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In Pasadena, high school students Costa and J.B. plan to throw a party for their friend Thomas' 17th birthday, aiming to increase their popularity. Thomas' 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 is reluctant to have the party at his house, but Costa continues with his plan and hires A/V student Dax to chronicle the night's events. While Costa, J.B., and Dax advertise the party throughout the school, Thomas himself invites his best friend Kirby, who has a crush on him, and Alexis, a popular girl in their school whom Thomas has his own crush on. While shopping for party supplies, the boys have a chance encounter with actor Miles Teller and nervously invite him to the party, only to discover that the celebrity has already heard about it through the grapevine and plans to attend.
The trio then visits drug dealer T-Rick to buy marijuana, wherein Costa steals his lawn gnome to use as a party mascot. As they leave, T-Rick discovers the theft 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, partygoers arrive en masse, Miles Teller included. Thomas intends to limit the party to the backyard and pool house, with the house guarded by Everett and Tyler, two young security guards hired by Costa. However, more and more people begin arriving, and the party moves beyond their control and into the house. 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. Things escalate quickly as the party draws the ire of the neighbors and the police arrive, responding to a noise complaint. However, the partygoers hide and remain silent, convincing the officers that the party has already ended. The police leave and the party resumes.
Miles Teller smashes T-Rick's gnome, revealing that it contains large amounts of ecstasy tablets, which the partygoers quickly consume, including Thomas and his friends. 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 noise and chaos of the party, including violence and mass property destruction, which has now spilled into the surrounding neighborhood, receive televised news coverage with helicopters flying over the house. A dwarf guest drives Thomas' dad's Mercedes into the pool after being put into an oven by other partygoers earlier.
T-Rick arrives armed with a flamethrower, setting fire to trees, cars, and houses in the neighborhood in search of Costa and in an attempt to reclaim his gnome, 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 (and the rest of the neighborhood) is left aflame, and the SWAT team moves in to retake the neighborhood. By morning, the boys return to their respective homes to discover what punishment awaits them. After his parents return, Thomas' 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.
The epilogue reveals that T-Rick survived the explosion and is arrested for his actions; Thomas is convicted for disturbing the peace, contributing to the delinquency of minors, and inciting a riot, but is also voted most likely to succeed by his classmates. Costa and J.B. are acquitted, the former having a competent and expensive lawyer but awaits results of three separate paternity tests, and the latter's parents convincing the court that he is mentally incapable and unfit to stand trial but is forced to ride the short bus for the remainder of high school. Dax, meanwhile, is under investigation for the disappearance of his parents. In a television interview with Jillian Reynolds, Costa invites her to his next party instead of giving an apology.
