Simon Kelleher has a particular talent for uncovering the secrets of his fellow students at Bayview High School. He uses a gossip app he created called About That to spread these secrets as well as rumors and nasty comments about his peers.

One September afternoon, Simon and four other seniors find themselves in detention. Addy Prentiss, Bronwyn Rojas, and Cooper Clay are rarely mentioned on Simon’s app, but Nate Macauley, who is often in trouble and who sleeps with a lot of girls, appears frequently. A teacher, Mr. Avery, has given all five students detention for having their cell phones in class—phones which they all claim were planted. When a car crash occurs outside, Mr. Avery leaves to go make sure no one was hurt. Moments later, Simon takes a drink of water and collapses almost instantly. After a frantic search for an EpiPen comes up empty, Simon is taken away by paramedics and soon is later pronounced dead.

Addy and her boyfriend, Jake, are friends with Cooper and his girlfriend, Keely. Bronwyn, whom Nate has known since kindergarten, has been an overachiever ever since her younger sister, Maeve, survived leukemia. Speculation about Simon’s death quickly takes over Bayview High School as the police seem to be treating it as not having been an accident. The four students who were in detention with Simon are approached by the police when someone makes an online post claiming to be Simon’s murderer. The police also discover unpublished About That content queued up by Simon implying that Addy, Bronwyn, Cooper, and Nate each has a secret to hide. The police believe that each benefited from Simon’s death because that prevented these rumors from being posted.

Jake and Addy have been together since their freshman year, but Addy fears what will happen when Jake finds out that she had sex with someone else last year. That is her secret.

Although generally a top student, Bronwyn once stole chemistry exam answers off her teacher’s computer. She fears what will happen if this becomes public knowledge. The police also confront Bronwyn with an old About That post. Simon wrote a humiliating account of her sister Maeve’s first high school party and made light of her bout with cancer, which obviously angered Bronwyn.

Cooper is a pitcher with a bright future playing baseball, has a popular girlfriend, and is widely liked. The rumor Simon had queued up, that Cooper used steroids, is untrue. However, the police discover a file Simon had encrypted, containing another rumor about Cooper, and this one actually is true. Despite protests from Cooper’s lawyer, Cooper is pressured into revealing his secret to his family: he is gay, and he has been cheating on Keely with a guy named Kris. 

As the only member of the Bayview Four with a criminal record, Nate seems to be the leading suspect in Simon’s murder. Nate has always had feelings for Bronwyn, feelings that seem to be mutual as they grow closer during the investigation into Simon’s death. However, Nate worries that Bronwyn just sees him as a project she can manage. Nate was known to have used and sold drugs in the past, and Simon’s unpublished post alleges that he is doing so again—which is, in fact, true. But since Simon’s death and the events following it, Nate has walked away from selling and taking drugs—at least partly because he knows that Bronwyn deserves someone whose life is less complicated. 

When the police seem focused only on pinning Simon’s death on one or more of the “Bayview Four,” Maeve begins an investigation of Simon on her own, and shares what she finds with her sister Bronwyn. Maeve is able to access Simon’s About That admin page and she also looks into Simon’s past. Maeve discovers some unsettling information about Simon, including some terrible comments he wrote and posted after a nearby school shooting. In spite of their lawyers and parents discouraging the Bayview Four from speaking with each other, they agree to work together as the investigation intensifies. Aided by Ashton and Maeve, they continue to dig into Simon’s past. 

After Addy reaches out to Janae, Simon’s only friend, Janae starts spending time with Addy and Bronwyn. Addy finally confesses to Jake that she had cheated on him, and she is bullied and ostracized at school by her former friends—who, it turns out, were just Jake’s friends and not hers. However, she finds new freedom once she is outside Jake’s influence. After Cooper tells his parents about Kris, he confides in Addy, as well. Nate comes to Cooper’s defense when the school learns that Cooper is gay. By now, everyone’s secret is out, and each of the four is increasingly intent on finding Simon’s real killer so they can move forward with their lives.

For Addy and Cooper, the urgency briefly lessens when Nate is arrested. Acting on an anonymous tip, the police have discovered incriminating items in Nate’s possession, including Simon’s EpiPen and water bottle. Nate’s fingerprints don’t appear on any of the items, however. Bronwyn enlists Addy and Cooper to help clear Nate, as well as a pro bono lawyer named Eli Kleinfelter. Since she is deeply involved in the investigation, Bronwyn has been warned not to contact Eli, who has officially taken on Nate’s case, so she feeds Eli information through Ashton. When the Bayview Four discover that Simon hired someone to stage the car crash, they conclude that Simon orchestrated his own death. However, the group don’t believe that Simon acted alone. Simon must have had an accomplice: whoever has been posting anonymously about the case.

Jake soon becomes their prime suspect, which saddens Addy. Addy goes to Janae’s home, and Janae confirms that Simon did indeed orchestrate his own death. He used his knowledge about Addy’s infidelity to persuade Jake to help him, including planting the fake cell phones that led the group being assigned to detention. After Simon’s death, Jake wrote the anonymous posts and forced Janae to plant evidence that would implicate his ex-girlfriend Addy—or so Jake thought. In the end, Janae couldn’t bring herself frame Addy because Addy had been so kind to her after Simon’s death, so she framed Nate instead. The framing of Nate was a deception that the police were all too willing to accept as fact.

Confronted by Jake in her home, Janae tries to stand up to him and get him to admit to his role in the framing plot while Addy hides and records their conversation on her phone. But when Addy’s phone rings at this inopportune moment, she is chased from the house by Jake. Addy falls, hitting her head on a rock, and Jake begins to throttle her. Cooper suddenly appears just in time to pull Jake off Addy and save her life. The authorities are called, and Jake is taken into custody.

Addy recovers from Jake’s attack, Nate is released from jail, and the Bayview Four, now exonerated, move forward with their lives. Nate decides that Bronwyn is better off without him, but she Bronwyn is hurt by his coldness toward her. A few months later, members of the group gather to watch Bronwyn perform in a recital. Afterward, Nate surprises Bronwyn with an apology. So many good things in Nate’s life have been ruined, but he doesn’t want to lose Bronwyn. They agree to resume their relationship.