After young Mark's (Elijah Wood) mother dies of cancer in the Southwest, his father sends him away to New England to stay with his aunt and uncle for awhile. The couple is raising their little daughter and her older brother "The Good Son" named Henry (Macaulay Culkin). Henry and Mark are both 12 year-olds, and at first, they really seem to get along well. They have fun playing together. But then Henry's idea of a good time starts to worry Mark. One day he throws a mannequin over the bridge onto the freeway below, causing a major traffic accident. Another day he uses a bow and arrow to shoot and kill the neighbor's dog. Mark tries to tell his aunt and uncle about Henry's activities, but they don't believe him. They think that out of grief over his mother's death, that maybe Mark is just trying to get extra attention by making up wild stories. Henry tells Mark that he drowned his baby brother in the bathtub and nobody knows. He says he's going to kill his little sister too, and he'll get away with that murder as well. Mark desperately tries to warn his aunt who still finds Mark's stories about her son Henry impossible to believe. But then she wanders into his playhouse and finds the missing bath toy of her drowned baby son and realizes that Mark must be right after all. Can Henry be stopped before he kills again?