February 1943: As the balance of the war slowly shifts in Britain's favour, Lieutenant-Commander Steven Marshall brings his battle-scarred submarine into home port. Captain and crew are exhausted after fourteen months' continuous service, but for most there can be no thought of leave. If the enemy collapse in North Africa is to be exploited, every experienced man will be needed. Marshall must return to the Mediterranean, but this time to a very different kind of war. For his new command is secret and extremely hazardous - a captured German U-boat...