Leading Seaman Hobson goes home to save his marriage, while a reluctant Torpedo Gunner's Mate Corrigan departs for his wedding in London.

Crossing the North Sea, the submarine picks up three shot-down Luftwaffe pilots from a rescue buoy, and prevents their radio alert to German forces.

When the Brandenburg is spotted, Sea Tiger fires all its torpedoes, but dives before assessing their impact due to German destroyers dropping depth charges.

Hobson, a former merchant seaman who speaks German and knows the port on the island, persuades Taylor to let him go ashore and search for oil.

Although Pincher (the cook) is killed and Oxford and Lieutenant Johnson are wounded, they get back to the re-fuelled submarine and start to leave the port.

John Mills prepared for his role as the captain of Sea Tiger by sailing in a submarine on a training mission down the Clyde.

He recalled a crash dive: The ship then seemed to stand on her nose and I felt her speeding like an arrow towards the sea bed; charts and crockery went flying in all directions; I hung on to a rail near the periscope trying to look heroic and totally unconcerned; the only thing that concerned me was that I was sure that my face had turned a pale shade of pea-green.

But with the outbreak of World War II, the four boats were requisitioned by the Royal Navy and designated the P611 class in the British Fleet.