After the tragic death of his mother, young Mark Evans is sent to stay with his Aunt Susan and Uncle Wallace for a few weeks while his father is away on business. There, he reunites with his cousin Henry, who initially seems friendly and welcoming. As time passes, however, Mark begins to notice unsettling behavior in Henry. What starts as minor mischief soon escalates to increasingly dangerous and malevolent acts, causing Mark to become suspicious and afraid of his cousin's true nature. Despite his attempts to alert the adults to Henry's behavior, they dismiss his concerns, believing him to be distraught over his mother's death.

Determined to uncover the truth, Mark follows Henry closely and discovers a series of chilling secrets that indicate an ominous pattern. Tensions rise as Mark realizes the scope of Henry's sociopathic tendencies and the potential danger looming over his family. Struggling to protect those he loves, including his little cousin Connie, Mark finds himself in a desperate race against time. Suspicion grows into confrontation as Mark battles not just Henry’s sinister manipulations, but also the disbelief of the adults around him. The escalating conflict sets a stage for a final tense showdown, where Mark must summon unprecedented courage to fend off the growing threat Henry represents.