When a party of schoolboys assembles for a moth-watching party, the occasion turns out to be fatal not only to the moths. What is the explanation of the ruthless and deliberate killing of a normal, cheerful schoolboy whom everyone liked? Inspector Ferriby realises that in this unusual puzzle he will need the help of Scotland Yard, and he is additionally pleased when Desmond Merrion turns up to assist in the investigation. Before long the police find themselves with a further mystery on their hands, when the body of a woman is found floating in the river. All the clues in the possession of the police only seem to deepen the mystery. The investigation is about to be abandoned when a third death, more dramatic and startling even than the others, opens the way to a surprising solution.