Paul Decker (Peter van Eyck) murders his wife in her Italian villa by drugging her milk and asphyxiating her by gas. He cleverly locks the bedroom from the inside and hides inside a trapdoor in the floor until after the body is discovered by servants. He uses a scuba snorkel connected to tubes on the outside to breathe during the ordeal. Decker's stepdaughter Candy (Mandy Miller) suspects him immediately, especially since no suicide note was found. She also is convinced that he murdered her father years before, but her accusations fall on deaf ears. The ruthless Decker even poisons the family spaniel when the pet takes too great an interest in the mask and realizes he will ultimately have to get rid of Candy too.
Audio commentary with film historian Michael Brooke and author Johnny Mains
Undercover Killer: Inside 'The Snorkel (2018, 21 mins): an analysis of the film and its production by Hammer expert Jonathan Rigby, BFI curator Josephine Botting and cultural historian John J. Johnston
Hammer's Women:Betta St John (2018, 11 mins): Diabolique magazine's editor-in-chief Kat Ellinger offers an appreciation of the American actress, singer and dancer
Original Script Ending (10 mins): a reconstruction of the final scenes as they appear in Jimmy Sangster's screenplay
Peter Allchorne and Hugh Harlow remember 'The Snorkel' (2018, 8 mins): the Hammer production-crew regulars recall making the film
Four-Note Fear (2018, 24 mins): an appreciation of composer Francis Chagrin's score by David Huckvale, author ofHammer Film Scores and the Musical Avant-Garde