In the smoke-wreathed gloom of a Parisian salon, Detective Bencolin has summoned his allies to discuss a peculiar case. A would-be murderer, imprisoned for his attempt to kill his wife, has escaped and is known to have visited a plastic surgeon. His whereabouts remain a mystery, though with his former wife poised to marry another, Bencolin predicts his return. Sure enough, the detective's worst suspicions are realised when the beheaded body of the new suitor is discovered in a locked room of the salon, with no apparent exit. Bencolin sets off into the Parisian night to unravel the dumbfounding mystery and track down the sadistic killer.