Michael Gambon narrates the story of the tiny undersea craft developed during the Second World War. The Italian Navy first developed these deadly vessels and used them to devastating effect against the Royal Navy. Irritated by the Italian success, Churchill ordered the development of British human torpedoes and mini-submarines. These scored their first notable success against Hitler's "unsinkable" battleship Tirpitz. The programme also tells of Japanese volunteers who piloted human suicide torpedoes into American ships.