At the beginning of World War II, the devastating impact of German submarines on both the Royal Navy and merchant shipping saw Britain on the brink of starvation and defeat.

The enemy was formidable. U-boat crews saw themselves as an elite and they preferred to scuttle their vessels at the end of the war rather than surrender. They suffered the heaviest losses of any branch of the German services: out of 40,900 men, 28,000 were killed and 5,000 taken prisoner; by 1945, the average age was 19 and the survival rate was only three missions.

This is the story of how the Allies redressed the balance of power, focusing in particular on the role of the wolfpacks of U-boats in the Atlantic, whose stealthy presence beneath the waves ensured that British ships diced with death every time they put to sea.