Mrs. Wilberforce is a sweet and eccentric old widow who lives alone in a gradually subsiding house, perched over a railway tunnel in Kings Cross, London. With little to occupy her days and a vivid imagination, she habitually visits the local police station to share fanciful suspicions about neighborhood activity, a quirk the officers treat with bemused patience rather than concern.
She is approached by a sly and sinister character, Professor Marcus, who wants to rent rooms in her house. Unbeknownst to Mrs. Wilberforce, Marcus has assembled a gang of hardened criminals for a sophisticated security-van robbery at the nearby railway station, planning to use her home as a convenient base of operations. The gang includes the jittery and gentlemanly con-man Major Courtney, the Cockney spiv Harry Robinson, the punch-drunk ex-boxer One-Round Lawson, and the cruel and vicious continental gangster Louis Harvey. As a cover, Marcus convinces the naive Mrs. Wilberforce that the group is an amateur string quintet using the rooms for rehearsal space. To maintain the deception, the gang members carry musical instruments and play records of Boccherini and Haydn during their planning sessions.
The criminals successfully carry out the heist and trick Mrs. Wilberforce into retrieving the disguised money from the railway station herself. As the gang departs her house with the loot, One-Round accidentally gets his cello case full of banknotes trapped in the front door. As he pulls the case free, banknotes spill forth while Mrs. Wilberforce looks on. After she learns from a visiting friend that a robbery has taken place nearby, Mrs. Wilberforce finally sees the gang's true colours and informs Marcus that she is going to the police.
Stalling, the gangsters try to convince Mrs. Wilberforce that she will be considered an accomplice for holding the cash. Marcus asserts that the heist was a victimless crime as insurance will cover all the losses and the police will probably not even accept the money back. Mrs. Wilberforce wavers, but eventually she rallies, and the criminals decide they must kill her. No one wants to do it, so they draw lots using matchsticks. The Major loses, but he tries to make a run for it with the cash.
While Mrs. Wilberforce dozes off, the criminals double-cross each other and end up killing one another in rapid succession. The Major falls off the roof of the house after being chased by Louis, Harry attempts to escape with the money, but he is killed by One-Round, and One-Round is killed by Louis after he leaves his gun's safety catch on and fails to shoot Louis and Marcus. Marcus kills Louis by luring him down a ladder by the bridge overlooking the railway and dislodging it, causing Louis to fall into a passing railway wagon. Before falling into the carriage, Louis fires a last shot at Marcus which nearly hits him. Within moments, Marcus himself is struck on the head by a changing railway signal, and his body drops into another wagon. All the other bodies have been dumped into railway wagons passing behind the house and are now far away.
Mrs. Wilberforce is now left alone with the plunder. She goes to the police station, but they do not believe her story. They humour her, telling her to "keep the money". She is puzzled but finally relents and returns home. Along the way, she leaves a banknote of large denomination with a perplexed starving artist.
> "keep the money"
