High school teacher Claire Peterson is still recovering from a painful separation from her cheating husband when a charismatic 19-year-old, Noah, moves in with his uncle next door. Noah befriends Kevin, Claire's teen son, but his attraction to Claire is palpable. Over the summer, Noah hangs out with Kevin and charms Claire, who indulges what seems like a harmless flirtation. He's fallen hard for her, though, and has now enrolled at the school where she teaches. After a dismal blind date, Claire is called over to Noah's house where, in a moment of weakness, she succumbs to their attraction. The next morning, realizing the full weight of her mistake, Claire apologizes. Noah's once charming demeanor turns to violent obsession, and Claire must fight to protect her world from this psychotic danger.