As Katie Lewis drives away from her beachfront home, she loses control of her station wagon and crashes. Frightened, she phones her husband, Ben, at his dockside boat repair shop and he returns home to find the initial “W” scrawled on Katie’s car. Sometime later, Katie is shocked by an electrical outlet and Ben tends to her wound, reminding her that she need not fear, nor remember, her past. The next day at the dock, Ben’s car smashes into a boat and, after jumping to safety, he sees the initial “W” on the trunk. Meanwhile, Katie walks her dogs on the beach and is disturbed by their frantic barking. When Ben returns home in a rental car, he notices an object floating just off shore, then loads his handgun and turns on music so Katie cannot hear. As Katie showers, Ben phones Soledad Prison to learn that inmate William "Billy" Caulder, Katie’s former husband, was recently transferred to a nearby penitentiary called McKenna. Just then, Katie screams and Ben rushes to her aid to find another “W” traced on the bathroom mirror. Speculating that William escaped during the prison transfer and is trying to kill them, Ben promises to seek legal advice from his friend, Arnie Felson, but Katie fears that she will end up in prison, herself; William was convicted of her murder because she remained in hiding and changed her identity from “Ellen Caulder” to “Katie Lewis.” Although Katie warns Ben that he could be implicated in the ruse, he phones Arnie the next morning and leaves a message with the lawyer’s secretary. Just then, a private investigator named Charles Jasper unexpectedly visits the Lewis home. Ben permits Jasper entry and holds the investigator at gunpoint. Jasper reports that the couple’s recent car problems are suspicious and offers his services, but the Lewises are unresponsive and he leaves. After a short sailboat excursion, the couple returns home to find their dogs bludgeoned to death in Katie’s art studio. That night, the Lewises bury their dogs and Katie implores Ben to contact the police, but Ben phones Jasper instead. When the investigator arrives, Ben alleges that William escaped prison, killed their dogs and stole his gun, but Jasper maintains that there have been no reports of jailbreaks. Jasper concludes that William has been paroled and suggests that the couple implicate the man in a crime so he will be sent back to jail, but Katie protests. Ben agrees to hire Jasper for home security and the investigator stages a phone call, leading the Lewises to believe that he is hiring guards instead of talking to his own answering machine. That night, the couple sits alone in their darkened home when a television news report announces their deaths. Recognizing William’s voice, Ben realizes the broadcast is a hoax and discovers an explosive device rigged to the television. The next morning, Ben calls McKenna Prison and learns that William has not been paroled. Although Ben suspects that Jasper orchestrated the assaults, Katie is convinced that her former husband is seeking revenge. Ben takes a tour of McKenna that afternoon and recognizes William from Katie’s photograph. Enraged, Ben rushes to Jasper’s office, where he seizes the investigator’s gun and threatens to kill him, but Jasper explains that William has a work permit and leaves the prison daily for his job in the community. Jasper procures a 1969 newspaper article reporting the murder of “Ellen Caulder” and accuses the couple of fraud. With the threat of exposure, Ben agrees to Jasper’s $5000 blackmail; however, he only gives the investigator half of the money, stipulating that the rest will come when William’s work permit is revoked. Ben then phones his lawyer, Artie, who meets the Lewises at the beach house. Katie explains that she married William in London, England, while he was stationed there as a U.S. military sergeant, but the man became mentally ill and physically abusive when they returned to America. After a brutal attack, Katie left her husband and met Ben, who helped her regain her life with a false identity. While Ben and Katie take a moonlit walk, police discover the burned wreckage of a McKenna Prison truck, and Lt. Whitfield arrests the couple for murdering William. At the police station, an investigator shows Ben two articles that were found in William’s toolbox, the 1969 report of Katie’s supposed murder and a headline showing Ben and Katie on the beach in La Paz, Mexico. In another office, Katie insists that her current husband did not murder William. When the Lewises reunite, Lt. Whitfield announces that the murder weapon, Ben’s gun, was found on the beach outside the Lewis home, but Ben argues that the weapon was stolen. Katie then recognizes Jasper from police photographs and an officer identifies the so-called investigator as Samuel Tenley, a recent McKenna Prison inmate. Although Ben concludes that Jasper and William worked together, Lt. Whitfield is unconvinced, claiming that Jasper had no reason to kill his fellow prisoner. When the lieutenant reveals that Ben’s fingerprints were on the gun and refuses to set bail, Artie promises to secure his client’s release but warns Ben that he might be convicted of the same crime as William, second-degree murder. Sometime later, Artie takes Katie home and she insists on remaining there alone. When the lawyer leaves, Katie is shocked to find William in her bedroom. William explains that he staged his own murder by attaching two fingers to Jasper’s burned corpse, leading the police to confuse their identities. William beats his former wife and packs her suitcase, announcing that he successfully switched places with Ben and now wants Katie to “disappear” in prison. Katie attempts to dissuade William, but his anger escalates into psychosis. Knocking Katie to the ground, William sees his reflection in a mirror, pounds his bloodied fist into a nearby window, and collapses. As Katie consoles William, Artie returns to the house and phones for help.