Evan (Ben Stiller) narrates the beginning of the movie, talking about how wonderfully diverse the town is, but says he doesn’t have any black friends yet. He has founded the running club, the Spanish club, and created the position of deputy secretary at city council. He wants to help his town as much as possible. He works at Costco and has worked there for 12 years now he is a manager.
A man named Bob (Vince Vaughn) is at the store buying a 3D TV from him. He loves his job. His friend and coworker Antonio (Joe Nunez) comes in to tell him that he is officially an American citizen. Antonio does security at Costco and celebrates by getting drunk and high at the store during the night. Suddenly the lights start flickering, and it’s clear someone is in the store. Antonio takes out his gun and doesn’t even know how to use it. He sees something and starts shooting and running away but can’t get out of the store and is caught. We don’t know what happens.
The next day Evan’s wife Abby (Rosemarie DeWitt) is packing to go to a baby shower in NYC. He doesn’t want her to go because they hate NY. On his way to work he sees cop cars and an ambulance at Costco. He’s told that his store is closed until further notice. They tell him that Antonio is dead, and it looked like he was ripped apart by a wild animal (his skin was ripped off). The cops are incompetent. They are asking Evan why he killed him, but are just joking. They go inside the store and Evan is shocked at what he sees. He cries on the way home. The cop on the news is blaming the citizens of the town since there are thousands of them and should be doing something.
The next night during a football game Evan runs out to the field at half time and makes a speech about being safe, the murder, and the fact that he is forming a neighborhood watch.
Bob, Franklin (Jonah Hill), and a British man named Jamarcus (Richard Ayoade) show up. Bob thinks the group is all about getting away from the wives for a night. Franklin wanted to be a cop after dropping out of high school but didn’t get hired so this is his second chance. Jamarcus seems normal but also has a dirty fantasy about neighborhood watch. They don’t want to hear what Evan has to say and want to drink, so they go to Bob’s house with all the “material.” His garage is the ultimate man cave. Evan can’t stand their immaturity, and so he leaves to put up fliers.
He sees a man named Paul outside his house. He’s new in the neighborhood and has some of his mail by accident and was just dropping it off. He’s kind of strange. The next day Abby comes home and wants to have sex because she’s ovulating, but Evan has the excuse that he has to meet the guys for neighborhood watch. Bob has made jackets with a new logo that Evan doesn’t like because he’s too conservative. At night, the men are all sitting in the car, but nothing is happening. Bob pees in a can, which obviously pisses Evan off. He also opens up beers. A cop car comes by and asks them what’s going on. Evan tells them that they’re on neighborhood watch, and the cops make fun of them with the flyers and accuses Evan again. They give him a ticket for having open alcohol containers in the car and for putting up flyers without a permit.
The guys get a noise complaint at the football field and leave to go there. They’re walking on the field and get egged by a kid. They get him and bring him to the police station. The kid leaves since he’s not really in trouble and gets scared as he sees a lot of shadows and hears noises, and he’s attacked. Meanwhile, the guys are driving home on a high from thinking they actually did something. They get into an accident and freak out because the other guys have been drinking. They don’t know what they hit, but it looks like an octopus tentacle on their car. Evan sees green slime on the car and realizes it was near Antonio’s dead body. They see some big basketball sized thing, and when they touch it, it starts to glow. A man comes out of his house because they are in his backyard and he starts screaming at them with a rifle. He leaves, and the guys put their fingers in holes on the orb. It shoots out power and a tree explodes. They decide to play with it and go into the woods shooting various things including a cow. Back at Bob’s house, they talk about all the things it could be alien, government weapon, etc. They think the old man could have something to do with it since it was in his backyard. Evan goes home and sees the strange man standing outside his house next door.
Bob spies on his daughter Chelsea’s (Erin Moriarty) facebook page and sees a video of her making out with some guy named Jason. He flips out on her and makes her take the video down saying that sex is for love. Meanwhile, Evan is at home looking at a pinboard he made of all things about Antonio’s death and the accident. Bob sends him a desperate text so he meets with him. He just needs to vent about his daughter. Evan reveals that he can’t get Abby pregnant, and he hasn’t told her. Evan gets a call from the old guy that there’s somebody there. They all show up and split up. Evan and Jamarcus see an alien type creature in the woods. He tosses it food and all of a sudden they see the old man dead on the ground. The alien goes after Jamarcus, but Evan smacks something over its head to knock it out.
They bring the dead alien back to Bob’s house. They take tons of silly pictures with it and a video of Bob making out with it. Suddenly it comes to life! Franklin starts throwing stuff at it. Evan jumps on its back, and they hear it talk to them saying “We are among you.” They get it outside and realize that they take the skins of the people so they can be among them. They don’t know who is and who isn’t an alien now. Evan is on edge and isn’t being nice to his neighbors anymore. They are basically watching everyone they see, thinking they are aliens. They’re at Costco watching everyone and Paul comes over to invite Evan over for a “gathering.” He tells him that he’ll think about it, and he tells them that he thinks he’s one of them. He complimented Evan’s skin, buys a lot of batteries, and said, “Before you know it, life ends.”
Evan goes home and sees Abby half naked trying to seduce him, not knowing that all his friends are with him. They argue because he’s always too busy to have sex with her. He tells her that he can’t explain right now and has to run off. They go to Paul’s house, and Evan can’t believe they’re all drinking. They see a couple go to Paul’s house, and Bob’s daughter calls asking to go to a party. Bob gathers the guys and says they have to go to her friend Mandy’s (Carissa Capobianco) house to stop the party. He takes Franklin with him to the dismay of Evan and Jamarcus. They hear screaming coming from inside Paul’s house and are approached by him outside. Paul takes them inside, and it’s a giant orgy going on inside his house. The batteries he bought were for tons of dildos. Jamarcus stays, but Evan leaves.
At Mandy’s party, Bob and Franklin see a guy named Jason taking Chelsea away. They go inside the house, and Bob is on the hunt for his daughter. She’s in the bedroom with Jason (Nicholas Braun) making out, and he wants to sleep with her. She tells him that he’s going too fast and wants to go back to the party, but he won’t stop. Bob walks in on them and is going to take her home. He starts yelling at Jason, who head-butts him and throws him outside through the glass doors. Franklin takes out a knife and yells at Jason and threatens the rest of the kids. Bob takes Chelsea home and Evan is waiting there for them. He tells Bob that what he did wasn’t cool, but Bob says that he had to help his daughter, and Evan needs to lighten up. Evan sounds like a jerk and Bob tells him that he doesn’t even have a personal life or friends. Evan kicks Bob out of the watch, and Bob says he will start his own and gets Franklin on his side.
Evan runs home and tells Abby (and her friend Carla (Liz Cackowski) is there) that he is sterile. Carla leaves and he comes clean about everything. He tells her that every time she wanted to have sex it’s all he could think about, and he was afraid to tell her. He says that there is one more secret, but can’t tell her because Jamarcus comes to the door and says there’s something important. Evan, Abby, and Jamarcus go to Bob’s. Jamarcus cuts himself with a knife and the green goo comes out. He reveals that he’s an alien, but he’s not there to hurt them. Their ship came there six months ago, and they have been hiding ever since. He says that in a few hours, their transmitter will become operational, and they will take over the world starting in Glenview. The transmitter is in Costco because they have everything they need under one roof. He tells them that they can get away and live in caves and things for the rest of their lives. He leaves, but tells them to save themselves while there’s still time.
They all try to decide what to do and decide to take out the transmitter themselves because “Costco is for members only.” They go to Franklin’s bedroom, and he has tons of weapons. His mother comes up, and he yells at her to stay out of his room. He apologizes, says he loves her and tells her to stay inside the house that night. In the car, Evan goes on the radio to the cops and tells them what’s going on, but they just make fun of him. Franklin goes on the radio and says Evan is the killer and to get to Costco quick to get him there.
They get to Costco and start using merchandise to create something to stop the transmitter. They see the alien in the store and Evan shoots, but the cops show up. They tell Evan to drop the gun and accuses him of killing everyone. They try to explain, but the cops won’t listen, until one of them is killed by the alien. Everyone starts shooting it while the cop is on the ground crying, trying to revive his dead friend (though his heart is out of his body). Franklin gives him a pep talk and he tells him that not letting him on the force was the biggest mistake he ever made. The alien comes back to life so Evan keeps shooting him. They walk to the transmitter and find Jason, who transforms into an alien. Franklin and Evan go after the transmitter while Bob fights Jason. Suddenly they are surrounded by aliens and are out of ammo; however, all the aliens start dying. It’s Jamarcus! He shot them in their penises that’s where their brains are. They let Bob know via the walkie talkie, and he kills Jason by grabbing his penis. They get inside the transmitter and Evan grabs the orb. Everything shuts down, but the aliens are still on the loose. They need to take the orb out before the aliens take over. They all start to run away, but the aliens are after them. They hit the security switch on the way out and just slide under the gates before the aliens can get out. They have the orb and aim it at Costco blowing it and the aliens up. Abby thinks Evan has died in the explosion, but the four men walk away fine. Evan says Costco will rise again from the ashes. Evan and Bob apologize to one another, and they are all best friends again.
It’s prom night, and Chelsea’s boyfriend asks her if it’s true Bob ripped her last boyfriend’s penis off. Franklin is a cop. Evan and his wife have adopted a little girl, and Jamarcus still goes to Paul’s orgies. They are all still on neighborhood watch together.
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