High school student Zoe Hull struggles with the death of her mother, Jennifer, and her relationship with her father, Todd, is strained. On the way to school, Zoe witnesses classmate Chris Jelick planting a strange device in a field, but writes it off as a prank. At school, Zoe's best friend Lewis attempts to ask her to prom, but it backfires and Zoe leaves for the bathroom when a drink is spilled on her.

Around town several incendiary devices are set off, distracting first responders. A van crashes into the school cafeteria carrying four school shooters. The leader, Tristan Voy, orders his accomplices Chris Jelick, Anna Jelick and Kip Quade to begin killing students. Lewis, who has access to the school's Facebook page, is tasked with live streaming by Tristan. Zoe crawls through the bathroom ceiling and escapes the school. She begins warning students and helping them escape.

Tristan calls the front office to alert them of the shooting. Because school protocol requires that the shooting be confirmed, the school's principal and security guard are sent to the cafeteria to investigate. The principal attempts to talk Tristan down, blaming himself for Tristan's rage. Tristan admits that the entire shooting was a ploy for attention and he only seeks to gain infamy. Tristan kills the principal but allows the unarmed security guard to escape, commenting on how unprepared the guard was for an actual shooting. With the situation finally confirmed, the front office calls a lockdown.

Sheriff Tarsy attempts to coordinate a response to the shooting, but it proves difficult. An explosion in the front office kills the first police officer to respond. Anna is sent to survey the damage, but she encounters Zoe, and is killed in the resulting fight. A police officer arrives at Tristan's home to find that he has murdered his mother. The media picks up the livestream, much to Tristan's delight, and he begins removing students from their classrooms where they are sheltered in place.

Kip encounters Zoe and chases after her, but he is quickly subdued. Handcuffed, Kip admits his motivation for the shooting was retaliation for a bullying incident that occurred in middle school. Zoe points out that Kip has murdered several innocent students who had nothing to do with the incident. Zoe contacts the Sheriff and tells him that he needs to direct the media's cameras away from the school so that she can safely evacuate students. Sheriff Tarsy calls Tristan live on-air to provide cover, but Tristan soon realizes the ruse. Tristan demands Zoe reveal herself to him, threatening to kill hostages every five minutes if she does not.

Zoe returns to Kip, who expresses remorse for what he has done. As partial atonement for his crimes, Zoe has Kip ambush Tristan, and in the ensuing chaos that leaves Kip dead and Lewis injured, Zoe escapes with Lewis. Zoe settles Lewis and reveals that she reciprocates his feelings for her, but that she has a hard time expressing them. Lewis warns her that the van in the cafeteria is filled with explosives. Chris is sent to kill Zoe, but he is instead killed by Todd, who has taken a sniping position outside the school, right before Chris can execute her. Zoe hijacks the livestream, telling Tristan that she is the one people will remember, not him.