One of Us Is Lying follows four different characters as their lives unfolds after their classmate Simon Kelleher dies during detention after drinking from a glass coated in peanut oil. Bronwyn Rojas, Nate Macauley, Addy Prentiss, and Cooper Clay were all in detention with Simon, and they are the four narrators of the book. They all went to detention for supposedly bringing their phones to class—however, the phones were not truly their cellphones, but were planted on them. During detention, Simon accuses them of being four different stereotypes: the jock, the brain, the princess and the criminal. The teens are briefly distracted by a car crash. Then Simon has an allergic reaction. The students look for an EpiPen in the school, but they are all gone. Simon dies.
The four narrators become suspects in Simon’s murder, as they each had a motive to kill Simon: he ran a gossip app called About That, and his next post was to feature their darkest secrets. When they are accused, the secrets gradually come out, in part because of Tumblr posts by the supposed murderer. Their lives change as a result: Addy’s cheating on her boyfriend Jake Riordan results in their breakup and her ostracization; Bronwyn’s cheating in chemistry class imperils her admittance to Yale and sends her into Nate’s arms; Cooper’s homosexuality puts his baseball future on the line and reveals fake friends; and Nate’s past criminal history including drug dealing ultimately lands him in jail, accused of Simon’s murder.
The four suspects grow closer and they start defending each other from the taunts of their peers. They all believe that none of them are guilty, and they try finding out what really happened. When Nate is arrested, the stakes become higher. Bronwyn tracks down the driver in the car accident that happened during detention, and she finds out that Simon had paid him to crash. This is when they realize the death might have been a suicide, as Simon was depressed. Simon’s one friend, Janae, is also acting weird, which suggests that she is hiding something.
After Addy confronts Janae, she reveals a “manifesto” in which Simon explained that he wished to die, while ruining the lives of those around him. He hoped their reputations would be forever stained. It is also revealed that Simon had an accomplice—Jake. He wanted to take revenge on Addy for cheating. Jake tries to kill Addy when she finds out the truth, but she is saved by Cooper. The truth is revealed to the police and they are all free. The epilogue explains how their lives are slowly but surely calming down, with Nate and Brownyn’s romance deepening.
