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Gilbert Roberts reports to Western Approaches Command. He needs to establish the Western Approaches Tactical Unit, or WATU, to decipher Dönitz’s U-boat tactics and develop countertactics to sink them. But he gets a frosty reception from Admiral Noble. They already have a tactic that works - “Buttercup”. Meanwhile the USA’s entrance to the war provides Dönitz with many new targets. He orders wave after wave of attacks.

The Royal Navy can’t spare any men to join Roberts’ team so he turns to a group of women from the Wrens. Roberts with Jean Laidlaw and a small team begin analysing the “Buttercup” tactic. They need to understand how the U-boats attacked, to try to decipher Dönitz tactics. But when they play through “Buttercup” in the war game, it doesn’t work. They are in a race against time to come up with a new tactic to fight back against the U-boats.