context: In the opening scene, there is an airplane flying to Moscow; on board many of the passengers are asleep as a stewardess does her rounds checking on passenger needs. Sean (Emile Hirsch) is playing a video game on his cell phone. The stewardess (Maria Romanova) tells him to shut off his phone, yet he questions her about the supposed need as he doubts that a phone could make the plane fall out of the sky. His friend Ben (Max Minghella) is embarrassed by his antics, but he does not tell him to stop. Sean asks her if she agrees, just keep her arms crossed. She looks at him and keeps her arms crossed and smiles. The lights go out suddenly due to an electrical short, and the two guys are startled. The plane lands safely at the airport in Moscow, Russia.As they exit they are trying to get ready for a presentation; Ben notices a hole in the sleeve of Sean's suit. Sean says he hasn't worn it since graduation: Ben points out Sean didn't graduate, but Sean says he dressed up in solidarity. Sean gets Ben to use a sharpie marker to colour Sean's shirt black in the area underneath the hole so it won't be noticeable. Sean and Ben are a pair of software developers, and they are trying to market a social media program that would make it easy for people to find parties, events, and friends all around the world. Sean mentions he found a hot Russian nightclub called Zvezda where he hopes to get drunk and meet hot models. Ben tests the app and finds a number of American ladies who are in Moscow. Sean comments that they did not travel this far to meet Americans; if they wanted to do so they could go to Applebees.They enter a boardroom and are taken aback to discover the investment meeting has already started, led by their Swedish associate Skyler (Joel Kinnaman), who speaks fluent Russian. Skyler states the guys are no longer needed, and that they should leave. They accuse him of stealing their project. Skyler tells them they should have had him sign a non-disclosure agreement, and that it is how business is done in Moscow. Ben and Sean are pissed off but leave after security arrives to walk them out.Natalie (Olivia Thirlby) and Anne (Rachael Taylor) are walking around Moscow talking about Anne's upcoming photography exhibition and marveling at how beautiful the city is at night. They mention the blog that Ben and Sean set up and comment that the guys look cute. They take a picture of themselves in front of the Kremlin. Ben and Sean find Zvezda and talk about the possibility of suing Skyler, but they are skeptical that Russia law would agree with them. They see Skyler at the club and confront him; he mocks Sean's shirt with the black sharpie spot, which is visible now he's not wearing a jacket.Skyler tells the boys once again that they are naïve and act like sheep to his wolf.Sean gets drinks and mentions no society exists without alcohol and religion. Ben uses his phone app and finds that Natalie and Anne are in the club; they meet up. They try to impress the girls by saying they invented the app that the ladies were using, and the ladies ask if they are internet millionaires. They say they are internet billionaires. Ben developed the app while Sean is the one who finds the exciting places to party. The group enjoy drinks together and get on great. Nat explains she's waiting to hear from her mom, as Nat left for Moscow with no notice, only leaving behind a note, after her original summer plans fell through, so she's helping Anne with her artistic endeavors. They all take a photo together. Suddenly the lights go out in the club.Cut to outside; our four are in a crowd of surprised Muscovites. Natalie notices her phone does not work. They look up in the sky and see a strong yellow Aurora followed by swirling balls of yellow light that are falling from the sky. The objects divide up, multiplying and falling all over the city. One of the lights lands in a clearing by our foursome, with people surrounding it curiously. The object is mainly invisible, just having a shimmer like a heat haze, with a faint corona of shifting yellow at times. A Moscow policeman (Oleg Poddubnyy) approaches it, holding his nightstick out towards it. As the nightstick touches the object the policeman is suddenly disintegrated into yellow light and gritty ash. His partner takes out his pistol and shoots at the object, but the bullet is shown to shatter on an invisible force field. The crowd panics as more of the alien lights appear and vaporize people. A group including our four runs screaming back into the club and shuts the doors. The windows of the club shatter. We see an alien pov; everything is visible as a network of interlaced sketchy lines; the architecture is silver-white and the people are glowing yellow (the alien pov occurs throughout the film, whenever our crew encounter them, so you can tell if they are successfully hidden or visible). The aliens attack the people at will; using yellow lines of energy to snag them and drag them back to their core, where they turn them to ash .Someone throws a Molotov cocktail at an alien and the fire keeps it at bay momentarily. Skyler saves himself by slamming a door shut on the girl he was with and watches as she is disintegrated. Ben, Sean, Natalie, and Anne exit the club through a hole in the wall and find a storeroom to hide out as more and more people are getting vaporized, taking with them the bartender, who has been wounded. Someone knocks desperately at the door, and although the group tells Natalie not to open the door, she does; it's Skyler.The storeroom acts like a bunker. Through a montage we see the bartender dies so they wrap his body in plastic and duct tape and there is escalating tension and fighting between the guys. A subtitle indicates the passing of each day until...FIVE DAYS LATERThe group is running low on food, and it has been 27 hours since they have heard any noises from outside. They all agree they need to leave, and Ben suggests that they might find help at the US embassy. Skyler suggests the Swedish embassy is closer, but the group says he can go there if he wants, but they want to get home. As they exit the storeroom they see the bar is empty; they grab some liquor bottles and look at a map of Moscow they got at a restaurant, which only has a few main landmarks on it, and few street names. Outside the bar, the whole city is empty; there are weapons and hundreds of cars lying abandoned, but no people. Natalie can't keep up in her high heels, so she takes her shoes off and carries them, barefoot.They walk along the streets, not really sure where they are going, because of their poor map quality. They hear some noises and they find a Russian lady (Alya Nikulina) in her apartment sealing up her window with bricks and mortar. They ask her for directions, but (not understanding them) she tells them in Russian that the "ghosts" are still out there, and they will all die. She does not offer any help and just wants to be left alone. They find a bridge that leads to the Embassy, but it has been destroyed by a Russian warship that crashed into it. They try to find another way to cross the river as they find themselves in Red Square. There is an abandoned police car and a dog barking. The girls and Skyler hide nearby while Ben and Sean go to the police car as they are sure it will have a better street map. They jimmy the boot to find road flares, a flaregun, and a map book of Moscow even though it is in Russian. The dog is barking at something they can not see, and they are surprised to see the dog vaporized by an unseen alien. Ben and Sean hide underneath the police car as the alien searches for more victims to attack. The car's siren and lights activate as the alien approaches. The alien floats over the car. From the alien's pov we can see that the alien can see through the metal body to the engine underneath, objects inside the car etc. The alien cannot see the guys under the car, and they surmise the creatures have weaknesses they can exploit. After it leaves they run into the GUM luxury department store (like an 19th century European arcade, not like a US-style department store) and meet up with the girls and Skyler. Inside the arcade they find a crashed airliner, and they begin to wonder if they can even survive. Skyler tells them that it is a terrible idea to head to the US embassy, but Sean and Ben feel they need to move on to survive. They surmise that the aliens must use some sort of electrical field for protection and that it seems to activate electrical devices. They find some light bulbs they make into necklaces to use as warning devices: the bulb will light up if an alien approaches.Skyler mentions they need to make smart choices and that you do not know who you are till you find yourself in a troubling moment, and that he keeps replaying the moment in his head (presumably the moment he sacrificed the girl to save himself). Ben suggests they need better clothes; Sean then makes a comment about a coat and tie being the clothes for the end of the world.Natalie searches in one store for something to wear and undresses. From the opposite side of the store Sean sees Natalie stripping to her bra and trying on a top, but they are interrupted when the light bulbs begin to light up. Sean is caught in a store display window, taking clothes off a mannequin. He freezes as the alien passes right by him, sure it will see him, however it passes him by without a thought. Natalie hides in amongst clothes racks as an alien walks through the store she is in. Sean grabs Natalie from behind and drags her to a square glass display case. The alien passes so close to them that its shield brushes Natalie's hair, but it does not see them. Sean deduces that the glass may have blocked its sensors since glass is an electrical insulator. The group flee the building and head for the US embassy. Natalie now has shoes and a jacket.They find the US Embassy, but it is empty, with multiple shells from a gunfight in the foyer. Skyler finds a semi-automatic rifle and goes berserk with it, firing at nothing. He stays in the foyer as the others climb up the floors, looking for survivors. They find a situation room. Natalie finds written reports that give them information on the events so far. Apparently global communications lasted for three minutes after the attacks. Most capital cities have been invaded, and no form of resistance seems to work against the aliens. The report does state that underground installations did survive the attack; so the aliens have some weaknesses in their attack plan. They find a map on the table with numbers, and these are estimates of survivors. Natalie reads further that an emergency action station has been setup on a radio frequency but the report ends there. They also find a radio under a birdcage but do not know why it is in there. Sean turns it on, and finds the signal, but the voice is in Russian so no-one can understand it. Anne gets very scared and begs him to turn it off, saying it might draw the aliens. Natalie checks her cell phone but, it still does not work. They look outside and, using a rifle scope, see large alien installations digging all around the city, with thick columns of yellow light and smoke streaming into the sky. Buildings surrounding the pits are collapsing into them as they get bigger. They hear gunshots, and see that Skyler has fled into the streets. The girls want to let him go, but Sean and Ben chase him as they need his ability to speak Russian to understand the radio message: they take the radio and map with them. As Sean and Ben make it outside the girls see from their vantage point that an alien is coming. They run outside to warn the guys. As Sean and Ben reach Skyler and explain he needs to come back with them the car alarms are set off and they realise they're caught in the open. Skyler tells the guys to run, and distracts the alien by shooting at it until it pays attention to him and not to the guys. Skyler is vaporized as the boys make their escape.Sean and Ben join the girls, and Sean notices a room lit with electricity in an apartment building off in the distance. They suspect that this is what made Skyler leave. They move carefully in the night not drawing the attention of the aliens. They are able to make it to the building as sunrise comes. Inside the building, they encounter a young lady with a gun who speaks to them in Russian. They answer in English and are happy that the lady named Vika (Veronika Ozerova) does answer back in English. She tells them to follow her, and she takes them in a room which is completely lined with a metal cage; over all windows and the door. They meet an older Russian man named Sergei who is happy to see more survivors. He has a diesel generator and a ginger cat. He locks the cage behind them and explains that the light is there to draw people to him; he was able to bring Vika and now them. Vika explains that they are in a Faraday Cage and the aliens can not see inside it.They turn on the radio from the embassy, and Vika translates the broadcast for them: the K152 nuclear submarine is in the Moscow River, and will pick up any survivors that reach it before the next morning. They also reported that a number of other submarines are doing the same around the world; Sergei comments that the submarines are perfect Faraday cages. Vika tells the ladies she will need their help getting supplies for their journey: they think she means in the apartment, but she takes them out into the surrounding apartments to scavenge. The latch on the door to the Faraday Cage sticks a little as Anne closes it.Sergei shows Sean and Ben a microwave gun he built that he believes can be used to disrupt the wave-form and electrical shield of the aliens. While getting supplies the aliens spot the three ladies in the building and move in to attack. Vika sees from the exterior lights that the aliens are coming and tells Anne and Natalie to run. Vika heads one way and Anne goes the other; Natalie is torn but follows Anne. They run inside the apartment but cannot lock the cage behind them; they try to block the doors, but an alien follows them in. Anne upends a glass coffee table and the girls hide behind it so the alien cannot see them. Sergei fires the microwave rifle at the alien; it appears to work, and disrupts the alien somewhat, but it doesn't actually damage it, so the creature is still able to attack, vaporizing Sergei.When disrupted, we can see the aliens: they look like the Pokemon Onix, inside the sphere from Contact, if that sphere was made from black vertebrae. Their eyes glow.Sean and Ben climb out through a fire escape and call to the girls to follow. Natalie sets some diesel fuel on fire to cover her escape. She calls to Anne from the window: Anne is too scared to move, and hesitates: by the time she finally leaps out from behind the table the alien sees her and vaporizes her as she reaches for Nat.The group moves on trying to reach the river, but they encounter a group of armed men dressed in metal and carrying makeshift shields. They are able to attack an alien with gun fire, a flamethrower and an RPG. They succeed in hurting the alien and a small piece of hard black metal or glass-like substance comes off it, before the alien retreats: the men explain they can hurt the aliens, but not kill them. They are led by a man on horseback who explains that the aliens are weakest when they use their lightning. Sean picks up the piece that was knocked off the alien, wraps it up and keeps it. The militia appear to be friendly and happy to offer help and protection; our heroes share what they know about the aliens and information about the location of the submarine. The leader of the group is skeptical and tells them that the river is too far, the area is infested, and it is right in the middle of the resource-collecting light towers.Sean says they will go, and is joined by Natalie arguing that this is not her home, and she wants a chance to go home. Vika says she will join them. Nat's impassioned speech persuades the militia guys to help them: they suggest that using the subway tunnels might be the best way to reach the sub. As they climb down to the station they scatter miniature light bulbs along the floor as they move to check if there are aliens in their path. On the platform an alien attacks. They all make it to the track in time except for Vinka, who is trapped hiding behind a pillar as the light-tendrils search for her. Ben saves her, but is grabbed himself, pulled along the platform, and vaporized.The rest are able to run along the tracks and reach the river, where they find a boat to hide inside; they untie it from the jetty so it will float downriver to the submarine. Sean is upset at losing his best friend, and Natalie comforts him as Ben had done for her earlier when she lost Anne. He tells Natalie about how he and Ben became friends when they were little: Ben was in the special advanced class, and his mom made him play with him at first. Sean says he wants to say goodbye to Natalie now, as he never got a chance to say goodbye to Ben.The boat runs aground on some debris, and they are all forced on deck to try and figure if they can free it. They witness a number of alien spaceships using their beams to lift up metals and resources (maybe this scene was deleted in my country; I saw only the same columns of light, no spaceships). The Russian surmises that they are strip mining the planet for metals that they can use; that Moscow is rich in copper and other metals. Vika can see the submarine just downriver, and states they can swim and make it. Suddenly a building next to the riverbank collapses as the extraction hole gets bigger: the debris crashes into the boat and all are tipped out into the river. Sean and most of the others are able to swim out to the submarine, but Natalie is missing. The Captain--------- welcomes them but tells Sean the sub needs to leave now. Sean tells him that Natalie is lost, and he knows she is still alive when they see a flare launch in the distance. The Captain agrees to wait; the technicians on the submarine look over the microwave gun and they offer to build more with better batteries. Sean and the militia leader go out with a group to search for Natalie, armed with two microwave guns and assorted weaponry. Vika wants to come too but they won't let her.As they move they throw out cell phones as warning devices, and proceed to a trolleybus depot that Sean thinks Natalie may be hiding near. One cellphone behind them rings; an alien is approaching. Sean trips over, and his weapon does not work; finally he fires his disrupter, and the rest of the crew are able to destroy the alien with small arms fire and an RPG. They setup an ambush zone: spilling water from a tank truck over a concrete pad to amplify the effect of the microwave guns: if they can disrupt an alien standing in the water then all the aliens standing in the water will be disrupted. Sean leaves them to search for Natalie.The militia group tries to lure three aliens over the water but they won't come close enough. Suddenly Vika appears behind the aliens, and with a few well-aimed molotov cocktails she forces them onto the water. The militia disrupt then kill all three aliens.Natalie hears Sean calling for her, and shouts out the number of the bus she is hiding in, so he can search among them, like an Amazing Race challenge. Finally Sean finds the bus and enters it; she stands up from her hiding place, at the same time as an alien arrives. The electrical surge makes the bus doors shut, trapping them inside with the alien, and the bus start to move. Nat steers the bus as it careens along the streets, bashing past abandoned cars. The alien light-ropes Nat. Sean disrupts it, but has no weapon. As a last resort he throws at it the piece of alien he picked up earlier, finally killing it.With the alien dead, the bus slows to a halt, at exactly the same location as the militia ambush, as this finishes. Reunited, they all head back to the submarine. Sean asks the militia if they will join them on the sub; the militia explain that this is their home, and they need to stay and fight. The leader refers to Napolean's defeat in Moscow in 1812. It is up to Sean to let the rest of the world know what they have learned about the aliens. The submarine leaves and sets sail for the open ocean (giving us the strong impression it will now be crossing the Atlantic to deliver Sean and Nat to the United States of America) .A Russian technician repairs Natalie's cell phone; she is able to get a text message from her mom. It states she is safe, hiding with survivors in Penn Station, and that she loves her and hopes to see her soon.Sean and Natalie almost kiss, but are noticed by Vika who is listening to music on her ipod, who says "Gross!". Later, with Russian naval minions in the background, Sam and Nat look at a map while listening to shortwave radio reports coming in about groups who are fighting back against the alien invasion. One group in Paris has even brought down one of the mining towers. Sean says, stirringly, "This is how it starts" i.e the takeback.The submarine sails on (on the surface of the ocean) as the movie ends. Yet the war between humanity and the aliens continues.
question: What did the policemen and Vika use to destroy the alien?
answer: Microwave guns amplified by sitting water