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On a Monday afternoon, five students walk into detention at Bayview High, a collection of stereotypes trapped in a stuffy science lab. There is Bronwyn, the Yale-bound brain who never breaks a rule; Cooper, the all-star baseball pitcher with a golden arm and a Southern drawl; Addy, the picture-perfect homecoming princess, rarely seen without her boyfriend, Jake; and Nate, the resident criminal, already on probation for dealing. And then there is Simon Kelleher, the outcast, the one they all fear. He is the creator of *About That*, the school's notorious gossip app, and he holds everyone's secrets in the palm of his hand.

The hour begins with a dull sense of injustice, a prank involving planted cell phones that landed the unlikely group in trouble. But the tedium shatters when, after a minor fender-bender in the parking lot below distracts them, Simon takes a drink of water from a plastic cup, grimaces, and collapses. He claws at his throat, his face turning a horrifying shade of red. Panic erupts. Bronwyn runs for help. Nate, surprisingly, takes charge. “An epinephrine pen,” he says, tearing through Simon's backpack. “He's having an allergic reaction.” But there is no EpiPen in Simon's bag. Cooper sprints to the nurse's office, only to find the emergency supply inexplicably empty. By the time the paramedics arrive, it is too late. Simon is gone.

The shock of his death quickly curdles into something far more sinister. The police determine it wasn't an accident; the cup Simon drank from was laced with peanut oil, a substance lethal to him. The investigation immediately focuses on the four students who were in the room, especially after a devastating discovery is made. On Tuesday, the day after he died, Simon had planned to publish his juiciest post yet, one that would ruin the lives of the four people who watched him die. All four of them had a motive. All four are now suspects.

They become the Bayview Four, pariahs linked by a crime they swear they didn't commit. Bronwyn, the perfect student, is hiding the fact that she stole chemistry tests to protect her flawless record. Addy, the devoted girlfriend, is terrified the world will find out she cheated on Jake over the summer. Cooper, the untouchable athlete, harbors a secret that could destroy his MLB dreams and shatter his family's expectations. And Nate, who has never pretended to be anything but a criminal, is exactly what he seems: a dealer violating his probation. They are all liars, but are any of them a killer?

Distrust and paranoia fester among them. They are pulled from class for interrogations, their secrets laid bare by detectives who see them not as teenagers, but as calculating murderers. Just as they begin to drift into their own private hells, a new voice emerges online. A mysterious Tumblr blog appears, its author claiming responsibility for Simon's death in a series of taunting, anonymous posts. *“Let's face it,”* one reads, *“everyone at Bayview High hated Simon. I was just the only one with enough guts to do something about it. You're welcome.”* The posts are chillingly accurate, filled with details only someone in that detention room could know.

The secrets Simon held are soon splashed across the internet for the entire school to see. Addy's world implodes when Jake learns of her infidelity, and she is cast out from her circle of friends, the word “WHORE” scrawled on her locker. Cooper's life unravels when the police uncover the real secret Simon intended to post - not about steroids, but about his hidden relationship with another boy. Bronwyn's academic future at Yale is thrown into jeopardy. Their lives are systematically dismantled, just as Simon would have wanted.

Forced together by their shared infamy, the four begin a fragile, unwilling alliance. They start to talk, sharing fragments of memory from that fatal afternoon, realizing that the police are only interested in one narrative. With the help of Bronwyn's sharp-witted younger sister, Maeve, they begin their own investigation, digging into Simon's past, his online life, and the enemies he made. They uncover a darker side to Simon - a lonely, resentful boy obsessed with school shootings and violent revenge fantasies, who felt cheated by the world.

The clues lead them down a twisted path of red herrings and old grudges. They revisit the car crash that served as a distraction, tracking down the driver who admits he was paid by Simon to stage the accident. The pieces begin to click into place, forming a picture more horrifying than they could have imagined. Simon wasn't murdered. He orchestrated his own death, a final, dramatic act of revenge designed to frame the four classmates he resented most. But he didn't act alone.

The truth culminates in a frantic chase through the woods behind a quiet suburban house. Addy, having uncovered the identity of Simon's accomplice, finds herself running for her life from the last person she ever would have suspected: her ex-boyfriend, Jake. Consumed by rage over her betrayal, Jake had eagerly helped Simon, manipulating events after his death to ensure the Bayview Four were ruined. Just as his hands close around Addy's throat, Cooper intervenes, saving her life.

In the aftermath, Nate is released from jail, and Jake is arrested. The Bayview Four are finally exonerated, but they are no longer the same people who walked into detention that Monday afternoon. Addy, shorn of her long blond hair and her toxic relationship, discovers a strength she never knew she had. Cooper, finally free to be himself, finds acceptance from those who matter. And Bronwyn and Nate, the brain and the criminal, find in each other an unlikely and unbreakable bond, forged in the wreckage of one boy's desperate, tragic lie.