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It is at a party that Richard Hedon first hears about the mysterious Paul Clyro and his unexplained disappearance. Clyro, a scientist, had been doing secret work on viruses with a colleague named Wolsingham. When Wolsingham committed suicide, Paul Clyro found the body, walked out of his house and vanished without trace. Richard follows Clyro's trail - all the way to Madeira, where a man known as Gavin Chilmark is living a comfortable and contented life, free from any breath of suspicion...