In World War II, the greatest threat to the British navy is the German battleship Tirpitz. While anchored in a Norwegian fjord, it is impossible to attack by conventional means, so a plan is hatched for a special commando unit to attack it, using midget submarines to plant underwater explosives.

John Mills
as Commandant Fraser

John Gregson
as Lieutenant Alec Duffy

Donald Sinden
as Lieutenant Tom Corbett

James Robertson Justice
as Admiral Ryder

Michael Medwin
as Smart

Theodore Bikel
as German Officer

James Kenney
as Abercrombie

William Russell
as Ramsey

O.E. Hasse
as Captain of the Tirpitz

Lee Patterson
as Cox
CinemaSerf
John Mills sets off to convince a sceptical Admiral (James Robertson Justice) of the merits of "midget" submarines in active warfare. His intention is to use these tiny craft to lay high explosives underneath the hull of the mighty Nazi battleship "Tirpitz" that is safely anchored in a steep-sided N...