Originally titled Neighborhood Watch, it was retitled and the marketing was changed to focus on the aliens after a controversial incident in Florida where an altercation between a teenager and a Neighborhood Watchman left the teenager dead from a bullet wound.
The story takes place in the small town of Glenview, Ohio. It begins when local Costco manager Evan Trautwig (Ben Stiller) forms the Watch after his store's night watchman is gruesomely murdered and the police don't seem to care. He's joined by three others: Bob (Vince Vaughn), a construction worker with a rebellious daughter, Franklin (Jonah Hill), a high school dropout who wanted to be a cop but failed the psych tests, and Jamarcus (Richard Ayoade), a recent divorcee. None of them are as dedicated as Evan, though.
Then they quite literally crash into the aliens' plot. The aliens are replacing people in the town as part of their preparation for a full invasion of Earth. Now its up to them to find out who has been replaced and stop them in time.
The Watch provides examples of:
- Achilles' Heel: Turns out the only way to kill the aliens is to shoot them in their brains, which are located in their dicks.
- Aliens Are Bastards: Why are the aliens invading Earth?Jamarcus: We're aliens, it's what we do.
- Alien Arts Are Appreciated: Jamarcus: "...also the ballsucking was a real eye opener."
- Alien Blood: The aliens have green blood which according to Bob, it has the consistency and smell of human cum. And according to Jamarcus, the taste.
- Big Damn Heroes: Jamarcus after his Heel–Face Turn, coming to the rescue with guns blazing.
- Bizarre Alien Biology: The aliens are Made of Iron and their brains are not located in their heads, but in the groin area.
- Brick Joke:
- Jamarcus mentions early on that one of his dream scenarios is an Asian woman coming up to him and offering to suck on his balls. When he and Evan happen upon an orgy in their neighborhood, this exact thing happens.
- Bob says "I swear to God I will rip his [his daughter's boyfriend] dick off with my bare hands." Her boyfriend turns out to be one of the aliens, and he does exactly that.
- The Cameo: From Lonely Island members Andy Samberg, Jorma Taccone and director Akiva Schaffer playing casual masturbators at the orgy.
- Chekhov's Gun: The alien orb powers the beacon that contacts the rest of the alien race and is used to blow up all of Costco to destroy the aliens.
- Evan's neighbor, who the Watch suspect of being an alien and torturing people in his basement. He isn't one, and is in fact a swinger hosting orgies in said basement.
- Also Jamarcus, who can act a little weird some times. Justified, in that he's an alien.
- Date Rape Averted: Played straight when Bob interrupts a party his daughter attends.
- Destroy the Product Placement: The Costco gets blown up sky-high at the ending of the movie, and lovely rendered close-ups of (flaming) flying products are included.
- Easily Thwarted Alien Invasion: All they really had to do was steal the "alien bowling ball" from the transmitter. Sure, a thousand aliens chased them and tried to get it back, but they were able to kill them thanks to the explosive power of the alien bowling bowl.
- Foreshadowing: At the beginning, Evan is seen briefly closing the latch that leads to Costco's basement. Which is where the aliens are holding the beacon.
- Four-Temperament Ensemble: Evan is Choleric, Bob is Sanguine, Franklin is Melancholic and Jamarcus is Phlegmatic.
- Groin Attack: This is how the aliens are killed.
- Guns Akimbo: Jamarcus. Yes, him. It might not even be unrealistic, since he's an alien and might even possess the physical ability to split his vision and look down both sets of sights.
- Happy Ending: Everybody gets one. Even Jamarcus, literally.
- Heel–Face Turn: Jamarcus, who was an alien all along. The reason? He likes human sex! Would have also counted as a Sex–Face Turn but it's also revealed that he was touched by Evan's kind nature and all the general attempts at teamwork.
- Hidden Disdain Reveal: Jamarcus talking about the aliens' leader, who committed the first murder in the movie.Jamarcus: Ugh, yes, I know him. His name is Flootrock. And believe me, the man is a cunt.
- Let's Get Dangerous!: Jamarcus
- Not Hyperbole: Earlier in the film, Bob mutters that he's going to rip the dick off his daughter's boyfriend with his bare hands if he gets touchy with her. Come the film's climax, Bob really does it, and by doing so Bob kills him. The boyfriend was one of the psychotic aliens that have been threatening the protagonists throuhout the whole film and their brain is in their groin.
- Parody: Of Alien Invasion movies.
- Police Are Useless: Not only do the local police have no leads on the killings or show any interest in investigating further, but Sergeant Bressman and his partner Chucho start accusing the Watch of being the culprits.
- Possession Implies Mastery: Subverted when Jamarcus is asked how the alien transmitter works.Jamarcus: What do you mean "basic"? Look at it, it's not "basic"! This looks complicated.
- Red Herring: Evan's neighbor isn't really an alien.
- Sir Swears-a-Lot: The shotgun-toting redneck farmer.
- Stepford Smiler: Evan's neighbor.
- Vulgar Humor: And how! Just about all of the humor relies on profanity, sexual references, gross-out gags, and sometimes, even combinations of them, but the aliens' one weakness turning out to be fatal damage to the groin is the part that really takes the cake.