Algy turned a trifle pale and shook his head. 'For God's sake be careful,' he whispered tersely. 'They'll shoot you like a dog if they spot what you're doing.' While on leave, Biggles is mistaken for someone else. He thinks little of it at the time, but when headquarters find out, they ask him to take the place of his double and work for the Germans as a spy, while secretly reporting back to the British. It's a difficult task and there's a very high price to pay if he's spotted - his life.