Jerry Blake wakes up in an insane asylum after having a nightmare of being stabbed by his stepdaughter while he attempted to murder the girl and her mother. The next day, he meets psychiatrist Dr. Joseph Danvers. Blake refuses to discuss having married into three separate families to murder them. To encourage trust, Dr. Danvers orders Smitty, a guard, to remove Jerry’s handcuffs. After a few more sessions, Jerry asks to see Danvers alone. Smitty protests, but Danvers tells him to wait outside the office. More sessions follow with Smitty waiting outside. One day, he brings in a wooden model of a house he built. A male figurine stands in front of the model house. He tells Danvers this should answer his question about what kind of man marries into a family just to kill them. Danvers does not understand, prompting Jerry to smash the model. Danvers hits an alarm, and Smitty runs in. However, Jerry calms them down by putting the model back together, explaining he is an eternal optimist who believes he can fix whatever is broken. After Smitty steps back outside, Jerry picks up the male figurine, which has a long nail protruding from its bottom. He stabs the nail into Danvers’s skull, killing him instantly, then presses the alarm. Smitty enters, and Jerry bludgeons him to death. Disguised as Smitty, Jerry walks out of the asylum, whistling “Camptown Races.” After killing a man for his car, he dyes his hair and buys contact lenses to alter his appearance. He sees the name Gene F. Clifford in the obituaries and decides to take the man’s identity. While hiding in a hotel, Jerry watches a game show in which the prize is a “dream house” in Pal Meadows Estates, Yorba Linda, California, and decides to start his new life there. Days later, Jerry, posing as “Dr. Gene Clifford,” a family therapist, is shown a house by Carol Grayland, a recent divorcee. After learning that Carol lives nearby with her thirteen year-old son, Todd, Jerry leases the house. Weeks go by and Jerry sets up a family practice. Matty Crimmins, a mail carrier, drags Carol to a group session. There, Carol voices her concern that Todd does not have a father figure. After the session, Matty accuses Carol of liking “Gene.” Jerry woos Carol by teaching Todd how to pitch a baseball and build a skateboard ramp. One day, Jerry sees Carol’s ex-husband, Phil, arrive at her house. Pretending he came to see Todd, Jerry visits Carol, but she tells him she will talk to him later. Upset, Jerry goes into his basement and beats a picture of Carol with a hammer. Matty comes to his house to retrieve the hat she left during the group session. Not getting an answer when she rings the doorbell, Matty comes in and peeks into Jerry’s notebook. Jerry appears, carrying the hammer. Matty explains she returned for her hat and rushes out of the house. The next day, Carol goes to Jerry to sort out her feelings about Phil wanting to get back together. Jerry suggests that he meet Phil to discern his true intentions. That night, Matty brings over a few bottles of rare wine her parents sent for her birthday and tells Carol to forget both “Gene” and Phil. Across the street, Jerry tells Phil that Carol wants him to leave her and Todd alone. Phil does not believe him, and puts his cigarette out on Jerry’s s rug. As Phil heads for the door, Jerry admits he was lying to test Phil. Then, he smashes a bottle over Phil’s head and stabs him to death with a bottle shard. As Jerry rolls Phil’s body in a rug, Carol calls. She asks Jerry to have Phil call her when he gets home. Jerry assures her that he will, then searches Phil’s jacket and finds a motel key. Jerry puts the corpse in Phil’s trunk, drives to a junkyard and crashes the car. After smashing the headlights and windows, he walks away, whistling “Camptown Races.” He is still whistling when he returns home to find Todd waiting to ask what happened with Phil. Jerry tells Todd he does not believe Phil is coming back, but that it is not Todd’s fault. He teaches Todd to whistle. The next day, as the junkyard demolishes Phil’s car, Jerry tells Carol that Phil only wants her for a hot meal, then declares his love for her. A few weeks later, Carol and Jerry throw a party to announce their engagement. The only one not celebrating is Matty, who questions Carol about “Gene’s” past. Matty reveals that “Gene” only gets bills and junk mail. Carol warns her to stop bad mouthing “Gene” or their friendship is over. That night, Carol gets frisky, but is insulted when Jerry suggests she is a loose woman for wanting premarital sex. The next day, as Matty delivers the mail, she notices a letter from Oregon addressed to “Gene Clifford.” Opening it, Matty screams, “I knew it.” When she confronts Jerry about the letter, he threatens to turn her in to postal authorities, but backs down when Matty shows him a picture of Gene F. Clifford, who is African-American. Jerry convinces Matty to let him inform Carol of his deception, and she agrees to give him until the evening to do so. He visits Carol, apologizes for his “bachelor jitters,” and carries her to her bedroom. Before they consummate their relationship, Jerry tells her he has a dark past. She hushes him with a kiss, but upon removing his shirt, sees the scars from wounds he received while trying to kill his last family. Jerry lies that a former patient attacked him. After they make love, Jerry sneaks out, goes to Matty’s house, strangles her to death, and tries to make it look like a suicide. As he leaves, he spots the last bottle of wine Matty’s parents sent and takes it. Walking out of the house, Jerry hears Matty's neighbor, Sam Watkins, call out. Jerry runs away. Returning to Carol’s, he wakes her up and pretends he went out for a bottle of wine. Days after Matty’s funeral, Jerry and Carol stand in her empty house when there is a knock on the screen door. Jerry is terrified to see Sam Watkins, until he realizes the man is blind. Sam tells them that he heard a man leave Matty’s house whistling the night she killed herself. He whistles the tune to "Camptown Races." Carol insists they go to police, but Jerry suggests it was probably a kid out for a night stroll. Matty’s death so upsets Carol that she asks to postpone the wedding. When Jerry suggests they elope, Carol changes her mind and agrees to a big church wedding. On the day of the nuptials, a case of wine from Matty’s parents arrives at the church. As Carol is dressing at the church, she realizes the wine has the same label as the bottle Jerry brought home the night of Matty’s death. She hears Todd whistling “Camptown Races.” Jerry stands outside and overhears Todd telling Carol that Jerry taught him the tune. Carol confronts Jerry with the evidence. He explodes and strangles her with her wedding veil. Todd comes in, sees his mother passed out and runs to get help with Jerry close on his heels. The boy runs into a janitor’s closet, and Jerry locks him inside. Finding some tools, Todd removes the door hinges. Carol comes to and searches for her son. Jerry catches her in the reception hall and beats her. When she is thrown into the buffet table, Carol grabs a serving fork and plunges it into Jerry’s hand. He backhands her, and she lands on the cake table, where she grabs a knife and stabs Jerry in the chest. Enraged, he hurls her to the floor, pulls the knife from his chest and is about to stab her when Todd appears and smashes Jerry’s arm with a hammer. Jerry drops to his knees. The boy slams the hammer claw into Jerry’s chest. As Todd and Carol flee, Jerry crawls to the wedding cake, grasps the bride and groom figures at the top of the cake, and dies.