The Mastermind: World War II breaks out and a force of U-boats attempt to starve Britain into submission. Meanwhile, Vera Laughton Mathews recruits talented women into the Wrens.

False Dawns: France falls in 1940 and Donitz's U-boats embark on a race to sink British merchant ships. Roberts' wargaming ideas are at last entertained by a desperate Admiralty.

The Wrens: 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.

The Game: 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.

Stalemate: With the Allies at crisis point, pressure mounts on the Wrens to prove their worth and the stage is set for a battle that will turn the tide of the Atlantic war.

The End Game: The battle hits a turning point and Donitz unleashes an advanced torpedo to flip the momentum. Roberts and Laidlaw must devise a strategy to combat the new threat.