In San Francisco, shipping magnate Matthew Cabot, though fatally ill, watches over his fleet and remains controlling and cruel to his wife Sheila and second-in-command, Howard Mason. As he does each day, Dr. David Rivera, who is secretly having an affair with Sheila, arrives to administer Matt’s injection of ether to control the pain, a procedure Howard considers suspicious. While Sheila listens in silence, Matt questions David about the position the physician has been offered as the head of a Zurich hospital. After dismissing David, Matt treats Sheila roughly, accusing her of having a “love deficiency” and demanding that she give up her plans to learn to drive. Sheila decides to go out, and although she has not left the house for days, her stepdaughter, Catherine, expresses disdain at her desire to escape Matt. Sheila asks her chauffeur, Cob O’Brien, to drive her to the store, but once there, sneaks out the back entrance and visits David. The two are desperately in love, but Sheila knows if she leaves Matt he will keep their son Peter and destroy David’s career, which is of utmost importance to him. David admits to Sheila that he must take the job in Zurich to escape his constant fantasies of murdering Matt with an untraceable air bubble in the hypodermic. Although despondent at the thought of David leaving, Sheila bids him farewell. The next day, however, David shows up at her house, and together they inject Matt with air, killing him instantly. Meanwhile, Cathy visits her boyfriend, Blake Richards, who is trying to rebuild his father’s shipping company, which Matt put out of business years earlier. Blake announces that Matt has awarded him a large shipping contract, the profits from which will allow the couple to marry. At Matt’s funeral, Howard unnerves David by implying that the doctor is unusually close to Sheila. Later that night, Sheila, plagued by nightmares, calls David for comfort, and although he is afraid to be seen with her, he agrees to visit soon. The next day, Howard declares his love to Sheila, and after she refuses him, he threatens to look into his suspicion that she was unfaithful to Matt. Among the corporate papers Howard urges Sheila to sign is the dissolution of Blake’s contract, which throws the struggling businessman into insolvency. Blake confronts Howard at his office, where Howard forces his secretary, Miss Lee, to lie to Blake that Matt did not want him to have the contract. Blake vows to fight against Howard, unconcerned that Howard is taping the conversation. Later, Cob, a clandestine gambler who is being hounded by loan sharks, checks the mailbox for a letter. David and Sheila meet inside the house, and after Cathy interrupts them, David fears that they are not being covert enough about their relationship. Before David can leave, Sheila opens a handwritten letter, postmarked from Carmel the previous Monday, which reads: “Congratulations on the success of your murder.” Soon, David is so wracked with anxiety that he can no longer perform surgery, and tells Sheila that they must find and kill whoever has written the letter. That night, Miss Lee, who loved Matt and does not want Blake to blame him for Howard’s treachery, presents Blake and Cathy with papers proving that it was Howard who systematically destroyed Blake’s father’s business. While Cob searches the mailbox again, Sheila grows concerned about the apparent disdain of her housekeeper, Tani. On the way to David’s office, Cob asks Sheila for another advance on his salary, and later she wonders to David if Cob’s remarks about his insolvency are a covert attempt to blackmail her. Just then, Howard calls Sheila and infers that he knows that she and David are lovers. Sheila is forced to admit to David that Howard loves her, after which David deduces that Howard sent the letter from his Pebble Beach golf club. He plans to murder Howard on the night of the upcoming longshoremen’s strike, and to this end has Sheila invite Howard to her house. Howard presents her with a corsage, after which she allows him to kiss her. After he leaves, she signals to David, who shoots into Howard’s car. Sheila is shocked when, minutes later, Howard appears at her door, unharmed. When David then calls, Sheila pretends he is Cathy in order to stall Howard, allowing David to return to the house. However, Howard discovers his corsage discarded in the fire pit and, after intercepting a real call from Cathy, deduces Sheila’s involvement in the attempt on his life. Howard is about to kill Sheila with a fireplace poker when David bursts in and shoots him. The sound wakes Peter, but Sheila tells him he has merely been dreaming. In order to get rid of Howard’s car and body, David insists that Sheila follow him in her car to the cliffs of Half Moon Bay. Unable to drive, Sheila tries to object, but with no other choice, she is forced to maneuver through the treacherous streets in the driving rain. After David pushes Howard’s car off the cliff, Sheila screams hysterically. Later, the police uncover Howard and Blake’s taped conversation and assume Blake is the murderer. As they question Sheila about Howard’s visit, Cathy realizes that her stepmother is lying about the evening’s timing. After Peter then reveals to her that he heard a gunshot that night, Cathy deduces that Sheila killed Howard, and races to David’s for help. David tries to convince her that Sheila, who cannot drive, could not have committed the crime and dumped the body. After Cathy leaves, David is consumed by guilt, and, hearing the Hippocratic Oath repeatedly in his mind, informs Sheila that he must go away. Soon after, Sheila receives a second handwritten letter of congratulations. David races over, and when he sees Cob sneaking out of the house, demands that Sheila detain the chauffeur, despite her protestations. At David’s questioning, Cob admits that he stole some money to pay off his bookie and now needs more money, and a reference. David tries to trick Cob into printing out his address, in order to compare the handwriting with that in the note, but Cob refuses. David then tricks the driver into admitting that he was in Carmel the night the first letter was mailed. When David attacks him, Cob cries out that he was driving Sheila that night, causing David to comprehend that Sheila sent the letters herself. Sobbing, Sheila begs David to understand that she deceived him only to keep him from going away, and in response he promises her that they will now start a new life together. Just then, however, they realize that Cathy has been listening and now knows their secret. When Cathy picks up the phone to call the police, David chases her upstairs, following the terrified girl into her room and onto the window ledge. Just as Blake arrives at the front walk, Sheila calls out to David, startling him. As Sheila watches in horror, David falls onto the walk to his death.