The party eventually turns into a riot causing tons of property damage and Thomas's house burns down. Thomas earns the begrudging respect of his dad for throwing the party and ends up reconciling romantically with Kirby (though he does some jail-time).
In this documentary-style film, we follow high school geek Thomas Kub (Thomas Mann) as he prepares for his 18th birthday while his parents go out of town. The parents aren't worried about leaving as Thomas' dad thinks Thomas is a social "loser" who won't get into any trouble. The party is being planned by his friends Costa (Oliver Cooper), who is a wealthy and gregarious teenager from Brooklyn, and JB (Jonathan Brown), who is fat and socially awkward. Costa wants this party to be a game-changer for them. Costa has hired a classmate named Dax to record the entire party.
Costa immediately begins spreading the word, sending out mass texts to everyone in the high scool (and as we later find out, posting party details on CraigsList and getting the party promoted on the radio). Thomas personally invites his long-time female friend Kirby (Kirby Blanton) as well as the hottest girl in the school, Alexis (Alexis Knapp). The trio then buy alcohol and convince local college sports hero Miles to attend. The three also buy pot from local drug dealer T-Rick and steal his garden gnome to serve as the mascot for the party (and enraging T-Rick in the process).
Kirby shows up early and she and Thomas begin making out before they are interrupted by a huge influx of people bringing huge shipments of alcohol. Thanks to Costa's investment in a professional DJ and moonbounce, the party hits high gear, with celebrants swimming naked in the pool, getting Thomas' dog high and doing hard-core body shots. The party grows larger once the gnome is smashed to reveal that hundreds of ecstacy pills were hidden inside.
More and more people show up and make their way into the house, causing massive property damage and driving Thomas's dad's sportscar into the pool. JB and Costa convince Thomas to try something "new" rather than go after Kirby, so he and Alexis begin having sex. Kirby walks in on them and storms out of the party. Eventually, the party gets so big that local news choppers begin filming the scene. The police arrive in riot gear and are repelled by the attendees. Then, T-Rick shows up bearing a flame thrower in order to recover his gnome (and the now long-gone ecstacy). T-Rick's violent arrival ultimately causes the party to break up, with the police ultimately stopping T-Rick's rampage (but not before he burns down Thomas's house and singes some of the neighboring yards too).
The teens split up to face the wrath of their respective parents (though Dax mentions that his parents are "gone" and he lives alone). When Thomas's parents return they can't believe the havok Thomas wreaked. Though the mom is upset and Thomas learns that his college fund will be used to pay for the damage caused, his dad expresses begrudging admiration that his son was capable of holding such a party. When Thomas, JB and Costa return to school, they are cheered by their fellow students in the hallway. Thomas apologizes to Kirby and the two make up and agree to begin dating.
In the epilogue, we learn that T-Rick survived his encounter with the police; JB was found to be a "special needs" child by the court and avoided prosecution (though he is now forced to ride the short bus); Costa was found not-guilty thanks to a high-priced attorney team but is facing several paternity suits stemming from the party; Dax is being investigated for the disappearance of his parents; and Thomas' college fund was drained and he alone was convicted of several crimes.
During the credits, Jimmy Kimmel (in a cameo appearance) jokes about the party. Costa is interviewed by real-life news anchor Jillian Barberie and promises the next party he throws will be even bigger.