When a gas station attendant on the Nevada-California highway is killed after recognizing hoodlum Joe Walpo, who is wanted by the FBI, all FBI offices are alerted. In Los Angeles, the case is assigned to agent Zack Stewart, who already is working on a case involving interstate car thefts. He then takes on a case involving a woman named Kate Martel, the victim of an extortionist who is threatening to kill her young daughter unless she hands over insurance money left to her by her husband. Stewart arranges to interview Kate at Ohrbach's department store, where she works as a fashion buyer. When she proves unable to identify the extortionist's voice, he places her house under surveillance and records her phone calls. Stewart's attention is diverted from Kate's case when Brenda Ralles phones and asks for an agent to come to her home late at night. John Ripley, Stewart's boss, accompanies him to the house but Ralles refuses to see them. Stewart then spots someone leaving by the back door and goes after him, but is shot and killed. The Los Angeles police are unable to get any information from Ralles, but keep her under observation. John is assigned all three of Stewart's cases and believes that one of them holds the clue to his killer. John and his partner, Greg Barker, interview Walpo's girl friend, Connie Anderson, who claims to know nothing about his whereabouts. They then visit Julie Angelino, the blind wife of Vince Angelino, a young man arrested for stealing a car. Angelino refuses to reveal the names of the car theft gang that hired him, and John is unable to link Angelino's and Kate's cases to Ralles. When John has lunch with Kate, they are interrupted by Dave Millson, an old friend of her husband, with whom she has recently become acquainted. Later, Millson stops by Kate's apartment, where she lives with her uncle Max and a housekeeper, to collect her for a date. Millson suddenly kisses Kate, but she reacts very coldly. After Ralles's body is found in a disposal drum, the FBI establishes that she was shot by the same gun that killed Stewart. In the hope that she may lead them to Walpo, John tells Connie that they have evidence that he has another girl friend. When Connie leaves her apartment, agents follow her to the subway terminal, where she boards a street car to Glendale and eventually leads John and Barker to the house where Walpo is hiding. John lures Walpo outside and, in a gun battle, shoots and kills him. Later, John learns that Walpo was in Las Vegas the night Stewart was killed. Kate, under John's surveillance, keeps a midnight rendezvous in a cemetery with the extortionist but he fails to appear. The Angelino case develops after Julie is roughed up by an assailant, Matty Pavelich, whom she thinks may be a boxer, as she felt a scar on his face and a cauliflower ear. John traces Pavelich through a gym and confronts him with Angelino, who claims not to know him. However, when John reveals that Pavelich has beaten Julie, Angelino tells all he knows about the car theft racket and reveals that he was in Arizona when Stewart was killed. John is invited to a birthday party for Kate's daughter and the FBI takes the opportunity to get fingerprints from all the guests's glasses, as they suspect the extortionist is someone known to her. Later, the extortionist calls and arranges a new rendezvous and warns her not to involve the FBI. After talking with her housekeeper, Kate withdraws the necessary cash from her bank. However, the bank tips off John and he learns the rendezvous point from the housekeeper. Meanwhile, through the fingerprints and a semantics expert, the extortionist is identified under various aliases. Kate finds a note from the extortionist at the rendezvous spot, telling her to leave the cash at the base of the "W" at the "Hollywood" sign. John and Barker are close behind Kate when she reaches the sign, but she stumbles and the money falls and is blown about. Suddenly, Millson appears and gathers it up. He tells Kate that he never knew her husband and had only posed as a friend, killed Ralles, who knew of his plans, and Stewart. When John and Barker arrive on the scene, Millson grabs Kate, but John shoots at him, scaring him off. Millson attempts to escape in his car but is stopped at a road block. Kate thanks John, and after she is driven away, he notes that sometimes a detective gets to meet the nicest people.