A forensic expert can become involved in crime in many ways. Often he will act for the police, but there are times when he can be called in by other interested parties. Dr. John Hardy is no exception and even when he has no particular liking for the people employing him he will always ferret out the facts. When these facts do not prove to be advantageous to his clients he can find himself in a difficult position. Troubles arise when Dr. Hardy discovers the 'unknown factor'. Three bullets, a dead man and £50,000 in insurance – this is the background to this intriguing opening case.