While 12-year-old Mark Evans is at his mother's side as she's dying of cancer at a hospital in Arizona, she makes him promise her that she won't die. When she does die, Mark is consumed with grief, and is also guilt-ridden because he couldn't keep his promise to her. Shortly after, Mark's father Jack is assigned to take a two week business trip to Tokyo, Japan. Thinking that the blustery Maine environment will do Mark some good, Jack leaves Mark with Jack's brother Wallace Evans in Maine while Jack is on his trip. Wallace and his wife Susan have a son Mark's age named Henry and an 8-year-old daughter named Connie. Wallace and Susan's 3-year-old son Richard died by drowning in the bathtub a few months ago. At first, Mark and Henry get along great, but Mark begins to notice that Henry's ideas of fun differ significantly from his own. Henry threatens to topple Mark from a 15 meter high tree house, and Henry shows Mark his dummy, named "Mr. Highway", which Henry drops from an overpass onto a highway, causing a 10-car pileup. Later, Henry uses his homemade crossbow to kill the neighbor's dog. Henry also insinuates that Richard's drowning was not an accident. But Henry's perfect son facade is so convincing that no one believes Mark. Everyone is convinced that Mark is just acting out the trauma of his mother's death. Mark knows that Henry drowned Richard in a fit of jealousy because it seemed to Henry that Susan and Wallace were giving Richard the most attention. Also, Mark believes that his mother has been reincarnated in Susan, and he makes the mistake of telling Henry about it. Fueled by more jealousy than ever before, Henry's reign of terror escalates, and he hints to Mark that he plans to kill Connie for the same reason why he killed Richard. But even after Connie survives a suspicious ice skating accident, Mark's fears are still dismissed. And then Susan discovers Henry's secret playhouse, and she finds the rubber duckie that was missing from Richard's bath on the day of the drowning. Now that Susan knows the truth, will she save her nephew or her own son?
Evil has many faces. In a quiet town...In a comfortable home...In a perfect body...Evil can be as close as someone you love. Visiting the relatives isn't always a treat.