The Twin

Sixteen-year-old Ivy rides home with her father and identical twin sister, Iris, two weeks after their mother's death. Their mother reportedly slipped off a bridge while running near a farm, hit her head, and died instantly. The twins have not lived together since they were ten, when their parents divorced: Ivy stayed with their father in a small town, while Iris moved to the city with their mother. Though identical in appearance, they are opposites. Ivy is reserved, driven, and logical; Iris is outgoing and social. Now, with their mother gone, Iris has moved back in permanently.

From the start, Ivy senses something off about her sister. On the day they arrive home, she spots Iris watching her from Ivy's bedroom window. That night, Iris makes noise rearranging furniture and claims to be private about her room, despite having already entered Ivy's space. Over the following days, Iris sits alone in the dark kitchen before dawn, refuses to discuss their mother, and shows no signs of grief, expressing far more interest in learning about Ivy's friends and social life.

Ivy begins therapy with Dr. Meera Rajan, who identifies Ivy's need for control and suggests that Iris may not know how to form genuine friendships. Iris declines all professional help. She convinces their father to let her start school early, and on the first day, she panics about being in classes alone. The school psychologist changes Iris's entire schedule to match Ivy's. The anxiety vanishes instantly, and Iris begins charming Ivy's classmates, befriending Ellie, the head cheerleader, and several football players.

Iris quickly inserts herself into Ivy's social life. She goes to the movies with Ivy's best friends and swim teammates, Haley and Sophie, on a night Ivy has a date with her boyfriend, Ty, and creates a group chat that excludes Ivy. She brings up the swim team's individual times in conversation, making Haley and Sophie acutely aware of how far Ivy has surpassed them, breeding resentment. In class, Iris disrupts Ivy's concentration by whispering, tapping her pen, and humming, behaviors Ivy suspects are intentional.

Iris manipulates family dynamics as well, convincing Ivy to confront their father together about his new girlfriend, Rachel, and pressuring him to stop dating. He agrees. Then Ty receives a photo from a blocked number showing Ivy and Logan, a football player, kissing at a party a year earlier. Ivy explains that Logan was drunk and kissed her, that she pushed him away immediately, and that they agreed to keep it quiet to protect Ty's friendships. Ty is hurt by the secret rather than the kiss itself. As he storms off, Ivy catches Iris watching from the school entrance with a faint smile. Ivy later discovers an old flip phone hidden in Iris's desk drawer, which she suspects Iris used to send the photo.

The situation worsens when Iris comes home early and finds Ivy and Ty kissing on Ivy's bed. Iris screams, exaggerating the scene by claiming Ty was fully on top of Ivy. Their father walks in during the argument, believes Iris's version, and grounds Ivy. Though Ivy later has an honest conversation with her father and he softens, the damage to his trust is clear, and Ivy notices he gives Iris far more leniency.

Meanwhile, Iris steadily takes over Ivy's identity. She cuts her hair to match Ivy's length and befriends Ivy's circle while distancing Ivy from them. She secretly trains with Haley and Sophie and joins the swim team. A dead mouse appears in Ivy's locker. During an early morning swim, Ivy sees what she believes is Iris standing at the pool's edge; startled, she inhales water and chokes badly enough to need a nurse visit, but no one else saw anyone there. When a stolen test paper is found in Ivy's locker during a school search, Ivy is suspended from competing, threatening her hopes for a Stanford scholarship and solidifying Iris's place on the team.

Ivy begins investigating. She buys a charger for the flip phone and confirms that Iris sent the kiss photo to Ty. She discovers that Iris has deleted every friend from her old school on social media and tracks down Kat, Iris's former best friend, arranging a secret meeting. Kat reveals that Iris systematically destroyed her reputation and that someone pushed her down a flight of stairs. Although no one was seen, Kat smelled Iris's perfume. Kat raises the possibility that Iris harmed their mother, noting that their mom had started dating a man and Iris had complained about the relationship.

Shaken, Ivy drives to her mother's apartment, where she finds a hidden book about mental health conditions in children, covering personality disorders and psychosis. She realizes her mother suspected something was wrong with Iris. At the bridge where her mother died, Ivy notices a safer public footpath through the adjacent field, a route her mother, who had a fear of heights, would have naturally chosen. Someone must have led her mother onto the bridge.

Shortly after, Ivy discovers that Kat has died, reportedly falling into a river while taking a selfie. Ivy believes Iris killed Kat after learning about Ivy's visit. Back at school, the pool is vandalized with dangerously high chlorine levels, and Ivy is questioned first given her removal from the team. Her father pulls her out of therapy with Meera, saying he needs to find more suitable help. Ivy's credibility is at its lowest, with only Ty and Logan, who recognizes parallels to his own past troubles, still supporting her.

Ivy confronts Iris in the school parking lot, asking directly if she killed Kat. Iris admits she followed Kat and pushed her into the river, blaming Ivy for the death because Ivy contacted Kat. Inside their house, Iris reveals she killed their mother because she was obsessed with Carl, their mother's boyfriend, believing he was secretly in love with her despite never having expressed romantic interest. Iris was furious that their mother planned to end the relationship, cutting off her access to Carl. When Ivy points out that Carl never reciprocated, Iris insists their mother deserved to die. Ivy attacks Iris physically. Their father walks in to find Ivy on top of her sister and, despite Ivy's frantic explanations, sides with Iris. Ivy collapses from a panic attack and loses consciousness.

Ivy wakes in Rose Haven Institute, a psychiatric facility, placed there for observation. Meera arrives and reveals she has been meeting with Iris for weeks; rather than advocating for Ivy, she suggests a supervised conversation between the twins. Once they are alone, Iris drops her act entirely. She confirms she orchestrated everything, including enlisting Sophie to scare Ivy at the pool and Ellie to plant the dead mouse. Iris reveals she has planted incriminating evidence in Ivy's room: shoes with dirt from the scenes of their mother's and Kat's deaths, Kat's sweater, and photos of Carl. If Ivy is ever deemed sane and released, the evidence will frame her for the murders. Ivy is left alone, fully aware she has lost her friends, her father's trust, her swim team, her therapist, and her freedom. She resolves to play along and earn her release, but the novel ends with Iris firmly in control.