A sheltered woman realizes that either her husband or his mysterious brother is a psychopath.

When industrialist Alan Garroway is introduced to Ann Hamilton, the unsophisticated daughter of chemistry professor "Dink" Hamilton, he falls instantly in love with her and proposes marriage. A short time after their wedding, Ann accompanies Alan on a trip to Washington, D.C., where she is introduced to Mr. Warmsley, Alan's manager, and many of Alan's socialite friends. Her plain clothes and lack of style embarrass Ann, who then tearfully asks Alan to show her how to dress and behave. One day, Alan tells Ann that after his father died he and his brother Michael took over their father's mechanical gadget business, and that he has not seen or heard from Michael since Michael was caught stealing from him. While visiting Alan's family home in Middleburg, Virginia, Ann is startled when a horse suddenly becomes wild in the stable, and later learns from the stablehand that Alan once beat the horse severely. Later, when Alan angrily orders Ann to cease playing a familiar tune on the piano, she begins to suspect that his odd behavior is symptomatic of some deep psychological wound. Her suspicions are confirmed when George, the estate caretaker, tells Ann that the song she was playing was one that Michael used to play. Believing that Alan has been deeply hurt by his brother's betrayal, Ann decides to help her husband overcome his problems. After Alan leaves for Seattle on a business trip, Ann visits his office and asks Warmsley to give her directions to Michael's ranch house near the ocean. At the house, Ann meets Michael, but she is not aware of who he is because he introduces himself to her as the caretaker. Later that night, Alan comes back to Seattle and surprises Ann at the house. He angrily demands to know why Ann is there, and Ann tells him that her only interest is to save their marriage and to help him by finding out more about his past. Alan returns to Seattle to resume his business while Ann returns to Middleburg. When Alan returns to Middleburg, Michael accuses him of having killed an engineer at their plant in order to steal his plans for a remote control explosive device. Michael tells Alan that he has decided to expose his crime now because he met Ann and believes that she is too fine a person to be involved with a murderer. When Alan informs Michael that he has reformed and that he plans to tell Ann the truth about his past, however, Michael decides to remain silent about the murder. Fearing that Ann will leave him if he confesses the truth, Alan keeps his past a secret and vows to let nothing and no person come between them. Ann later confesses that she is obsessed with thoughts of Michael and that she wants to meet him so that she can forget him. As Alan grows insanely jealous of Ann's obsession with his brother, she becomes fearful of him and tries to escape from the house. Alan prevents Ann from leaving, and realizes that he must kill her to prevent her from leaving him again. While Alan and Ann ride on horseback to visit a friend, he makes a failed attempt to push her off a steep ledge. Ann races ahead for help but falls off her horse when she hits a branch. Alan catches up with Ann, but when he raises his arms to knock her on the head with a rock, his horse pounces on him with its hooves and kills him. Ann recovers at her father's home, where, one day, she is visited by Michael. Ann tells Michael that she knew who he was a short time after they met, and that she wanted to see him again. After Michael tells Ann that he has special feelings for her, the two play a duet on the piano.