Four years after the events of Stepfather II, Gene F. Clifford escapes from a Puget Sound institution and pursues a new life by changing his appearance. He arranges for a surgeon to alter his looks, and when the doctor is killed by a blade, he makes his way to Deer View. There, he assumes a new identity: Keith Grant [Robert Wightman], and he secures a cottage and a job at a nursery, beginning a fresh sequence of carefully staged roles.
Keith Grant [Robert Wightman] soon meets Christine Ann Davis [Priscilla Barnes], the town’s principal, and her son Andy Davis [David Tom], who uses a wheelchair after an accident, though doctors believe his condition is psychosomatic. A wary romance blooms as Keith and Christine begin dating, and the quiet life starts to take shape. But danger shadows this fragile new arrangement: a recent boyfriend of Christine, who has been watching them, confronts Keith at his house to warn him off. Keith responds with lethal force, killing the man with a shovel and burying the body in his garden.
The nightmare intensifies as Keith and Christine marry, despite Andy’s misgivings. Andy’s father Steve Davis [Jay Acovone] offers Andy the chance to spend the summer with his family and attend a school for the gifted, and Andy accepts, despite Keith’s protests. Keith pivots his attention toward Jennifer Ashley [Season Hubley], beginning a new courtship while harboring a plan to murder Christine so he can marry Jennifer. The tension spikes when Keith murders his boss, Mr. Thompson [Dennis Paladino], with a rake after Thompson teases him about the possibility of Keith cheating on Christine with Jennifer.
The would-be killer’s plans are complicated by family loyalties and a crisis of trust. Keith returns home with an axe, intent on finishing Christine, but Andy unexpectedly returns and disrupts the attempt. In a moment of breach and confusion, Keith calls Andy “Nicky,” which mystifies Christine, even though Keith offers a convoluted explanation. Jennifer meets Christine while enrolling Nicky in school; Christine is struck by the name Nicky, and Jennifer mentions the remote road outside town where she and Nicky have rented a house—an address Christine suspects might be Keith’s former cottage, now subleased.
Keith slips by the school and spots Jennifer with Christine, panicking and denying any knowledge of Jennifer. Andy seeks answers about Keith’s past and turns to Father Ernest Brennan [John Ingle], who agrees to help, knowing Andy will try to uncover the truth if left on his own. At dinner, Andy slyly handles Keith’s fork to help Father Brennan test fingerprints. Keith realizes Brennan may have learned something about his history and leaves the scene, pursuing the priest and staging an accident to remove the threat. He also uncovers the fork.
Keith rushes to the nursery, calls Jennifer, and Jennifer contacts Christine to cancel their meeting. Keith reveals that he is married and tries to remove Jennifer from the equation. He knocks Jennifer unconscious and plans to feed her to a chipper, a grim act that would finally end the rival. Christine and Andy arrive, and Christine realizes Keith is Jennifer’s lover. In a brutal confrontation, Keith is knocked into the chipper after a chase through the nursery, hanging on the edge as Andy pushes him. Christine helps Andy to safety, while Keith is torn apart by the shredding machine. Jennifer hacks the power cable with a hatchet, halting the chipper’s operation, and she surveys the grisly remnants of Keith in the machine. Andy stands, Christine embraces him, and as the scene closes, Andy turns to Christine and utters a chilling, haunting line.
> Happy Father's Day.
