On the surface, Richard Mason Humphrey Bogart and Kathryn Mason Rose Hobart appear to be a happily married couple. But on their fifth wedding anniversary, Kathryn accuses Richard of having fallen in love with her younger sister, Evelyn Turner Alexis Smith, who is visiting them. He does not deny it, but has resigned himself to leaving things as they are, since Kathryn certainly would not give him a divorce. Kathryn knows he knows it, and derides him further. At a party celebrating the couple's anniversary, hosted by family friend and psychologist Dr. Mark Hamilton Sydney Greenstreet, Evelyn meets with Mark's handsome young colleague, Professor Norman Holsworth [Charles Drake]. On the way home Kathryn slyly mentions to Evelyn that their mother is lonely, knowing that Evelyn will feel obligated to move back home. Angered, Richard crashes their car and suffers a broken leg.
Richard decides to take desperate action. He pretends to require a wheelchair even after his leg has healed. His puzzled physician, Dr. Grant [Grant Mitchell], diagnoses the problem as psychological, not physical, and suggests exercise, so a car trip to a mountain resort is arranged. At the last minute, Richard contrives to stay home to finish some work.
Going on ahead, Kathryn stops by Hamilton's home and asks him to check in on Richard. Resuming her journey, Kathryn comes upon an abandoned parked car blocking the narrow, deserted mountain road. Unexpectedly, Richard walks threateningly out of the fog. The audience is left to imagine him killing her. Next he pushes her car down a steep slope; it dislodges some logs which crash down and hide it. He returns home in time to set up an alibi by meeting with an employee he had summoned to finish the work. In his presence he twice phones the resort, only to be told she has not arrived. He then notifies the police that she is missing.
Subsequently, things happen to make Richard wonder if Kathryn somehow survived. First, the police find a pickpocket in possession of a cameo ring that Richard and Evelyn identify as Kathryn's; the man admits to stealing it from a woman matching Kathryn's description, after her disappearance. Then Richard smells Kathryn's perfume in their bedroom, finds her key to a home safe, and opens it: her wedding ring is inside.
Mark suggests Richard and Evelyn join him on a fishing vacation to relieve the strain. Mark also invites Holsworth, who takes the opportunity to ask Evelyn to marry him. She is undecided. When she tells Richard, he believes her hesitation is because of him. He tells her he loves her, and that she must feel the same about him, but she strongly denies it. Later, realizing his mistake, he encourages Holsworth to try again.
Then a pawn shop claim ticket is mailed to Richard, addressed in what appears to be his wife's handwriting. At the pawn shop, he finds Kathryn's locket and her signature in the register, but when he returns with the police, the register is different and there is no locket. Finally, he sees a woman on the street who looks and is dressed like his wife. He follows her to an apartment, only to find that it is vacant, with no one inside.
Unable to reconcile these occurrences any longer, Richard returns to the crime scene to see once and for all if Kathryn's body is inside the car. But Hamilton and the police are waiting for him. Kathryn's body had been found and removed long before, and now Richard is arrested. Hamilton reveals that he had been onto Richard since Richard's initial interview with the police. Since Hamilton's suspicion wouldn't be enough to secure a conviction in a court of law, Hamilton and the police worked together to stage the events that made Richard suspect Kathryn was still alive, hoping he would return to look for her body, and thus prove he had known all along what happened to her.
