ONE ON TOP OF THE OTHER

George Dumurrier is a wealthy and self-centered San Francisco doctor who runs a clinic with his younger brother Henry, but leaves care of his asthma-stricken wife Susan to her sister Martha and a local nurse. He is engaged in an affair with Jane, the personal assistant to Larry, a trendy photographer. Although very much in love with George, Jane is fatalistic about the future of their relationship.

George and Jane travel out of town for a romantic break in Reno. But after arriving at the casino, George receives a phone call from Henry, telling him that Susan has died during a violent asthma attack. Returning home, George is consoled by Henry but frozen out by the hostile Martha, who always disapproved of George marrying her sister. However, a $1 million life insurance policy left by Susan is a timely bonus for George's recklessly extended business enterprise. An insurance agent begins tailing George, discovers his affair with Jane, and brings his suspicions to a local police detective, Inspector Wald.

Meanwhile, an anonymous tip-off leads George and Jane to The Roaring Twenties, a strip club where they are both astonished at the appearance of Monica, a stripper who, although a blonde, bears an uncanny resemblance to Susan. George is morbidly attracted to her and soon embarks on an affair that is part-detection, part-willing seduction. When the police, who have been tailing him, arrest Monica, she tells them that she was paid to pose as Susan by a woman calling herself Betty. Monica, as the police discover, is a popular fixture lately among the city's high-class prostitutes. She has a devoted wealthy client, Benjamin Wormser, whose hopeless passion she toys with. When Benjamin hears about Monica's arrest, he arrives at the police station with her exorbitant bail, but soon discovers that she has already been sprung by someone the police will not name.

A police search of Monica's apartment turns up an envelope containing money. When George's fingerprints are found on the envelope, he is charged with murdering his wife for the insurance money. Monica goes missing, and George is tried, convicted and sentenced to death. Some months later, on the eve of George's execution, Henry arrives for a visit where, in the privacy of an interrogation room, he reveals the truth: he and Susan hatched up this entire plot to get him out of the way and get the insurance money all for themselves. Monica was really Susan all along and faked her own death to implicate him. After Henry leaves, George tries to get a stay of execution by informing his lawyer about what Henry said. But despite some last-minute investigation by Inspector Wald, George is unable to clear his name. Only Jane continues to believe his innocence, but she is held in check by Larry.

The day arrives as George is taken out of his cell to the gas chamber to be executed, still protesting his innocence. At the last moment, a phone rings where the governor orders the execution halted as a telex arrives at the local FBI office that is forwarded to the prison authorities. In a twist of fate, police in Paris have informed the US authorities that Susan and Henry have been shot dead in a local café by the spurned and jealous Benjamin.

George is convicted of his wife's murder and sentenced to death, only to learn on the eve of his execution that Susan and his brother Henry orchestrated the entire plot to frame him and claim the insurance money.

The ending reveals that George was the victim of a meticulously planned conspiracy by those closest to him. The doppelgänger is exposed as Susan herself, who faked her death and manipulated events to implicate George. Fulci uses this final twist to underline the film’s themes of betrayal, duplicity, and the dangers of unchecked desire—leaving George powerless as the ultimate pawn in a decadent game.