It is at a party that Richard Hedon first hears about the mysterious Paul Clyro and his sudden disappearance. Clyro had been a scientist, doing secret work on viruses with a colleague named Wolsingham. But when Wolsingham committed suicide, Clyro found the body and promptly vanished without a trace.

Richard's curiosity is piqued, and he follows Clyro's trail - all the way to Madeira, where a man known as Gavin Chilmark is living a comfortable life free from any breath of suspicion . . .