The Royal Navy is concerned about constant attacks on convoys and seeks a way of removing support from German submarines. Despite official misgivings, Commander Fraser (Mills) seeks and receives permission to plan and launch an attack on Tirpitz using miniature submarines to attach mines.
The initial operation, using "chariots" (manned torpedoes), is a failure and the crew are forced to abandon ship and land in Norway, walking to the border of neutral Sweden, from where they are returned to their base in Scotland.
Another operation is launched using three small submarines which the crew rode inside. Two of them successfully attach their mines and then scuttle the submarines, and are picked up by the crew of Tirpitz and taken away as POW's. The third submarine is too badly damaged to surface and explodes, killing the crew.
The film is based around the true-life attacks on the Tirpitz, first using using manned torpedoes (Chariots), and then the subsequent Operation Source, using the Royal Navy's midget X class submarines, and uses much original equipment. See British commando frogmen#1942.
The original book was written by C.E.T Warren and James Benson, who between them wrote five books in total about their experiences at war.
Commander Donald Fraser, who took part in the operations (as a Lieutenant) was an advisor on the film
The events in the movie have some minor differences from the real events, e.g. the boat that carried the chariots was called Arthur in reality but Ingebord in the movie.