Spring, 1941. The armies of the Reich are masters of Europe. Britain stands alone, dependent on her battered navy for survival, while Hitler’s submarines – his ‘grey wolves’ - prey on the Atlantic convoys that are the country’s only lifeline. Lieutenant Douglas Lindsay is amongst just a handful of men picked up when his ship is torpedoed. Unable to free himself from the memories of that night at sea, he becomes an interrogator with naval intelligence, questioning captured U-Boat crews. He is convinced the Germans have broken British naval codes, but he’s a lone voice, a damaged outsider, and his superiors begin to wonder - can he really be trusted when so much is at stake? As the Blitz reduces Britain's cities to rubble and losses at sea mount, Lindsay becomes increasingly isolated and desperate. No one will believe him, not even his lover, Mary Henderson, who works at the very heart of the intelligence establishment. Lindsay decides to risk all in one last throw of the dice, setting a trap for his prize captive - and nemisis - U-Boat Commander Jürgen Mohr, the man who sent his ship to its doom...