A night watchman is being bludgeoned, as a safe is cracked open in the offices of the District Food Controller. A list of wartime foods to be rationed is stolen, and the police fear gangsters are planning to sell the foods on the black market. As the office charwoman, Old Mother Riley's fingerprints are all over the safe, and she becomes the police's number one suspect. To prove her innocence, Mother Riley turns detective, adopting various methods and disguises to track down the villains.