Plot
Ten-year-old Mark Evans has recently lost his mother, Janice. Before leaving on a business trip to Tokyo, Mark's father, Jack, transports him from Arizona to his Uncle Wallace and Aunt Susan's house in Rock Harbor, Maine, where he will stay during winter break. Mark is reintroduced after a decade to his extended family, including his cousins 6-year-old Connie and 10-year-old Henry; there was a third child named Richard who drowned a year before. Mark and Henry get along initially and Henry seems to be nice and well-mannered. However, Henry displays an abnormal fascination with death, making Mark feel uneasy.
Henry begins to display clear signs of psychopathic behavior, which Mark is unable to raise with Wallace and Susan due to Henry's threats. One of his violent actions is throwing a dummy he calls "Mr. Highway" off a bridge and onto a highway, causing a massive vehicle pileup. Henry then plans to kill Connie, and Mark spends the night in her room as he is afraid something might happen to her. The next morning, Mark awakens to find that Henry has taken Connie ice skating. At the pond, Henry purposely throws her towards thin ice, which collapses. Connie is rescued but ends up in a coma.
Although Susan does not initially believe Mark, she becomes suspicious and interrupts Henry's attempt to suffocate Connie in her hospital bed. Susan then finds a rubber duck that Henry has hidden in his shed — it belonged to Richard and was with him in the bathtub before he drowned. When Susan confronts Henry, he coldly reminds her that the toy had belonged to him first. Henry then flips and kindly asks for the rubber duck back. After a violent tug-of-war, he takes the toy and throws it down the well.
As Susan and Mark grow closer, Henry insinuates that he will kill Susan rather than let Mark continue to develop a relationship with her. When a fight breaks out between the two boys, Wallace locks Mark in the den. Henry asks a suspicious Susan to go for a walk with him, while Mark escapes and chases after them. Susan confronts Henry, asking him if he killed Richard, to which Henry simply replies, "What if I did?" Susan realizes that Mark was right about Henry's true nature. She tells Henry that he needs help, but Henry refuses and flees.
Susan gives chase, and upon arriving at a cliff, Henry pushes her over the edge. As Susan dangles from a branch on the cliffside, Henry tries to drop a rock on her, but Mark arrives and tackles him. Susan manages to climb back up just in time to grab hold of the boys as they roll over the edge, one in each hand. Henry holds on with both hands, but Mark's one-handed grip begins to slip. With only enough strength to save one of them, Susan releases Henry and he falls to his death. Susan pulls Mark up and they look down at Henry's corpse being washed away into the ocean.
When Mark returns home to Arizona, he reflects upon Susan's choice to save him instead of Henry. Mark wonders if she would make the same choice again, but knows it is something that he will never ask her.
