Bryan, the son of the Galesburg, Illinois mayor, is brutally murdered in his home, his body later found stuffed and posed as a scarecrow. Local policeman John Brady begins investigating the murder. Meanwhile, John's son Pete, a high school senior, sits in on a course at Galesburg University with his friend, Oliver. During the course, professor Gwen Parkinson screens a lecture by her late mentor, Dr. Le Sange, whom Pete's late mother, Catherine, once worked for. After the lecture, Pete agrees to become one of Gwen's test subjects in order to earn money for his college applications. Later, Pete attends a house party. During the party, one of his classmates, Waldo, is stabbed to death outside by a masked assailant, and Waldo's girlfriend Lucy is attacked and falls into the swimming pool. Pete and several others rush to save her, and the masked attacker flees; in the distance, he removes his mask, revealing himself to be Oliver. John subsequently questions Oliver, who says he cannot recall the events of the party as he was drunk.
Medical examiners observe that Waldo's corpse has a bizarre surgical incision near his eye, and, upon scrutinizing the evidence, John concludes that two different people are responsible for the murders of Waldo and Bryan. Meanwhile, Pete attends one of Gwen's studies at a large laboratory, which hosts both human and animal tests. Gwen has Pete swallow a pill and repeat several words before dismissing him. After, he invites Caroline, a college student who works the front desk at the laboratory, on a date, and the two quickly begin a romance. The following day, a woman finds her son, Timothy, being dismembered in her bathroom by an unknown young woman; she phones police before being stabbed, and manages to tell her friend Mildred over the phone sparse details about the girls' appearance before having her throat slashed.
John, suspecting the female assailant may be one of Gwen's test subject, confronts Gwen at the laboratory, unaware of the fact that Pete is in one of the test rooms, tied to a chair. After John leaves, Gwen resumes the session, in which she injects a fluid into Pete's eye. After he awakens and is dismissed, Pete goes to have dinner with Caroline, but becomes violently ill, and begins urinating blood. Meanwhile, John has a conversation with his girlfriend, Barbara, about the murders: He deduces that each of the victims are sons of men who previously collaborated with John to investigate the unethical experiments of Le Sange, and believes Le Sange is in fact alive, enacting revenge. Barbara follows John to the cemetery, where he breaks into Le Sange's crypt, and finds the casket empty. John and Barbara return home and find a confused Pete along with Caroline.
John retrieves a shotgun and heads to the university. Barbara meanwhile recounts to Pete and Caroline how Pete's mother, Catherine, acted strangely during her employment under Le Sange, and that her subsequent unexplained death spurned John's initial investigation into the program. Pete and Caroline decide to follow after John to the university. Pete enters a chamber where John is communicating with Gwen, who appears on a small television. Gwen orders Pete, now in a daze from her mind control methods, to take his father's gun. Gwen appears in the room with Le Sange, who has been disguised as an elderly man. He expounds that his methods of mind control will help the world, before proceeding to instruct Pete to slash his own wrists, which he does, before instructing him to stab his father to death. Pete responds by stabbing Le Sange in the throat, and declaring that he "is his father," revealing that Pete's mother, Catherine, had an affair with Le Sange, and John is not actually Pete's biological father. Police subsequently arrive and Gwen is arrested.
Some time later, Pete, healed from the experiment, attends his father's wedding to Barbara with Caroline.
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