In September 1939 a German U-boat fires its first torpedo setting in motion the longest continuous battle of the war. German Admiral Doenitz is innovating U-boat technology and tactics and now has the opportunity to enact his vision: control and cut off allied supply lines from below in a brutal war of attrition. A cat and mouse game of epic proportions ensues as German U-boats and allied forces engage in a 5-year battle for supremacy in the Atlantic. The U-boats hunting for merchant vessels are able to inflict huge losses and Churchill himself speaks of how the U-boat peril brought fear to his heart. However as combating this new hidden enemy becomes a top allied priority, new tactics and technologies allow them to turn the tides and with the cracking of the infamous German 'enigma’ naval codes the hunter soon becomes the hunted.