Dundee, 1915. When twelve-year-old Nancy suspects one of her teachers is a German spy, she ropes in the reluctant Jamie Balfour to help her uncover the scheme. Midshipman Harry Melville is aboard HMS Argyll in the stormy North Sea, unaware of both hidden rocks and German plots that threaten the ship. Nancy and Jamie discover HMS Argyll is in deadly danger and they are drawn into a web of espionage, secrets, and betrayal, where no-one is as they seem and no-one can be trusted.