The story is told through the perspective of Eve, Kevin's mother, in the form of letters she writes after the series of brutal murders that Kevin commits. As a child, Kevin hows classic signs of a psychopath and shows little to no affection to his family, but appears to reserve special loathing for his mother. He tortures her from an early age by deliberately refusing to be potty trained, spraying ink over the walls of a room that Eve decorates with maps. The only thing Kevin shows any real interest in is archery after reading Robin Hood.
As Kevin grows up, his behavior worsens as he gets older, playing the part of a loving son in front of his father but continuing to manipulate and torture his mother, much to the horror of Eve and to the denial of his father, Franklin. Eve has another child, Celia, in an attempt to feel close to someone in her family; however, a tragic accident when Celia is 6 years old only causes the rift between Eve and Franklin to grow as Eve believes Kevin (who was babysitting at the time) is to blame for Celia losing an eye. Franklin's continued denial over Kevin's behavior leads to their estrangement; eventually, Franklin asks Eve for a divorce.
Not long after, Kevin attacks nine classmates, a cafeteria worker and a teacher at his school with a bow, after shooting and killing both his father and Celia. Though the book never reveals why, Eve speculates that this is a response to him overhearing news about her and Franklin's impending divorce. The story ends with Eve visiting Kevin, now in jail, on the second anniversary of the massacre. Kevin is about to turn 18 and will be transferred to a maximum security prison. Nervous, Kevin displays seemingly genuine emotion for the first time in his life when Eve asks him why he committed the crime. Kevin replies that he is no longer sure of why; he returns Celia's fake eye and apologizes. Eve and Kevin embrace and she leaves.
