Deborah, a wealthy American, and her Italian husband, Marcel, are honeymooning in Geneva when they meet Marcel's friend Philip, who belligerently informs them that Susan, Marcel's former fiancée, has committed suicide. The couple stop at Susan's deserted villa, where Marcel receives a death threat over the telephone. In Nice, he continues to receive menacing phone calls, and Deborah begins taking tranquilizers; one evening she accidentally takes too many and is revived by Robert, an artist who lives in the adjacent villa. Later, Philip attempts to murder her to avenge Susan's suicide, but Marcel appears, stabs Philip, and buries his body in the garden. The next morning, when Marcel leaves to buy two plane tickets to the United States, Philip and Susan suddenly appear; when the terror-stricken Deborah faints, the couple drug her, slash her wrists to make her death look like suicide, and drive away. On the highway, they pass Marcel and exchange knowing glances with him. Marcel returns to the villa and encounters Robert, who tells Marcel that he knows of the suicide-murder plot and demands payment for his silence. Unaware that Robert has taken the phone off the hook and that the police are listening in the next room, Marcel agrees to pay, but upon seeing the receiver, he draws his gun and is shot dead by Robert. Later, Deborah and Robert meet and congratulate each other on the success of their scheme to collect Marcel's insurance money.