Dr. Diane Stevens Sybil Danning, an English literature professor at Ocean View College, hires a student, Jay Richard Eric Brown, to varnish her yacht, the Lillian. During a break, the affair begins, and Diane plans to use Jay in a scheme with her husband to secure a large inheritance from Lillian and Lettie Stevens. Diane’s husband, Professor Michael Stevens Andrew Prine, would rather wait for the relatives to die of old age, but Diane grows tired of their ordinary life and makes clear she expects the money soon. To that end, the two convince Jay to scare Michael’s aging relatives out of the house and into a retirement home.
Their plan falters when the family dog starts barking at Jay, and Lillian KT Stevens chases him away, firing a rifle in his direction. Moments later, violence strikes: Lillian and Lettie are assassinated. Jay flees to the boatyard, and Michael grows suspicious, calling Lillian and Lettie and soon learning they’re dead. He accuses Jay, who counters by accusing Michael of trying to frame him. Jay swears he won’t go to the police and vows to uncover the truth himself.
The next day, Michael harasses Jay. Jay tells Diane to get Michael off his back and admits that he knows she was using him all along. Jay’s girlfriend, Cynthia Beth Scheffell, arrives on the scene to threaten him with a blackmail scheme—photos of Jay and Diane aboard the yacht—but Jay brushes her off. George the gardener is curious about Lillian and Lettie, and Michael lies that they’ve gone to Hawaii. Jay searches the yacht and finds a fragment of television screen glass, blood on trainers, a gun, and invoices for a Swiss psychiatric hospital for children. He presents the findings to Diane, who warns him against going to the police—he would simply be suspected of the crime—and cautions that Michael is capable of violence. This warning deepens Jay’s resolve, and the two continue to work in secret.
Diane later tells Jay that George is Lillian’s cousin. Cynthia visits Michael’s house to reveal the incriminating photos, but she is killed by the murderer, now dressed as Santa Claus. On campus, Diane sees Michael paying off George, and Michael accuses her again, ending their relationship. Diane later visits Jay and they resume their affair.
Michael suddenly appears at their rendezvous, and Diane and Jay flee to the yacht, where Jay declares his love for her. Michael tracks them down and intends to shoot Jay, but the killer arrives and murders Michael. Diane and Jay chase the killer to the mansion, where George confronts him. The killer is Martin, who suffers from Huntington’s chorea and is revealed to be George and Lillian’s illegitimate child, capable of inheriting the fortune. Martin coerces the family to fly him from Switzerland so he can claim the inheritance instead of Michael and Diane, but George is killed by the man he helped create.
Martin, who also murdered Michael, then aims at Diane. Jay intervenes, firing the rifle to defend her, and kills Martin—who turns out to be Paul Clemens in the guise of his former school friend and roommate, Bird. In the aftermath, Diane visits Jay at his workplace, gifts him her car, and asks him to come with her to Hawaii. He agrees, and the couple drives off together, leaving behind the tangled web of jealousy, deceit, and murder for a brighter future.
