What would it be like to be unsure whether you could recognize your own brother? This is the terrifying quest that Julie Davidge, a young free-lance artist living in London, suddenly and bewildering finds herself confronted with. Her brother Richard, 10 years her senior and always a remote figure to her, has disappeared, presumably kidnapped as a British agent by the enemy. One day a man claiming to be Richard appears in London, and almost simultaneously Julie receives a postcard in Richard's handwriting, saying he is alive in France and needs her help. Is the man in London her brother or is he an enemy plant, waiting for any clue to the whereabouts of the real Richard in order to have him killed?
Only Julie and Stephen Archer, a man she has barely met, but to whom she is strongly attracted, know of the second Richard, and together they set off to the Continent to find him. But even before they leave, she has reason to feel her life is in danger, and the watchful eye of Mr Robinson, the mysterious new tenant in her building, does nothing to reassure her. Soon Julie realizes there is no one at all whom she can completely trust. Her safety and the safety of whichever Richard is the real one rest entirely upon her own wits and ingenuity as even Stephen, despite her love for him, suddenly seems suspect. A wild chase through northern France, a manhunt and murder in the ancient city of St. Malo, a perilous smuggling trip to the Channel Islands, and the violent death of the one person who might have helped her all occur before Julie's final confrontation with the false Richard, who will kill her now that he has used her.