“Gentlemen! What stands between us and strangling Britain’s merchantmen and bringing her to her knees, is not the fury of the sea, or the violence of her Navy, but the obstinate stubbornness of the British sailor.”

Cousins Jack Lee and Emma Pickney and the attractive and charismatic Ranulph form an awkward love triangle. This explodes as they sail the Northern seas off Shetland in the summer of 1939 with the war clouds gathering. Jack, a Merchant Navy deck officer, finds himself in the front line of Hitler’s U-boat war, battling for survival in the turbulent waters of the North Atlantic, while clinging to his love for Emma. Ranulph, a salvage diver, is determined to evade the war. However, of the three, he is the first casualty, driven mad by the carnage wrought on the Battleship HMS Royal Oak, torpedoed in Scapa Flow in a daring attack by U-boat No.47.

Joachim Endrass, U47’s 1st Officer and the man who fired the torpedoes that sowed the destruction Ranulph witnessed, feels destined for great things. However, instead of his own U-boat, Endrass has foisted on him the angry Lieutenant Elrich and a new commander who for him, symbolises everything that is wrong with Hitler’s Third Reich. Disgruntled, he returns to the Atlantic as Germany’s attempt to strangle Britain's Atlantic lifelines reaches a climax, with no inkling of the dramatic and incredible events that lie ahead.