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Three high-school seniors throw a birthday party to make a name for themselves. As the night progresses, things spiral out of control as word of the party spreads.

It's Thomas Kub's 17th birthday and all he wants to do is throw a small party with some friends to help raise his social status and maybe even get lucky. But when his best friend Costa starts calling radio stations and putting ads up on Craigslist, you can be sure that this party is going to get really out of hand really fast.

High-school senior Thomas Kub wants to have a small get-together while his parents leave town. His two friends convince him not to worry, and to invite a couple more people than intended. Once people start showing up, the party begins to spiral out of control, leading to a uncontrollable situation.

Three seemingly-anonymous high-school seniors attempt to finally make a name for themselves. Their idea is innocent enough: let's throw a party that no one will forget, and have a camera there to document history in the making. But nothing could prepare them for this party. Word spreads quickly as dreams are ruined, records are blemished, and legends are born.—Warner Bros. publicity

In Pasadena, high school students Costa (Oliver Cooper) and J.B. (Jonathan Daniel Brown) plan to throw a party for their friend Thomas's (Thomas Mann) birthday, aiming to increase their popularity. Thomas is turning 18. Costa names the party "Project X". Costa is a prankster by nature. He walks in on Thomas while he is taking a shower and even rubs his toothbrush on a bar of soap that leaves a bitter taste in Thomas' mouth. Costa is hoping that the party will finally help him lose his virginity.

Thomas's parents go away for the weekend, leaving him alone in the house, but warn him not to have too many people over, and not to drive his father's Mercedes. Thomas's father specifically tells him not to allow anyone into his study as he is working on 3 projects simultaneously. Thomas' father knows that Thomas is not the most popular kid in school and believes that at most Thomas would have a couple of friends over. Thomas's mother is against giving him permission to host a party while they are away. Thomas shares his birthday with his parent's anniversary. They gift him a mini-van before they leave for the weekend.

Thomas is reluctant to have the party at his house, but Costa continues with his plan as he had heard that even complete losers get laid at such parties. Costa says that to make it as big time players they need to make a big-time play. Thomas says that he is the one who has to take the entire risk. Costa is relentless and Thomas agrees to have maximum 50 people at the house.

Costa hires AV-student Dax (Dax Flame) to chronicle the night's events. Costa and J.B. advertise the party throughout the school. Thomas himself invites Alexis (Alexis Knapp), the hottest girl at school, and his best friend Kirby (Kirby Bliss Blanton), who has a crush on Thomas. Despite their efforts most students at school have no clue who Thomas is, and have no plans of attending the party.

As the boys shop for party supplies, they have a chance encounter with Miles Teller and nervously invite him to the party, only to discover that Teller has already heard about it through the grapevine and plans to attend. Thomas tells Costa that the party needs to be restricted to the back lawn, and that he would keep the house locked. He says that he would give Costa, JB and Dax full access to the house, in case they score with a girl and want to have sex.

Costa, Thomas, and J.B. go to buy marijuana from drug-dealer T-Rick (Rick Shapiro). While T-Rick gets the marijuana, Costa steals his lawn gnome to use as a party mascot. As they leave, T-Rick discovers his gnome stolen and chases after them, but they escape in Thomas' mini-van. As night falls, the party's start time passes but no one turns up and Thomas worries that the party will fail.

Suddenly, party-goers arrive all together, Miles Teller included. Thomas intends to limit the party to the backyard and pool house with the house guarded by two young security guards, Everett and Tyler, hired by Costa. However, more and more people begin to arrive, and the party moves beyond their control and into the house. Thomas begins to have a nervous breakdown in the bathroom and Costa and J.B. find him there. Thomas questions Costa on how he advertised the party, forcing him to confess that he put ads on Craigslist and on a local radio station, worried that no one would attend.

As the party grows, T-Rick's gnome is smashed, revealing that it contains a large amount of ecstasy tablets, which are quickly consumed by the party-goers. The police arrive, responding to a noise complaint, but the party goers remain silent, convincing the officers that the party has already ended. After the police leave and the party resumes, Thomas kisses Kirby and reveals that he loves her. Alexis meanwhile flirts with Thomas throughout the night and eventually seduces him. Kirby walks in on the pair as they are about to have sex and leaves the party upset, as does Alexis, upset with Thomas running off and being secretly filmed.

The party has now escalated into violence, property damage and vandalism, which has now spilled into the surrounding neighborhood. This receives televised news coverage with helicopters flying over the house. A dwarf guest (Martin Klebba) drives Thomas' dad's Mercedes into the pool after being put into an oven by other party-goers.

The police return in force but are overwhelmed and repelled by the guests. They decide to let the party end naturally before moving in. T-Rick arrives armed with a flamethrower in an attempt to reclaim his gnome. He begins setting fire to the trees and cars in search of Costa, forcing the guests to flee and the party to end. The police shoot his flamethrower pack, and it explodes. Thomas, Costa, J.B., and Dax flee with the other guests as Thomas' house burns and the SWAT team moves in to retake the neighborhood. The neighborhood is left aflame.

By morning, the friends return to their respective homes to discover what punishment awaits them. After his parents return, Thomas's father commends him for throwing such a large party because he thought he was a loser but reveals that Thomas is very much in deep trouble as his college fund will be used to cover the damages. At school, the boys are cheered by the students, and Thomas reconciles romantically with Kirby.

In the epilogue, T-Rick is recovered alive following the explosion, Thomas is convicted for disturbing the peace, contributing to the delinquency of minors, and inciting a riot, and Costa and J.B. are acquitted. Costa because of his expensive lawyer and J.B. because his parents convince the court that he is mentally incapable and unfit to stand trial.

Costa, however, is currently waiting for the results of three paternity tests. Dax, meanwhile, is under investigation for the disappearance of his parents. In an interview with Jillian Reynolds, Costa promises his next party will be even better.