When a merchant in Exeter hears an intruder in his home one night, his first thought is to conceal his adulterous lover. But then he witnesses a sinister figure stooping over the bed of his only child, a figure who seems to vanish into thin air.

Two years on and the identity of the intruder has become common knowledge: the idiot of the city who lost his own children many years ago, and who seems doomed to wander the town searching for them. But when a boy then disappears, suspicion immediately falls on him.

The local constable is determined to solve the mystery, as his own son disappeared some years ago and he always suspected the fool. Sir Baldwin is asked to follow a lead to the manor of Bishop's Clyst to try and find out what has happened. While he is there a body is found under the stone bridge - the body of a boy, but not the one who recently went missing...