but it started me thinking how each of us might have turned out, had history been just a little different. I wanted him to take me with him because, whatever it's like where he's from, it's gotta be better than this. There's got to be something better than this."
 - Miles "Smiley" O'Brien

"We've got ships from here to New Bajor out looking for you. Where have you been?"
"Through the Looking-Glass, Commander. But it's good to be back."
 - Sisko and Kira

Background information

Story and script
 The original title of "Crossover" was to be "Detour". (Star Trek: Deep Space Nine Companion - A Series Guide and Script Library)
Michael Piller commented: "We've been pitched  sequels since  began, and I wasn't interested. But I couldn't get away from the fact that it would be interesting to know what happened after "Mirror, Mirror" finished. I couldn't escape the idea that Kirk's influence in the world that he left might have been profound and changed history. What would be more of a gross violation of the Prime Directive? Ira [Behr] said, 'What if he actually screwed things up?' Spock listened to what he said and then they turned this evil empire into a much more gentle empire that was conquered and taken over by the Klingons, the Cardassians and others. I was watching Schindler's List and I was thinking if I were a little older, I could have been in one of those camps in Poland. If Germany had won the war, I would not be here doing what I am doing today. I guarantee you. I was very pleased with the way the script turned out". (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages) 
 Writer Robert Hewitt Wolfe wrote the fall of the Terran Empire into the script as an analogy for the fall of the Roman Empire to barbarians and the Chinese Dynasty to the . He also wanted to illustrate that if an Empire is as brutal as the Terran Empire was in , there were probably reasons why it was so brutal and he wished to convey the message that, in such circumstances, one cannot change things overnight, and even the actions of Captain Kirk can have severe consequences; "Empires aren't usually brutal unless there's a reason. There are usually external or internal pressures that cause them to be that way. So I just thought that if the parallel  was that brutal, there had to be a reason. And the reason was that the barbarians (the Klingons and the Cardassians) were at the gate." ()
 Wolfe elaborates on the political/historical metaphor contained in this episode when he says, "My analogy was to the Roman Empire. The Roman Empire was as brutal and as nasty as it was because all around it, it had very aggressive barbarians that it was afraid of. The Chinese had the same thing, the Mongols were always there. So if you suddenly make the Romans nice guys, or the Chinese nice guys, well that's great and everything, but then the Mongols come across and it's all over. So that was kind of the idea, what was the mirror universe like a hundred years [after ]. Well, it might not be a very nice place." (Hidden File 03, DS9 Season 2 DVD, Special Features)
 Worf was to appear in this episode, but Michael Dorn was unavailable as he was filming the series finale of , . His lines were given to Andrew Robinson and the lines intended for Elim Garak were used to create the character Telok. (Star Trek: Deep Space Nine Companion - A Series Guide and Script Library) When Dorn did appear in later mirror universe episodes, he played Regent , leader of the Klingon-Cardassian Alliance.

Production

David Livingston commented on the filming of the episode that it was: "a killer. I don't usually call Michael Piller to comment on the scripts, but when I read the draft, I just called him and said, 'Cool', and hung up. It was not a pleasant experience for me because of all all the pressure. It was just not fun. I wanted it to be a lot more than it is, but I'm glad to do it. It's one of the most expensive episodes we've ever done. We had more prosthetic makeup because the station is inhabited by Klingon, Cardassians and Bajorans, and the humans are basically slaves so they're wearing different costumes. We had swing sets that were huge, like the mining set, and very complicated to shoot. The shooting time was long because of the look and because of special effects, like smoke and steam and that kind of thing. Plus, we did make changes in the sets themselves, like Quark's, in which we replaced the big graphic in the bar with the symbol that represented the Bajorans, Klingons and Cardassians melded together". (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages) 
 The teaser scene in which the two Klingon soldiers board the runabout is intentionally shot from extreme angles, which director David Livingston compared to essentially shooting up the actors' noses. He did this intentionally to emphasize how different things were in the mirror universe. During the preproduction period of this episode, Livingston watched the 1949  movie  for inspiration, although he has admitted that he may have gone too far in using radically different angles. (DS9 Season 2 DVD.)
 The knife wielded by the Terran when he is knocked over by Telok was designed by Rick Sternbach for an episode called "The Beast", which became the basis for the episode , and it was originally intended to be wielded by a Klingon. (Deep Space Nine Sketchbook, DS9 Season 2 DVD, Special Features)

Reception
 After this episode aired, there was a great deal of discussion about how sexy and alluring Nana Visitor was as the Intendant, with the costume she wore garnering a great deal of attention. According to costume designer Robert Blackman, the general consensus amongst fans was that it was the revealing costume that gave Visitor this new level of seductiveness, but Blackman disagrees; "It's not that I've exposed more of her body – it's exposed pretty much the same way it always is. What's the difference? She's the difference. It's how Nana wears it. It's what she does. She walks like a provocative woman, with her legs crossing in front. She uses her hips, and a whole other kind of body English than she normally uses." ()
In Star Trek 101 (p. 125), Terry J. Erdmann and Paula M. Block list "Crossover" as one of the "Ten Essential Episodes" from Star Trek: Deep Space Nine.

Trivia
 This is the only  mirror universe episode in which the crossover is made via the Bajoran wormhole. All subsequent episodes show the crossover being made by way of a specially adapted transporter. The novel Fearful Symmetry posited this is because the crossover in this episode was a deliberate attempt by the Prophets to connect the two universes. 
 In later mirror universe episodes, the Terran Rebellion captured  from the Klingon-Cardassian Alliance, which is what  alludes to when he says he and his men will "stir things up on this side." 
 Cirroc Lofton (Jake Sisko) does not appear in this episode. Later episodes would establish that Jake Sisko was never born in the mirror universe, making him the only character in the Star Trek universe known not to have a mirror counterpart.
 While Avery Brooks, Rene Auberjonois and Colm Meaney all appear throughout the episode as their mirror counterparts, their regular characters of Benjamin Sisko, Odo and Miles O'Brien only appear in the final scene. Terry Farrell (Jadzia Dax) also only appears in the final scene. 
 Quark does not appear in this episode, only his mirror counterpart.
 The mirror Odo's uniform sports a belt and collar, which Auberjonois liked so much that he began using it in his regular series Odo uniform as well.
 Prior to the establishment of the history of the televised mirror universe, a mirror version of O'Brien appeared in the novel Dark Mirror, depicting a mirror universe in which the Terran Empire had survived well into the 24th century. That version of O'Brien served as a conn officer aboard the ISS Enterprise (ICC-1701-D) from 2364 until at least 2367.
 According to the novel "Disavowed", Bashir steals the weapon he uses to kill Odo from the  of . The Desca of the "prime" universe is a long running character in the novels.

Awards 
 This episode was nominated for an ASC Award for Outstanding Achievement in Cinematography in Regular Series (Marvin V. Rush).

Video and DVD releases 
 UK VHS release (two-episode tapes, CIC Video): Volume 22, 
 As part of the DS9 Season 2 DVD collection
 As part of the Star Trek: Fan Collective - Alternate Realities collection

Links and references

Guest Stars 
 Andrew Robinson as ""
 John Cothran, Jr. as Telok

Co-Stars 
 Stephen Gevedon as Klingon #1
 Jack R. Orend as Human
 Dennis Madalone as Marauder

Uncredited Co-Stars 

 René Auberjonois as  (mirror)
 Pam Blackwell as Terran slave
 Avery Brooks as  (mirror)
 Mark Lentry as Terran slave
 Colm Meaney as  (mirror)
 Estella Russell as Terran slave
 John St. John as Klingon patrol guard
 Armin Shimerman as  (mirror)
 Nana Visitor as  (mirror)
 Unknown performers as
 Bajoran guard
 Bajoran musician
 Bajoran personnel
 Dabo girl
 Female Cardassian guard
 Female Klingon guard
 Four Terran marauders
 Male Klingon guard
 Nine Terran slaves
 Vulcan servant 1
 Vulcan servant 2

Stand-ins 
 Ivor Bartels as stand-in for Siddig El Fadil
 John Lendale Bennett as stand-in for Avery Brooks
 Mark Lentry as stand-in for Rene Auberjonois
 Robin Morselli as stand-in for Nana Visitor
 Randy Pflug as stand-in for Colm Meaney
 Unknown actress as photo double for Nana Visitor

References 
2267; 2368; 0413 Theta; ability; Alpha Quadrant; amusement; arrest; Bajor; Bajor (mirror); Bajoran; Bajoran music; Bajoran sector; Bajoran wormhole; bath tub; barbarian; Bellows Breath; ; Boldaric; bury the hatchet; Cardassian; career; chief of operations; college; commander in chief; composer; confession; crossover; dabo; Deep Space 9; deflector grid; disruptor rifle; dizziness; docking clamp; Drathan puppy lig; driver coil; Earth (mirror); electro-plasma; emergency stabilization control; euphemism; expression; fairy tale; Fowla system; Ferengi; flag; freedom; Gamma Quadrant; gift; heart; ; ; impostor; Intendant; joke; jumja tea; ; ; kiss; Klingon; Klingon-Cardassian Alliance; Klingon cruiser; laborer; ; latinum; leader; long range sensor; love; ; marriage; medicine; meditation; mines; mirror universe; mirror universe cultures; mirror universe history; New Bajor; ; ; nap; Ops; ore processing center; pagh; parallel universe; pig; Promenade; plasma injector; Promenade; Quark's; Quark's (mirror); revolving door; rhythmic breathing; Rules of Obedience; runabout; Smiley; soul; ; spoon; spitting; Starfleet; Starfleet (mirror); Starfleet Academy; supervisor; tax; ; Terran Empire; Terran Rebellion; Terok Nor (mirror); theta (classification); thorium; thorium containment cell; transporter; thruster; toad; Tor Jolan; transporter accident; transporter technology; ; Vulcan, warp field; worker; wormhole

External links 
 
 
 
 

de:Die andere Seite
es:Crossover
fr:Crossover (épisode)
nl:Crossover
DS9 episodes