After Paul Decker (Peter van Eyck) seals windows and doors in a stylish European sitting room, he attaches rubber hoses to a snorkel. He lets gas escape through the room's lighting fixtures, allowing the gas to kill his wife, Madge, who is lying prone on the couch while he hides under the floorboards, using the snorkel to breathe while concealed. Decker remains under the floor as household servants discover Madge's body. Since the room has been locked and sealed from the inside, it appears to the local Italian police and British Consulate Mr. Wilson (William Franklyn) to be a suicide.