Michael O'Hara meets a beautiful woman by chance in Central Park. She drops her purse but before he can return it, she's attacked by three robbers. He rescues her and escorts her to her car. Two men in her garage keep an eye on them. Mike reveals he killed a man once. They talk about a recent murder trial in the newspapers and the famous but shady defence lawyer Bannister. The woman Elsa offers Mike a job on her boat cruise to San Francisco. Mike asks Elsa why she threw down her purse since it has a gun in it. She says she wanted him to find it. Mike finds out she's Mrs. Bannister. The next day the handicapped Bannister, who is much older than Elsa, asks Mike to come work for him. Finally Mike agrees.
At sea one of the men from the New York garage, Grisby, joins the cruise. Grisby, Bannister's business partner, asks Mike about the man he once killed and if Mike would kill him. He seems to leave for his motorboat. Thinking themselves alone, Elsa and Mike kiss but Grisby lets them know he has seen them.
Mike wants to quit but his boss Bannister derides Mike for not wanting the money he's offering. The second man from the New York garage, Bannister's private detective, warns Bannister of a plot to kill him but he doesn't care. Elsa warns Mike about the detective spying on them. She suspects Bannister wants to divorce her and leave her penniless. Bannister squabbles with Grisby and needles Elsa about Mike. Bannister hints to Mike that he blackmailed Elsa into marrying him. Mike rebukes all of them for their fighting by likening them to a pack of sharks he once saw devour each other in a feeding frenzy.
Grisby offers Mike five thousand dollars to kill him. Mike tells Elsa about Grisby's suicidal plot. She's thought about killing herself by taking Bannister's painkillers. Mike wants Elsa to run away with him, but since she's tried and failed at that before, she refuses.
Grisby reveals he isn't suicidal. He wants to fake his death to collect money from the partnership insurance on either Bannister's or his life, get away from his bad marriage and avoid WWIII. He wants Mike to sign a confession that Mike killed him. He assures Mike that he can't be prosecuted for murder if no body is found.
At home in San Francisco, Elsa changes her mind and decides to run away with Mike after all. Mike tells her about Grisby's plan and the fake confession. She thinks Bannister is somehow behind the plot but asks Mike to carry out the plan anyway!
Bannister's detective Broome accuses Grisby of being involved with Elsa and planning to kill someone and frame Mike for his death. Broome offers silence for a fee, but Grisby shoots him. Claiming he was doing target practice to Mike, he puts the gun in Mike's car before they drive to the harbour. At the harbour, Grisby leaves on a speedboat.
Hearing the gunshot, Elsa discovers the dying Broome and finds out Bannister might be killed. When Mike calls the Bannister residence, Broome also warns him that Grisby intends to kill Bannister at the office and frame Mike. Mike rushes to the office but the police have Grisby's body. The police find the fake confession on him and arrest him.
Bannister defends Mike for Grisby's murder but doesn't believe Mike's innocence or that Grisby was going to fake his death. Elsa believes Mike killed Broome and expresses doubt about Grisby faking his death because he wasn't married.
Bannister weakly defends Mike. He tells Mike he wants to lose the case because Mike and Elsa are involved (but he sent her to visit Mike alone). Elsa signals to Mike to swallow Bannister's painkillers, so he does. He's taken to the judge's chambers, where he overpowers the guards and runs away. Elsa tracks him to a Chinese theatre. He finds Elsa's gun and accuses her of killing Grisby but faints from the painkillers. She stashes him in a closed amusement park. He confronts her in a hall of mirrors. She confesses she and Grisby planned to kill Bannister but since Grisby freaked and shot Broome, she killed Grisby. Mike asks if she was going to run away with Grisby or him, but she says she loves him. Bannister enters the maze of mirrors. In a shoot-out, he and Elsa kill each other. Mike walks out on a dying Elsa and thinks to himself that he's going to try and forget her.