Dundee, 1915. Twelve-year-old Nancy Caird is desperate to do her bit for the war. So when she suspects one of her teachers of being a German spy, she's determined to foil his plans, and ropes in the reluctant Jamie Balfour to help her uncover the scheme. Meanwhile, Midshipman Harry Melville is on his first voyage aboard HMS Argyll as it forges through the black and stormy North Sea, unaware of both hidden rocks and German plots that threaten the ship. When Nancy and Jamie's suspicions are confirmed, and they discover HMS Argyll is in deadly danger, they're drawn into a web of espionage, secrets, and betrayal, where no-one is as they seem and no-one can be trusted.