One day, after the stern American publisher of a London newspaper, William T. Marshall, learns from Inspector Madden that his ward, a young woman named Melissa, has committed suicide, Marshall recounts how their relationship went wrong: In her early twenties, rebellious Melissa insists on dating singer and philanderer Larry Shaw despite Marshall's disapproval. Melissa believes Marshall's domineering personality is only a facade and challenges his authority. One night when Marshall finds Larry and Melissa kissing at his mansion, Craighorn, he orders Larry out. In the ensuing fight between Melissa and Marshall, Melissa vows to run away with her lover and Marshall retorts that he will never allow her to return home if she does so. Marshall's secretary, Katie Whiteside, warns him to let Melissa have her way, but Marshall insists that Melissa forsake Larry, prompting her to leave with few belongings and make no contact with Marshall for over two years. After Melissa's death, Marshall receives a letter from her explaining that she does not want forgiveness for her choices. Instead she asks Marshall to take revenge against the man who caused her so much pain that she felt she had no reason to live. After months of research, Marshall has a plan to undo Melissa's last lover. At an international auction, Marshall outbids Schteigel, the representative of criminal mastermind Emile Karnak, for a precious stamp. He then offers to trade the item to Karnak, an avid stamp collector, in exchange for a meeting at Craighorn that night. Reluctantly, Karnak arrives at Craighorn, where Marshall asks him to help perfect a murder plan in exchange for the rarest stamp in the world. After Schteigel establishes the stamp's authenticity in a battery of tests, Karnak reluctantly agrees to the trade. Marshall wants the man tortured before he is killed and insists on keeping the identity of the victim secret. Days later, Katie, realizing Marshall is distressed about Melissa's death, offers her sympathy and love to him. They kiss, but Marshall tells Katie that it is too late for him to begin a romance. Meanwhile, Karnak discovers an article identifying Larry as Melissa's last boyfriend and assumes that Marshall wishes to kill the playboy. At the club in which Larry is performing, Madden questions the singer about Melissa's death, insinuating that he blackmailed Marshall. Larry claims Melissa was insane and he had broken up with her before her suicide. Meanwhile Karnak, obsessed with keeping his plans secret, kills Schteigel by running over him with his car. Later at the club, Karnak, posing as a journalist, threatens to expose a secret that will ruin Larry’s stage career. The next morning after newspapers report that Larry is to be deported shortly, Karnak tells Marshall that he knows Larry is his intended victim. He then explains the plan: Larry will find himself alone, realize he is trapped and be told he is going to die without meeting his murderer. Panic will set in, leaving Larry a desperate “animal” seeking escape. Although he will escape, when he has his "life is sweetest" moment of freedom, Marshall will kill him. Karnak states that he will complete the plan in a few days and suggests that Marshall decide the exact technique for the final execution. Later, after Marshall tells his butler Bennett to take a vacation, insisting the obviously healthy man is ailing, Bennett enlists Katie's help in investigating Marshall's strange behavior. Katie drives to Craighorn, where she begs Marshall to tell her where he is going and what he is running away from, but he refuses to reply. Marshall orders her to contact his lawyer in case of an accident during his absence, gives her a beautiful keepsake brooch and makes her promise not to ask any questions. Noting that Katie never fails him, Marshall hands her an envelope to be delivered to Madden at 10:00 the next evening. Katie reluctantly agrees to follow the orders, but tells him to stop obsessing over Melissa and then criticizes his need to follow through with any plan regardless of the potential harm to others. Hours before the 10:00 deadline, Katie gives the letter to Madden to help save Marshall. Back at Craighorn, after Karnak hands him the final plan in trade for the stamp, Marshall leaves the room to fetch some bourbon, but as time passes, Karnak becomes increasingly nervous. Deciding to leave, Karnak finds the front door locked. Searching for another exit, Karnak quickly realizes that all the doors and windows are bolted with solid shutters Marshall has recently installed. Clambering from floor to floor, the desperate Karnak surmises that he is the murder victim and is caught in his own plan. Upon finding a room with Melissa's portrait illuminated over a mantle, Karnak recognizes Melissa as his former lover, whose name he never knew, then hears Marshall's voice over the intercom. Marshall explains that Melissa is the person seeking revenge and describes how the vulnerable woman's anguish over being used and "kicked aside" by Karnak caused her to commit suicide. Marshall then concludes that Karnak was the real cause of her demise. As Marshall repeatedly plays a recording of Karnak's own description of how he will die, Karnak races through the house to find an exit. Breaking an attic window, Karnak climbs down the stone mansion walls and escapes in his car. Meanwhile, as Marshall stands in the middle of the road with a loaded gun preparing to shoot Karnak, he hears Katie's voice in his head calling him to stop, and decides to stand aside as Karnak's car passes him. Soon after, Katie and Madden, racing to Craighorn, pass Karnak and realize, to their relief, that Marshall did not follow through with his plan. Spotting him in the road, Katie runs to Marshall's side. Meanwhile, Karnak is just realizing "life's sweetest moment" when he begins to laugh and loses control of his car. Karnak is killed and the stamp is destroyed in the ensuing crash.