When WW2 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.

Gilbert Roberts sets up a tactical school to counter the U-boats. He enlists the help of Jean Laidlaw and a small team of Wrens to wargame battles and develop new manoeuvres.

Roberts and Laidlaw have uncovered a flaw in the Royal Navy’s anti U-boat tactic ‘Buttercup’. They must now convince Noble that their new manoeuvre will sink Dönitz’s U-boats.

In 1943 pressure mounts on WATU to prove their value. When convoy ONS.5 catches Dönitz’s attention the stage is set for the battle that will turn the tide of the Atlantic war.

After ONS.5 Roberts and Laidlaw must do everything in their power to counteract Dönitz’s acoustic torpedo and recapture the waves before the impending D-Day landings.