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 war gamer.
The WRENS finally get the break they have been looking for and are posted to the newly formed war gaming room, to work out and develop tactics for the Navy.
Roberts and Laidlaw have uncovered a flaw in the Royal Navy's anti U Boat tactic called 'Buttercup'. They then set about modelling a counter tactic, by wargaming various scenarios, and finally develop 'Operation Raspberry'.
Frustration rise on both sides. For every new tactic, the enemy comes up with a countermeasure. This places ever more pressure on Roberts and Laidlaw to ensure that their gaming produces the best tactics for the Navy.
WATU faces a crucial encounter for ONS.5 in the battle of the Atlantic. Donitz unleashes the new torpedo targeting on escort ships. Roberts and Laidlaw must again develop new tactics to retake the advantage.
