The Sound of Thunder (episode)
When a new signal appears over Saru's home planet, Burnham, Saru and the crew embark on a perilous mission that puts Saru in danger and raises questions about the Red Angel's intentions. Hugh struggles to come to terms with his new reality.

Summary

Teaser 
"We all come from somewhere. We carry that place with us wherever we go. That never leaves our hearts. Not entirely. But none of us can predict where our voyage will lead. We may suffer losses along the way, but we can hope to learn and grow from those experiences… and from those who accompany us on our journey."

While Michael Burnham and Sylvia Tilly continue to go over the records recovered from the ancient sphere, Saru undergoes a physical examination in the 's sickbay, where he notices Dr. Hugh Culber still looking somewhat bewildered from his resurrection. When Saru comments that it seemed "inconceivable" to have Culber back, the doctor remarks that there is not a test or scan that Dr. Tracy Pollard has not run on him, and that while he looks normal, he does not feel that way. Thinking to his own transformative experience, Saru counsels that "perhaps, in feeling less like you were, you are more like who you were meant to become". Dr. Pollard goes over the results of Saru's examination, remarking that the most evident anatomical change from enduring vahar'ai was that the cartilage that had housed his threat ganglia was reforming to accommodate a new keratin-based tissue that appears like "spikes growing in like teeth". She asks if this sounds right, to which Saru replies that he would not know. He asks if the change would affect him psychologically. Dr. Pollard reviews his neurotransmitter expression patterns from a year earlier compared to that day. With Saru as her only Kelpien specimen (a term for which she apologizes), she could not say for certain what the changes meant, but her guess was that his fear responses were being repressed. This leaves Saru to wonder: "What is a Kelpien without fear? I am losing the very thing that defines me. I am stumbling in the dark, and yet I cannot help but feel that, somewhere within the unknown, there is purpose... and that we are being guided toward it." 

In the ready room, Section 31 liaison  meets with Burnham and Captain Pike, informing them that Section 31 believes that if the seven signals were being generated by the "Red Angel", it may also be capable of temporal incursions, and might also be dangerous. Pike points out that the two signals that the Discovery has already encountered resulted in rescue missions, while Tyler counters that they could also have been catastrophes, and they had to consider both options. Pike recalls that the Red Angel had brought the Human colonists to Terralysium before they could be killed by nuclear weapons, while Burnham recalls Spock finding her on  when she tried to run away to Earth, after the Red Angel told him where she was. Nonetheless, she agrees with Tyler; they know too little about its nature or motivations to draw conclusions. What they need, Pike says, is Spock; Tyler mentions that Captain Leland has put considerable Section 31 resources into the search and has come up empty, and asked Burnham, as his sister, if she could think of anywhere he would go, somewhere they might not have thought to look. Before she can answer, Saru calls Pike to the bridge; they have discovered another signal.

As Pike enters, Saru reports the signal is outside of Federation space. Joann Owosekun pinpoints the coordinates to an M-class planet... and to Saru's utter shock, he recognizes the planet as his homeworld of Kaminar.

Act One 
The Discovery is at maximum warp en route to the Kaminar system, while Saru briefs Pike, Burnham, and Tyler on the two species that call Kaminar home – his people, the Kelpiens, and their predators, the Ba'ul. Burnham reports that the Ba'ul achieved warp drive capability twenty years earlier, and made a hostile first contact with the  in response to a signal from Kaminar. Tyler asks why they attacked if they sent the signal; Saru admits that the signal had come from him, and that he had created a transmitter with Ba'ul technology in the hope someone would receive it. That someone ended up being then-Lieutenant Philippa Georgiou, who brought Saru with her to the Archimedes with the promise that he would not return. Both Pike and Burnham believe this is not a random occurrence; the Discovery has taken an interest in the Red Angel, and it is possible the entity is returning the favor. Tyler is skeptical, and asks if there are any "angels" in Kelpien culture; Saru replies that his people believe only in the "Great Balance", and are taught that the Ba'ul are the keepers of the Balance. The Ba'ul hide behind the technology that maintains their mastery over the Kelpiens, so no Kelpien has ever seen a Ba'ul in living memory. Burnham adds that the technology was also enough to keep Starfleet away, and that the Ba'ul are by nature isolationist and committed to preserving the status quo on Kaminar. Tyler remarks that they likely also saw the signal, to which Pike remarks that perhaps they might be open to sharing if they "knock on the front door and see if they answer". Tyler sarcastically asks if Pike plans to ask nicely; Pike replies that as unexpected guests, it wouldn't hurt to be polite.

Keyla Detmer brings the ship out of warp above Kaminar, and reports that the signal disappeared before they arrived. Pike is unsurprised, as this had happened at the other signal sites. He orders R.A. Bryce to hail the Ba'ul on all channels, and instructs Burnham to conduct planetary and full spectrum scans to find anything resembling the "Red Angel". The Ba'ul do not respond to hails, but Owosekun reports that the Ba'ul are scanning Discoverys weapon systems... and only their weapon systems. Burnham has not found any trace of the "Red Angel", to which Pike wonders why they were there. Tyler remarks that Pike is assuming the signals only appear for a reason; Pike confirms that is exactly what he is assuming, then wonders why the Ba'ul do not respond. Saru becomes indignant, asking why they would trust the Ba'ul, who he condemns as "oppressors" who enslave his people. When Pike asks if he has any idea how he could make inquiries, Saru mentions that every village on Kaminar has a priest who acts as an intermediary between the Ba'ul and the Kelpiens, and also connects to the extended communities; if the "Red Angel" has been seen on Kaminar, the priests would be the ones to ask. Tyler asks if that would violate the Prime Directive, to which Burnham replies that the Kelpiens are aware of warp technology and spaceflight through the Ba'ul. Making official first contact with the Kelpiens would be a judgment call, but she believes the mission to investigate the signals warrants it. Pike ultimately agrees, and believes that Burnham, as a xenoanthropologist, would be the best choice to make contact. Saru protests, as he feels he is the more logical choice because of his experience, and to disregard it would be to disregard the suffering of generations of Kelpiens. Pike believes that suffering makes him the most complicated choice. Saru becomes increasingly angry at what he sees as Pike's distrust of his abilities, particularly at the implication that he would not follow Starfleet regulations out of a desire to share the truth about vahar'ai with his people. As it appears Saru and Pike are about to come to blows, Burnham steps between them, saying that the Kelpiens have no knowledge of Starfleet or other inhabited worlds, so her appearance would be a shock to them; Saru would be invaluable to have at her side. Pike concedes, but makes clear that they were there to investigate the Red Angel, not start a war between the Kelpiens and the Ba'ul. Saru makes clear that his foremost loyalty was to the ship and its mission. Pike suggests that Saru start with his own village.

In sickbay, Culber is undergoing another round of tests, with his husband Paul Stamets at his side; Dr. Pollard explains that while his scans appear normal, his body was reconstituted from his own DNA, and was, for lack of a better word, "pristine". Culber notices that a scar on his left shoulder is gone, remembering he had gone hiking alone on the cliffs of Cabo Rojo when he was sixteen ("and apparently indestructible," Stamets adds) when the path gave way under his feet, and he had fallen fifteen meters (which Stamets remembered as thirty) and received a puncture wound in his shoulder. A doctor named Kashkooli, on the path half a kilometer behind him, risked her life climbing down to save him, and stitched the shoulder wound with fishing line. The incident left Culber with a "very sexy scar" and inspired him to go to medical school. Dr. Pollard remarks that any abnormalities with his senses are the result of his new nervous system, and that while she would continue monitoring him, she believed he would be able to return to his normal life.

Saru and Burnham beam down to the beach outside of Saru's village, noticing a Ba'ul pylon in the village center. Saru explains the Ba'ul have placed one in every village to observe the Kelpiens, who refer to them as the "Watchful Eyes". Burnham's scans show that the pylon is part of a vast network, and remarks that it must have taken generations for their technology and ideology to integrate into Kelpien society; Saru agrees, saying that their species had become so intertwined that, outside of the Ba'ul's technology, it was difficult to discern "what is purely us and what is distinctly them". The village appears abandoned, but Saru explains that it was nearly nightfall, and Kelpiens rarely venture out in the dark. Burnham remarks that if she didn't know Kelpiens were an oppressed people, she'd think Kaminar was a paradise. In many ways, Saru replies, it is, as his people live in harmony in a world without hunger or poverty, with the only violence they know being the culling by the Ba'ul. He remarks that his father, Aradar, had been the village priest when Saru left, an unwitting collaborator enforcing the Ba'ul's mastery over his people. For eighteen years, Saru had dreamed of coming home to his village, and while it has not changed at all, his experience has left him to see it differently. As they enter the village, the Ba'ul pylon activates.

In the woods nearby, Burnham recognizes the fredalia from the ones growing in Saru's quarters, to which Saru remarks they do not grow quite as well on a starship. At that moment, they spot a Kelpien priestess gathering flowers. Saru approaches, greeting her in the Kelpien language – and is astonished when he recognizes her as his sister, Siranna. Siranna is equally surprised to see him alive, as she and the others had thought he had been taken by the "Watchful Eye" for questions he should not have asked. When she asks where he has been all this time, Saru elects to tell her the truth; that he had sought his future in the stars, and that he had been welcomed by Starfleet. Burnham introduces herself, identifying herself as a Human from Earth, and that she worked with her brother on the Discovery. Siranna is surprised that they can understand one another; Burnham had activated the universal translator in her communicator, which could translate a thousand languages. Siranna seems overwhelmed at the idea of different lifeforms and space travel... then asks if Humans from Earth drank tea. In her tent, Siranna pours them fredalia tea, remarking that they had had a bountiful harvest. Saru remarks that she sounds like their father did, and asks about him. Siranna explains that Aradar underwent vahar'ai not long after Saru left, then demands to know how he could leave and not tell them he was alive. Saru believed it safer for them to believe he was dead. Siranna is indignant, as they believed the "Watchful Eye" had punished Saru, and they had spent years fearing retribution. Saru apologizes profusely, but Siranna – remarking that it would be a "longer conversation" – senses that he is here for another reason. Burnham and Saru explain about the Red Angel, and that they know little about it only that it appears in conjunction with a "red light" in the sky (a "fiery sign" as Siranna calls it), and in places of crisis. Siranna is again outraged that Saru had returned following the Red Angel, not coming back for her, and accuses him of not being brave enough to face the Balance as the rest of them did. Just then, her threat ganglia extend, and the ground begins to shake. Siranna believes the Ba'ul must have seen them, and demands that they leave; when Saru tries to explain himself, she tells him to go back to the stars, as there was no place for him on Kaminar anymore. The Ba'ul pylons begin activating, which Saru recognizes as a precursor for the Ba'ul harvest. Burnham contacts the Discovery to beam them back.

Act Two 
As they materialize in the transporter room, Burnham notices that Saru appears distraught, just as the computer signals yellow alert and calls them to the bridge. As they enter, Pike remarks that whatever they did on the surface, the Ba'ul are hailing them now, audio only. Pike suggests it might be best for Saru to stay out of the conversation, before he orders the channel open and identifies himself. The Ba'ul claim that they have taken something that belongs to them – Saru – and that Starfleet had promised not to interfere with the internal affairs of their planet. Pike explains they were investigating the signal that appeared over the planet, which is typically followed by the Red Angel, and that it might pose a threat to their "Great Balance". When Pike expresses a "mutual interest" in keeping the peace, the Ba'ul reply that they would allow the Discovery to leave if they returned Saru to them. Saru is outraged, shattering the monitor display in his grip, and shouts that he is not theirs to be returned. Over Pike's objections, he identifies himself without fear as brother to Siranna, "who lives a life of oppression", and son of Aradar, "who died needlessly at your hands", and that he knows the truth about vahar'ai. When the Ba'ul reply the "Great Balance" is the only truth, Saru calls it "lies", because he survived his vahar'ai and knows that it means not death, but evolution – an evolution the Ba'ul murdered his people to suppress. "You think you know everything, Kelpien, but you do not even know what you are," is the Ba'ul's ominous reply. Detmer then reports that ten Ba'ul sentry ships are on an intercept course; Pike orders red alert. The Ba'ul ships are each much larger than the Discovery and surround it on all sides, and Gen Rhys reports they are powering up their weapons; Pike orders him to warm up the phasers. He advises the Ba'ul to choose their next move carefully; when they again demand Saru's return, Pike refuses, as Saru is a political asylum seeker and a Starfleet officer, protected by the Federation. The Ba'ul question whether Pike would risk his own people for one Kelpien, to which Pike replies that Saru is one of them, and to protect him, he will do what he deems necessary. The Ba'ul reply they will do likewise. Burnham's scanners show that the Ba'ul have activated the pylon in Saru's village. Saru knows the pylons are weaponized and begins to plead that they should help when Pike orders him to remove himself from the bridge. Saru silently enters the turbolift and goes below decks.

As the crew scrambles to their stations, Saru goes to the transporter room, relieves the transporter officer on duty, and sets the transporter on a timed delay. Burnham then enters, knowing what he planned to do, and tells him to step off the transporter pad. Saru is belligerent, demanding to know what Burnham would do if he didn't. In response, Burnham points a phaser at him, telling him she didn't want to use it, but that Saru was not thinking clearly. Saru retorts he has never felt more clarity than in that moment, that this was the only way. He asks if she would not do the same for her brother, which leads her to lower her phaser as the transporter activates, beaming Saru to the surface. Owosekun detects the transporter activity, and Burnham confirms that it was Saru, surrendering himself to the Ba'ul. Rhys reports the Ba'ul ships were powering down weapons. Owosekun detects Saru's signal for a moment aboard a Ba'ul ship, but loses it just a second later.

Act Three 
Burnham goes to Tilly and Airiam, who is still working on the information collected from the sphere to see if the sphere's archives had anything on Kaminar that would explain why the Ba'ul were so determined to capture Saru when they found out he had survived vahar'ai. Burnham thinks the Ba'ul know more about the biological changes caused by vahar'ai than the Kelpiens do, and they would do anything to keep it secret. Airiam finds thousands of years of statistical data on Kaminar in the sphere's archives. Burnham said that the Ba'ul had told Saru they did not know what he was, so the question was: What was Saru?

Meanwhile, aboard the Ba'ul ship, Saru regains consciousness. A moment later, Siranna is transported aboard, confused as to why the "Watchful Eye" had taken her, as it was not yet her time. She asks Saru why he had to come back; everyone else had "accepted" that he was dead, but Siranna had seen the light in the sky (the Archimedes going to warp) the night he disappeared, and became a priest to seek that light. She said his face was "beautiful" to her, but that she also hoped she would never see it again, because that would mean that Saru was free. At that moment, a trio of Ba'ul drones float into the room, one of which slams Saru up against the wall, where he is shackled at the wrists and neck. From a pool in the center of the room, a figure rises from within... a Ba'ul.

Reviewing the historical biological data, Burnham and Tilly realize why the truth about vahar'ai threatens the Ba'ul: According to the sphere's data, 2,300 years earlier, the post-vahar'ai Kelpiens actually greatly outnumbered the Ba'ul, driving them to the verge of extinction. The Ba'ul were originally the prey species... which meant that the evolved Kelpiens had been the predators.

Struggling against his restraints, Saru's evolution results in spikes protruding from where his ganglia had once been, which then shoot out at the Ba'ul, who is protected by a force field. The Ba'ul remarks that Saru was the first Kelpien to pass vahar'ai in more than two thousand years, but his "primal, feral responses" remained the same as centuries ago, and that the Federation did not know what it had welcomed into their ranks. The Great Balance, the Ba'ul claim, is the only way to keep the Kelpiens from destroying everything in their path; Saru rejects the Great Balance as a lie meant to defend the Ba'ul from the Kelpiens, and to absolve themselves of murder, calling them "frail" and helpless without their technology. The Ba'ul make clear they will not allow the past to be repeated, and as Saru and Siranna are the only two Kelpiens to know the truth, they will be analyzed... and then neutralized. The Ba'ul then retreats into its pool as the drones move in, including one with a bladed drill to bore into Saru's skull; another pins Siranna to the wall, locking her in restraints. With a newfound strength fueled by rage, Saru breaks free of his own restraints and smashes the drones to pieces, before freeing Siranna. Siranna asks if it was true, that he had survived vahar'ai. Saru confirms that he has; Siranna realizes, as he did, that the Great Balance was a lie, meant to stunt the Kelpiens' evolutional growth. "And it must end," Saru says, as he searches the wreckage of the drones for parts.

Back on the Discovery, Burnham and Tilly report their findings to Pike, explaining why the Ba'ul thought Saru a threat, just as Bryce reports the Ba'ul are hailing them again. They are relieved to hear Saru's voice, reporting that he is with Siranna in a Ba'ul structure of some sort, but with no indicators as to their location; Owosekun can't locate him on sensors, either. Burnham reports what was found in the sphere's archives, which confirms Saru's suspicions of his own biological abilities, as well as the Ba'ul's intent to maintain their "Great Balance". Saru believes that he must somehow demonstrate, to both his people and the Ba'ul alike, what they could become, just as Saru had been. Tilly recalls that the sphere had triggered Saru's vahar'ai, but Saru believes the process must be accelerated before the Ba'ul could take action. Burnham believes if they could isolate the frequency of the sound waves the sphere transmitted, it could speed up the biological response, just as it had for Saru. Pike has reservations; what if history repeated itself? The Federation could not be responsible for the extinction of the Ba'ul. Burnham points out that the Ba'ul still had vast technological superiority, which would take the Kelpiens generations to catch up to - generations, Saru adds, for the Ba'ul to come to terms with the thing they feared most about Kelpiens: Not their so-called "baser instincts", but their rage. But the Kelpiens could prove they could move beyond it, not perpetuating the cycle, but creating a new balance. Saru admits he cannot prove it, but he believes this is why the signal brought them to Kaminar. Pike ultimately agrees, and pledges that Discovery would help amplify the signal. Burnham asks how they would broadcast the sphere's transmission to the entire planet. "The same way I made first contact with Captain Georgiou," Saru replies: "by using the Ba'ul's own technology against them." Meanwhile, Tilly finishes setting up the signal, and transmits the frequency to Saru.

Saru plugs his makeshift transmitter into the Ba'ul's communications system, but before he activates it, he explains to Siranna that this will trigger vahar'ai for everyone, including her, and that it would be incredibly painful. Siranna asks him to promise her that whatever happens, it will not be the end of their people. Saru replies that it would not be an end, but a new beginning. "Then let it begin," Siranna tells him. Saru activates the transmitter, which begins sending out the signal through the Ba'ul pylons. Next to him, Siranna begins screaming in agony, collapsing into his arms as she begins to undergo vahar'ai.

Act Four 
Aboard Discovery, now at yellow alert, Burnham reports that the sphere's transmission has triggered vahar'ai in 63% of Kelpiens, and rising. Across the planet, Kelpiens begin collapsing in anguish, the pain of their transformation taking hold; back in the Ba'ul stronghold, Saru holds Siranna, telling her it would be over soon. Meanwhile, Owosekun detects a massive disturbance from the lake near Saru's village, at least fifty kilometers in diameter. The Ba'ul stronghold begins to rise from the waters of the lake, protected by a powerful force field; Burnham believes that could possibly be where Saru and Siranna are being held. Pike again calls for red alert, and orders Rhys to lock weapons on the stronghold's shields, to see if they can't break through them to beam Saru and Siranna out. Inside, Saru holds the remnants of Siranna's ganglia in his hand; she has survived. However, the Ba'ul stronghold activates the pylons in every Kelpien village, with an energy buildup capable of wiping them all out at once; they are willing to commit genocide against the entire Kelpien race rather than allow them to evolve. An indignant Pike orders Bryce to hail the Ba'ul, and instructs Rhys to shift targeting to the pylons. The Ba'ul do not respond, but Pike orders a channel open anyway, declaring that he will not allow the Ba'ul to wipe out an entire race. Starfleet would help negotiate a new balance between the Kelpiens and the Ba'ul to preserve their world, but if they chose genocide, they would become the Federation's enemies. Detmer reports that all 4,056 pylons, in every Kelpien village, has been activated, with too many for Rhys to target at once. Pike nonetheless orders photon torpedoes armed.

As the Ba'ul stronghold begins sending energy pulses through to the pylons, a burst of energy flies into it from the signal in orbit. Within the stronghold, Saru sees the Red Angel with his own eyes. A moment later, a powerful electromagnetic pulse disables the stronghold's weapons and shields, and deactivates the pylons all across Kaminar. Aboard Discovery, the crew is stunned; Detmer remarks that the technology required for a pulse that wide should be "impossible". Pike and Burnham exchange glances, knowing that this was the work of the Red Angel. Within the stronghold, Saru and Siranna stare for a moment at the Red Angel before it disappears in a burst of flame; in the distance, the Ba'ul pylon in Saru's village explodes. Returning to the village the following morning, Saru and Siranna find the villagers confused, some still holding their ganglia. One asks Siranna how they had survived vahar'ai, to which Siranna replies the truth had survived, and it was time to restore a true balance - one without fear.

In the mess hall, Pike slides a PADD across the table to Tyler – Saru's report, "shared in the spirit of cooperation". Thanks to Saru's enhanced eyesight, he was able to get a good look at the Red Angel, describing it as a humanoid in a mechanized suit with access to extremely advanced technology, beyond current Federation capabilities. Tyler remarks that Control, which models Section 31's threat assessments, is probably "rightfully alarmed" at what could possibly be a time traveler with its own agenda, manipulating the fates of entire species. Pike remarks that when the Ba'ul were forced to decentralize their power across the planet, the Red Angel took advantage of the Ba'ul's exposure to save the Kelpiens from genocide. Tyler asks what would happen if that power was turned against the Federation; Pike considers Section 31, and Control's threat assessment models, to be paranoid, asking if Tyler was aware they were no longer at war. Tyler replies that the idea was not to end up in another one; the last war had taken a toll on those who had fought it (a subtle jab at Pike, as the  had been on an extended mission during the war), and "some of us are still torn apart", before taking Saru's report and leaving Pike alone in the mess hall.

In Saru's quarters, Siranna looks down at Kaminar, mesmerized by the sight; Saru remarked he had felt the same way, the first time he had seen his world from orbit. Siranna notes that he took home with him, noting the various native plants he had grown in his quarters. For many years, the plants grown from the seeds he had taken with him were all he had of home, but he also knew there was more out there, and invites Siranna to come with him. Siranna refuses, as her people needed her, with a long road ahead before Kelpien and Ba'ul would accept each other. Nonetheless, she is glad that Saru did not leave in fear, as she had earlier accused him, but in hope, and that he had brought that hope with him – he, and the Red Angel. In the transporter room, Siranna invites Saru to return home whenever he would like, which Saru promises he will, before she is beamed back to her village. Saru turns to Burnham, standing behind him, and thanks her for her help. Burnham is reminded of Aeschylus, the Greek tragedian, who wrote that "He who learns must suffer. And even in our sleep, pain that cannot forget falls drop by drop from the heart." Knowing his sister had a chance to evolve, to live a full life, made Saru feel that he could endure anything, and that given the opportunity, he believed Burnham would do the same for Spock. "If I could find him," she replies. She admits that seeing Saru with Siranna has taught her one thing: She, too, needs to go home – to Vulcan.

Memorable quotes 
"Burnham, as a xenoanthropologist, I think you're uniquely qualified to talk to a village priest."
"Sir! I am the logical choice.  I offer my experience. To disregard it is to disregard the suffering of generations of Kelpiens!"
"Given all that suffering, you're the most complicated choice."
"Do you doubt my ability to carry out the mission?"
"Of course not, but at present, we lack a full understanding of the biological, and... behavioral changes you're undergoing."
"I can assure you, captain, I am still very much myself."
"I can only imagine the pressure you might feel to share the truth with your people about Vahar'ai."
"Are you suggesting that I might be incapable of following Starfleet regulations, or that I would disobey your orders?!"
- Christopher Pike and Saru, in an unsettling confrontation regarding contacting the Kelpiens

"If I didn't know your people were oppressed, I'd think Kaminar was a paradise."
"Well, in many ways it is. We live in harmony with each other in a world without hunger, or poverty. The only violence we know is the culling, which Kelpiens have been misled to accept as a mercy by the Ba'ul."
- Michael Burnham and Saru

"For eighteen years, I dreamed of returning to my village. It has not changed at all. But I see it... quite differently now."
- Saru

"How is it that we can understand one another?"
"This device. It's able to translate over a thousand languages."
"You mean to say that there are a thousand lifeforms out there?"
"Oh, not just a thousand. Hundreds of thousands."
- Siranna, Burnham, and Saru

"Do humans from Earth drink tea?"
- Siranna

"I am not yours to be returned! I am brother to Siranna, who lives a life of oppression! I am son of Aradar, who died needlessly at your hands!"
"The Kelpien speaks."
"I am Commander Saru, the first and only Kelpien to join Starfleet, and I know the truth about Vahar'ai!"
"The Great Balance is the only truth and it must not be disturbed."
"Your lies no longer work on me. I survived my Vahar'ai, and I know that it means not death, but evolution; an evolution you murder my people to suppress!"
- Saru and the Ba'ul

"Whoever is listening to my voice, I advise that you consider your next move very carefully."
"Give us the Kelpien."
"Commander Saru is a political asylum seeker and a Starfleet officer, protected by the Federation."
"You would risk the lives of your own people for one Kelpien?"
"This Kelpien is our people, and to defend him, I will do whatever I deem necessary."
"As will we."
- Pike and the Ba'ul

"Saru, I know what you're doing. Move off the transporter pad, now!"
" ...Or what, Michael?"
"I don't want to use this, but you're not thinking clearly, Saru."
"I have never felt more clarity than in this moment! This is the only way! Would you not do the same for your own brother?"
- Michael Burnham and Saru

"The Great Balance is a lie you created to defend yourselves from us! And to absolve yourselves of murder. In truth, you are frail! You would be helpless without your technology!"
- Saru, confronting the Ba'ul

"It is true then? You have survived Vahar'ai?"
"Yes. I am what we used to become. The form we were meant to take."
"The Great Balance, our sacrifice to the Watchful Eye, it was all a lie."
"Forced upon us by the Ba'ul."
"They stole our mother and father from us, and generations of Kelpiens, beyond reckoning!"
"And it must end!"
- Siranna and Saru

"This is Captain Pike. I will not allow you to wipe out an entire race. Your fear of the Kelpiens has blinded you to a peaceful solution. Starfleet can help you negotiate a new balance between your two species, protecting everyone on your world. However, if you choose to murder the entire Kelpien population, you will become our enemies. Choose wisely."
- Pike, issuing an ultimatum to the Ba'ul

"That is what you came to find. A savior."
- Siranna, after the Red Angel saves the Kelpiens

"The truth has survived. It is time for a true balance to be restored. You do not have to be afraid anymore."
- Siranna, to her villagers

Background information 
 This episode was produced simultaneously with the  episode , utilizing  Park in Toronto as the Kelpien village. ().

Cast and characters 
 Hannah Spear (Siranna) first appeared in .
 David Benjamin Tomlinson portrays two different characters in this episode. He portrays his normal recurring role of the  Linus and also reprises his role as a Kelpien Villager from "The Brightest Star".

Continuity 
 The stardate for this episode was given in 
 Saru mentions that Georgiou had been serving aboard the Archimedes when she brought him with her from Kaminar. This appears to contradict , in which Georgiou was aboard a shuttle with the graphic "SHN 03", identifying it as with the . The archive footage from "The Brightest Star" used in "The Sound of Thunder" was altered to remove the SHN designation.
 The Kelpien language, first heard in , was created by Marc Okrand, who previously crafted the Vulcan language and Klingonese.
 The Ba'ul writing seen on the right side of the main viewscreen during the first communications attempt with the Ba'ul reads "Ba'ul High Council Kelpien Great Balance". 
 Likewise, the symbol next to "Ba'ul" during the Kaminar population history simulation is the Ba'ul letter "B", while the letter next to word "Kelpien" (unevolved and evolved) is the letter "K" in the Ba'ul alphabet created for this episode.

Reception 
  Executive Producers Heather Kadin and Alex Kurtzman had teased the existence of the episode and its setting of Kaminar in .

Production history 
 : Title publicly revealed 
 : Premiere airdate on CBS All Access
 : International release date (outside Canada and the USA)

Links and references

Starring 
 Sonequa Martin-Green as Michael Burnham
 Doug Jones as Saru
 Anthony Rapp as Paul Stamets
 Mary Wiseman as Sylvia Tilly
 Shazad Latif as 
 Wilson Cruz as Hugh Culber
and
 Anson Mount as Christopher Pike

Guest starring 
 Javier Botet as the "Old Creature"

Co-starring 
 Hannah Cheesman as Lt. Cmdr. Airiam
 Emily Coutts as Lt. Keyla Detmer
 Patrick Kwok-Choon as Lt. Gen Rhys
 Oyin Oladejo as Lt. Joann Owosekun
 Ronnie Rowe Jr. as Lt. R.A. Bryce
 Hannah Spear as Siranna
 Julianne Grossman as Discovery Computer (voice)
 Mark Pellington as Ba'ul (voice)
 Raven Dauda as Dr. Tracy Pollard
 David Benjamin Tomlinson as 
 Linus 
 Kelpien Villager #1
 Michael Ayres as Transporter officer

Uncredited co-stars 
 Pamela Mars as Discovery bridge crewmember
 Clayton Scott as Kelpien villager
 Michelle Yeoh as Lt. Philippa Georgiou

Stand-in 
 Stacy-Ann Buchanan as stand-in for Sonequa Martin-Green

References 
2237; 2239; Aeschylus; Aradar; ; Archimedes shuttle 03; Ba'ul; Ba'ul drone; Ba'ul High Council; Ba'ul pylon; Ba'ul sentry ship; Ba'ul ship; Cabo Rojo; Cal Rae; captain; cartilage; collaborator; commander; Control; ; doctor; electromagnetic pulse; extinction; fishing line; force field; fredalia; fredalia tea; General Order 1; genocide; Great Balance; Greek; Hindmar; isolationism; Kaminar system; Kashkooli; keratin; lake; language; lieutenant; lieutenant commander; meter; political asylum; predator; prey; priest; quarters; Red Angel; road; Saru's mother; scar; Section 31; SHN 03; ; tea; Terralysium; threat ganglia; time travel; tragedian; transmitter; vahar'ai; village; Watchful Eye; Zimja

Sphere data references 
albedo; axial tilt; chemistry; escape velocity; mean density; mean radius; north pole right ascension; sidereal rotation period;

External links 
 
 
 

de:Donnerhall
es:The Sounds of Thunder
fr:The Sound of Thunder (épisode)
pl:The Sound of Thunder
Sounds of Thunder, The