A Relative Stranger — Anne Stevenson

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?