When WWII breaks out between Britain and Germany, the Royal Navy must find a way to sink the German U-boats before they cut off their vital transatlantic shipping routes.

Dönitz's U-boats continue to terrorise the transatlantic routes. In Britain, the WRENS prepare to frustrate his plans, while the Royal Navy turns to a retired wargamer.

Though the Admiralty are convinced the Allies have the expertise to defeat Germany, Roberts and the Wrens start devising wargaming tactics to combat U-boat formations.

Roberts and Laidlaw uncover a fatal flaw in the Royal Navy's anti-U-boat strategy, and develop their own countertactic. Their next task is winning over the Admiralty.

Following a promotion, Dönitz now has the power to fill the Atlantic with more and more U-boats. The Battle of the Atlantic is about to reach a turning point when convoy ONS.5 catches Dönitz's attention.

When convoy ONS.5 enters the most perilous part of its journey, an area of the Atlantic that could not be reached by conventional air cover, Dönitz unleashes his wolf-packs.