Through a series of letters to her estranged husband, Franklin, Eva Khatchadourian recounts the events of her and Franklin’s family life leading up to their son Kevin’s recent mass murder. Eva tells Franklin that she misses him and that she lives in a new house close to Kevin. Eva’s community ostracizes her and vandalizes her new house with red paint.
Eva recalls the time that she and Franklin dated before they had Kevin. At this time, she is an avid traveler, and she owns a successful company that publishes travel guides. Eva and Franklin discuss whether they should have a child—Eva doesn’t want to because she thinks a baby would detract from the things she enjoys in life, but Franklin wants to be a father. One day, Eva decides that if she ever loses Franklin, she will want a child to keep her company. She and Franklin try to get pregnant, but Eva has second thoughts as she considers the inconveniences of pregnancy. Eva does get pregnant, and she immediately regrets it. She and Franklin argue about whose last name the baby should take (they decide on Eva’s) and about how Eva treats her body during pregnancy.
In the present, Eva writes to Franklin that she recently found a new job, which was difficult after Kevin’s murders. She is undergoing court proceedings because the mother of one of Kevin’s victims is suing Eva for parental negligence. Eva sometimes feels completely at fault for the murders, but she also knows that she can’t bear the full responsibility. She writes to Franklin that she is not sure if she loves Kevin. Eva visits Kevin in prison regularly. He doesn’t say much at the visits, but when he and Eva do speak they are both snappy and hostile to each other.
Eva’s letters to Franklin return to chronicling her pregnancy. When Eva is pregnant, she imagines that holding Kevin for the first time will bring a life-changing sensation. She has a painful and difficult childbirth, and when she finally holds Kevin, Eva feels nothing. As a newborn, Kevin won’t breastfeed. Eva believes that Kevin—though he is an infant—doesn’t like Eva and can tell that she doesn’t like him either. Eva takes a leave from work to stay home with Kevin. He screams all day long but stops when he sees Franklin. Franklin begins not to believe Eva about Kevin’s behavior when she’s alone with him, and tension builds between the couple. While Eva is ill, Franklin hires a nanny for Kevin, but she quits before her first day is over. Franklin finds a new nanny, Siobhan, who stays with the family while Eva returns to work. Siobhan quits after a year, reporting that Kevin intentionally hurts her. Kevin stops screaming at this point, but at one and a half years old, he doesn’t speak or play at all. One day, he suddenly begins to speak in full sentences. Eva thinks he was pretending not to be able to talk before this so that he could eavesdrop on others.
Eva and Franklin discuss moving to the suburbs—Eva wants to remain in New York, but Franklin thinks it is best for Kevin to leave the city. They make a compromise that they will move to the suburbs, and Eva will take an extended trip to Africa for work. On the trip, she decides to leave work again so that she can try to form a better relationship with her son. When she returns home, feeling reinvigorated, she goes out to eat with Franklin and Kevin. At the restaurant, Kevin constantly mocks Eva and their waitress. Eva’s frustration grows until she slaps Kevin in the face. A little while later, the family is moving into their new house in the suburbs. Kevin relentlessly shoots the movers with a squirt gun. Eva tells him to stop, but Franklin encourages him. Eva confiscates the gun, angering Kevin—though he acts unbothered. Eva can see that Kevin is learning to detach himself from things that might hurt him. Once they’re moved in, Eva undertakes a long project to cover the walls in her study with maps. She explains to Kevin that the maps are important to her. When she leaves the room, Kevin fills his squirt gun (which Franklin returned to him) with red ink and sprays it all over the maps. Eva returns, and, enraged, stomps on the gun until it is destroyed.
When Kevin enters kindergarten, the other children seem afraid of him. One day, Eva picks up Kevin and finds him in the bathroom with a girl who is bleeding all over her body. The girl has severe eczema, which she usually resists scratching. Eva is certain that Kevin somehow caused the girl to bleed. Eva is upset with him and doesn’t change his diaper that day (Kevin is still in diapers at six years old). Franklin is appalled that Eva refused to change Kevin’s diaper, and the tension between Franklin and Eva grows. Eva is convinced that Kevin refuses to use the bathroom on purpose to bother his parents. One day, Kevin soils himself three times in a row right after Eva changes him. Beside herself with frustration, Eva throws Kevin across the room into the changing table, and he breaks his arm. To Eva’s relief, Kevin lies about the incident to everyone, saying it was an accident. In light of this, Eva loses any power she had over Kevin and she begins to oblige all his demands. Franklin and Eva continue to argue about Kevin regularly. Eva wants another baby, but Franklin rejects the idea because he thinks Eva is a bad mother. Eva stops using birth control without telling Franklin, and she becomes pregnant.
Eva tells Kevin about sex, and that he is going to have a sibling. He is not curious about or surprised by sex, and he is upset by the prospect of a sibling. Eva gives birth to a girl and names her Celia. Celia is much easier to care for than Kevin. As a baby she never cries, and she sleeps easily. As she gets older, she and Eva become very attached to each other. Kevin tortures Celia by scaring and hurting her on purpose; one time he ties her to a chair and forces her to eat a mixture of mayonnaise and curry until she vomits. As Celia gets older, Kevin becomes more hostile to Eva and more pleasant towards Franklin. However, Kevin becomes very sick when he is 10 years old, and his behavior suddenly (but temporarily) flips. He seems to lack enough energy to either act nicely towards Franklin or to be angry and hateful towards Eva. Eva reads Kevin Robin Hood, and Kevin loves it. Eva is glad that he is showing interest in something, so she buys him an archery set.
Kevin’s hostile behaviors continue into adolescence. He bullies other students at school and once gets in trouble with the police for throwing bricks at cars. Shortly after this, Celia’s pet shrew mysteriously goes missing. When Kevin is 14, Celia badly injures her eye and face with drain cleaner. She has surgery and loses an eye. Eva is sure that Kevin hurt Celia with the chemical on purpose, but Franklin thinks the accusation is outrageous. Celia later gets a glass eye replacement.
School shootings become very common in America throughout Kevin’s adolescence. Kevin defends the shooters and hates it when Eva criticizes them. Around this time, Kevin accuses one of his teachers of touching him inappropriately. Eva is sure that he is lying and wants to send him to boarding school. Franklin doesn’t trust Eva’s opinion of Kevin at all anymore—he thinks she is delusional, and he says he wants to separate. Kevin overhears this conversation.
A short while later, Eva stays late at work and hears that there was a shooting at Kevin’s high school. Eva rushes to the school, scared for Kevin’s safety. She sees Kevin walk out of the school in handcuffs. Eva drives to the police station, calling Franklin in a panic, but he doesn’t pick up. She drives home after talking to the police. She finds Franklin and Celia dead in Kevin’s archery range, with multiple arrows in their bodies. Eva later learns that Kevin locked nine students and one teacher in the school gymnasium, and shot them with arrows, letting them slowly bleed to death.
A while after the murders, Eva sees a documentary about Kevin. Kevin tells an interviewer that he is tired of people blaming Eva for what happened, and Eva sees that the only thing in Kevin’s cell is a picture of her. Exactly two years after the murders, Eva visits Kevin in prison. He is almost 18, and he is terrified because he will soon transfer to an adult prison. Eva tells him how much she misses Celia and Franklin. Kevin says that he doesn’t know why he committed the murders. He gives her a tiny coffin containing Celia’s glass eye. Eva hugs Kevin, and she thinks she hears him apologize.