Elderly, widowed, doddering but strong-minded Louisa Wilberforce, all alone in the world except for her three pet parrots and a handful of similar friends, routinely rents out two rooms in her house, her most recent lodger being Professor Marcus, who has taken both rooms available so that he and his four friends - Major Courtney, Mr. Harvey, Mr. Robinson and Mr. Lawson - can rehearse as a string quintet. That is all a guise as Marcus is a criminal mastermind, who, with the other four, his current team all using aliases, are planning their latest crime out of the house, the crime namely an armored vehicle robbery. Marcus is betting on Mrs. Wilberforce being true to what he knows of her nature, which will allow him to use her as a dupe in the plan. But as certain aspects individually come to light of the crime, Marcus and the four have to decide what to do even if they believe that Mrs. Wilberforce has figured out what's going on. What they decide to do may take into account that Mrs. Wilberforce is friendly with the police, they not knowing that the police in turn largely humor her in being the proverbially harmless little old lady.—Huggo