Mrs. Wilberforce (Katie Johnson) likes to report suspicious behavior to the police. Unaware of her reputation, the dapper thief Professor Marcus (Alec Guinness) rents rooms in the elderly widow's home for himself and his band of cohorts. Posing as a string quintet, the thieves pull off a bank robbery, but slip up in front of the old woman as they try to escape. Agreed that they need to murder her, the bumbling crooks wind up double-crossing each other and slowly killing themselves off.
