Richard Mason (Humphrey Bogart) tells his wife Kathryn (Rose Hobart) that he no longer loves her, and is in love with her younger sister Evelyn (Alexis Smith). Kathryn tells him, she will never grant him a divorce, and anyway Evelyn will never go with him while Kathryn is alive. Richard fakes a leg injury, where he is in constant pain and can not walk. Kathryn goes on her own to a remote lodge they visit. Richard secretly rushes ahead to block the desolate road. He kills Kathryn and rolls her car, with her in it, over the incline. When she never arrives at the lodge, Richard reports her missing. The police can not find the car, or her. Richard expects them to think she drove off the road and died in an accident. He wants her found, as he can't go after Evelyn until she is known to be dead. He tries to give the police clues.
Richard tells Evelyn he loves her, and she is horrified by these advances from her sister's husband. Odd events make Richard suspect Kathryn may still be alive. A thief is found with her ring, and describes taking it from a woman who looked just like her. Richard finds her key to the home safe on his desk, and someone has put her wedding ring in the safe. He gets a pawn ticket in the mail. At the pawn shop, the ticket is for her necklace. The clerk describes a woman just like Kathryn, and her name is signed in the ledger.
In the middle of the night, Richard climbs down to the car, to see if Kathryn's body is there. There is no body. As he steps back, police floodlights shine, and he is arrested. It turns out their doctor, Dr Hamilton (Sydney Greenstreet) tipped off the police. After leaving for the lodge, Kathryn stopped at his office, to ask him to check in on Richard while she was gone. Dr Hamilton gave her a rose from his desk to pin on her jacket, and watched her drive up the desolate road. Hamilton was with Richard when the police interviewed him, and Hamilton heard Richard describe her to the police with a big red rose on her lapel. Hamilton knew Richard had to have seen Kathryn on the road to see the rose. He told the police, but that was not enough evidence to convict. Hamilton and the police planned the whole ruse, to prove Richard knew where the body was, and he was really healthy and capable of committing the murder.
Thanks Joseph C!