Simon, age 14, is obsessed with weaponry, survival tactics, and hunting. Against his mother's wishes, he secretly orders a BB gun over the Internet and begins to practice with it. During a long, hot summer, he discovers a new obsession: Leah, the older girl next door. To her, he's little more than a "project," but their relationship intensifies. Meanwhile, Simon's mother has started a mortifying affair with his art teacher, Matt. When Simon realizes that Leah has begun modeling semi-nude for Matt, he snaps and makes use of his gun. The results are deadly, leaving Simon with a new understanding of himself and his capacity for violence.