Harold Crossley is a barrister and respected intellectual, but proves no match to his scheming young wife

A group of crooks accidentally kill an elderly shop assistant while stealing the keys to the jewellery shop where she works. When his superiors think that the case is better handed over to Scotland Yard, the local detective inspector, Sparrow, decides to go solo to investigate the crime himself.

An international criminal arrives in London, and a detective is anxious to pin something on him for some unprovable murders of former lovers. The villain starts to suspect servants and henchmen of being a police spy known as "Number Six".