One of Us Is Back is a YA thriller novel, the third in the One of Us Is Lying series. The first novel, One of Us Is Lying, begins when five students are given detention at Bayview High School for having cell phones despite the no phones rule. The students are Bronwyn Rojas, Nate Macauley, Cooper Clay, Addy Prentiss, and Simon Kelleher. The teacher leaves the room to check on a disturbance in the parking lot. Simon drinks water from a cup and dies of anaphylaxis due to his peanut allergy. The police determine Simon’s death is homicide. The investigation reveals that Simon ran an app called About That that revealed secrets about other students, and he had secrets about each of the four students in detention with him. These students are investigated as suspects and are dubbed the Bayview Four, or the Murder Club.

The Bayview Four initially accuse each other, but soon they become friends. Nate is arrested for Simon’s death, but the Bayview Four continue to investigate on their own. Addy befriends Simon’s friend Janae and when Janae stops showing up at school, Addy goes to her home to check on her. There, Janae confesses to helping Simon orchestrate his death in an assisted suicide.