Addy, Bronwyn, Cooper, and Nate, have been put in detention with Simon Kelleher, creator of Bayview High's gossip app. Barely halfway through detention, all five of them witness a car crash outside the classroom's window.
When they turn back from the window to call the police, they see Simon drink water from a glass and choke, dying from a severe reaction to his peanut allergy before Cooper can find EpiPens.
All four of them get questioned by the Bayview police, but cannot give them a lot of information about Simon's death. Later, Nate discovers a Tumblr post that has been written in the murderer's perspective and is available for all the school to see.
A few days later, another Tumblr post gets released, still in the murderer's perspective, and writes that he/she took all the nurse's EpiPens so that Simon would for sure die. Later the Bayview four attend Simon's funeral service.
From Simon's funeral, detectives find and question the Bayview four separately, showing them a third Tumblr post that has posts about them that Simon was going to release in his gossip app, About That.
The third Tumblr post gets released to the school, where all the students find out that the Bayview four all had motives for killing Simon, making them murder suspects.