Diane Stevens, an attractive English professor at Oceanview College, hires one of her students, Jay Richard, to repair her yacht. Her husband Michael, also a professor at Oceanview, is not onboard, and Diane lures the young man into a bedroom cabin and seduces him. Driving Jay home after their tryst, Diane offers to loan him money to fix his car, then mentions she needs a “favor.” That evening, Diane and her husband discuss how to secure an inheritance from Michael’s wealthy, but stingy mother and grandmother. They want the old ladies declared incompetent so they can become executors of the Stevens estate. Diane believes Jay is the right person to help them with the scheme. Later, Diane and Michael meet with Jay and ask him to fake a burglary in order to scare Michael’s mother and grandmother from the family mansion. Diane explains the old ladies need to be convinced they would be safer and happier in a retirement home. Although reluctant, Jay is captivated by Diane and agrees to help. Meanwhile, at the mansion, Michael’s mother, Lillian Stevens, and his grandmother, Lettie Stevens, complain about Diane and plan to disinherit Michael if he does not divorce the woman. Returning from an afternoon stroll, Lillian quarrels with George Johnson, the gardener. Jay sneaks into the house through a basement window and accidently knocks over an object, alerting the dog to his presence. The feisty Lillian grabs a shotgun and fires at Jay as he escapes the property. When she picks up the telephone to call police, another intruder confiscates the shotgun and kills Lillian and Lettie. Unaware of what transpired after he fled the estate, Jay rendezvous with Michael and Diane to inform them that Lillian tried to shoot him. When the three arrive at the estate, the mansion is dark and nobody is home. Although the bodies of Lettie and Lillian have been removed, Michael finds evidence that his mother and grandmother were murdered and immediately accuses Jay of foul play. The young man is angry and believes the two professors are responsible, but agrees to keep quiet about the incident until he can find out more information. Later, Jay sneaks on the Stevens’ yacht and discovers a pair of bloodstained tennis shoes, a gun, and bills paid by Lettie and Lillian to a children’s psychiatric hospital in Switzerland. He shows Diane the items, describing them as proof that Michael murdered Lillian and Lettie. Diane is not completely convinced and tells Jay she remains determined to inherit the Stevens fortune. At the campus pub, Jay enjoys a beer with his roommate, Martin “Bird” Johnson, who is eager to hear about Jay’s affair with the beautiful English professor. Meanwhile, Diane informs her husband that Jay is snooping around and may have evidence against him. Michael confronts Jay at the pub and reminds the young man his fingerprints are all over the Stevens mansion, which should deter him from contacting police. Jay returns to the mansion in an attempt to cover his tracks, but an intruder wearing a ski mask tries to attack him and once again, Jay escapes the property. He runs to Diane, telling her that Michael tried to kill him, but she says her husband was at home and wonders if the gardener, who has a secret connection to the Stevens family, might have been hired by Michael to murder Jay. Later in the evening, Diane notices Michael with the gardener George, confirming her suspicions that the two are scheming together. After watching Michael give the gardener money, she confronts her husband. Michael denies any wrongdoing, and accuses Diane of having sex with the young student. He declares their marriage is over. Meanwhile, Jay waits for Diane at his apartment, anticipating their romantic rendezvous. Before leaving for the evening to give the couple privacy, Bird warns his roommate to end the fling, stating that the scorned husband, Michael, is “bad news.” When Diane arrives, she reassures Jay that her marriage to Michael was in trouble long before their affair started. As Jay and Diane make love, Michael pounds on the apartment door. Jay convinces the jealous husband that Diane is not there and the two lovers sneak off to the yacht. Michael later arrives on the boat and observes the pair having sex. He aims a gun at them, but is unable to pull the trigger. Suddenly, the intruder with the ski mask appears behind Michael and fatally stabs him. After discovering Michael’s dead body, Jay and Diane see the masked man flee the boat and chase him to the Stevens property. Inside the house, George the gardener confronts the masked man, calling him Martin. George tells him that Lettie and Lillian were planning to change their will and name Martin as heir, rather than Michael. Martin, however, argues George is only interested in the money and not him, then kills him with a sword. Meanwhile, Jay grabs a shotgun and walks upstairs to confront the killer as Diane follows him. In Lillian’s bedroom, Diane and Jay find a photo album of Martin Johnson, who is Michael Stevens’s illegitimate brother. Diane surmises that George, the gardener, fathered the child. They also find a recent medical report on Martin, stating that the patient has Huntington’s chorea, an incurable disease with tendencies toward violence and paranoia. In the attic, they come across disturbing paintings by Martin and a list of people he plans to kill. They also uncover the corpses of Lillian and Lettie. When Martin confronts them with the sword, Jay shoots him dead. Diane removes the ski mask, and Jay sees that Martin is his roommate Bird. Sometime later, Diane arrives at the gas station where Jay works part-time. Thanks to her inheritance of the Stevens fortune, she is driving a new sports car and announces they are leaving for Hawaii. Grabbing the keys, Jay leaves his job behind and takes off with the professor.