Two high school nobodies make the decision to crash the last major celebration before the new millennium on New Year's Eve 1999. The night becomes even crazier than they could have ever dreamed when the clock strikes midnight.
Best friends Eli, Danny, and Garrett talk about what they want to do on New Year's Eve in 1999 while their parents are away. Despite Danny's encouragement to kiss Laura at midnight, Eli is too shy to approach her because he has a crush on her. Laura and her pals Trevor, Madison, and Raleigh decide to smuggle out some wine and attend a party hosted by Laura's boyfriend, Soccer Chris, while they watch.
Eli is at a party when he witnesses Chris and Laura sharing a midnight kiss, which he finds shocking. A student is discovered dead with a fan fragment stuck in his brain after the power goes off. Trevor is killed when a toy automobile shows up out of nowhere and burns his face. After realizing that all technology has turned sentient, the frightened party-goers try to flee, but many of them are brutally murdered. In the turmoil that follows, a Tamagotchi murders Raleigh, a VCR uses a VHS tape to drill Madison's head, and a microwave melts Chris' brain.
Along with juvenile offenders Farkas, CJ, and Ash, Eli, Danny, and Laura make their getaway; however, Farkas dies when he strikes the ground while trying to rail a broken pole on his skates, and Danny is mortally stabbed by a blade. The gang visits an abandoned mill without electricity, where the sentient robots murder a number of locals. They also locate Jonas, Garrett, and Laura's ex-boyfriend and discover that the threat is a global conspiracy by all technological gadgets to enslave people.
Laura finally develops a kill code to stop the algorithm, which is now known as the "Amalgamation," but she is attacked by a machine. Eli turns it off by dousing it with water. The Amalgamation, which is utilizing electronics to enlarge itself, then corners the group. Garrett gets beheaded while attempting to repel it. The group rides the Amalgamation down a hill while hiding in makeshift restrooms. They find safety at a VHS store and invite musician Fred Durst to join them.
When the gang gets to the nearby high school, they discover that the machines are there because a new internet connection has been set up. They enter covertly and find that brain-implanted chips have turned the residents of the town into mindless slaves. As the gang discovers that the Amalgamation has now expanded to an enormous extent, CJ gives his life to save Fred from a machine. Laura is electrocuted by the Amalgamation when Fred, Ash, and Jonas divert the robots protecting the villagers.
The Amalgamation continues to ridicule and electrocute them, so even when Eli gets there in time to save Laura and input the death code, it is useless. Then, like a protective glove, Eli puts a condom that Danny had previously given him around Laura's hand. Eli and Laura are successful in inserting the kill code, which shuts down the algorithm globally and destroys the amalgamation, while Fred, Ash, and Jonas continue to divert the machines. As daylight approaches, Eli and Laura kiss and reconnect with the remaining residents of the village.
Five years later, in 2005, Ash, Laura, and Eli-who are now in college-visit the graves of their pals. Ash's new iPod begins to malfunction as they are leaving the graveyard, indicating that the algorithm has not been completely obliterated.
