Father Brown and Mrs McCarthy are at the local bank when armed raiders force the manager to open the locked safe using Father Brown as a witness. The robbers flee when the vault is opened and the manager's son-in-law is discovered dead inside. As the only key-holder Inspector Sullivan arrests the manager. Father Brown uncovers fraud at the bank, by the manager, financing a seedy nightclub owned by his son-in-law; but he needs Sid's help to break into the vault to discover how the murder was committed.