 In the midst of World War II, the battle below the seas rages. The Nazi s have the upper edge as the Allies are unable to crack their war codes. That is, until a wrecked U-boat sends out an SOS signal, and the Allies realise this is their chance to seize the  enigma coding machine . But masquerading as Nazi s and taking over the U-boat is the smallest of their problems. The action really begins when they get stranded on the U-boat.