Mrs. Wilberforce is a sweet and eccentric old widow who lives alone in a gradually subsiding house, built over the entrance to a railway tunnel in
Kings Cross, London. With nothing to occupy her time and an active imagination, she is a frequent visitor to the local police station where she reports fanciful suspicions regarding neighbourhood activities. The officers there humour her but give her reports no credence whatsoever. She is approached by a sly and sinister character, "Professor" Marcus, who wants to rent rooms in her house. Mrs Wilberforce is not aware that Marcus has assembled a gang of hardened criminals for a sophisticated security van robbery at the
nearby railway station, who plan to use Mrs. Wilberforce's house as a 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. One-Round tries to shoot Louis, but leaves his gun's safety catch on and Marcus kills him. The bodies have been dumped into railway wagons passing under a catwalk bridge behind the house and are now far away. Marcus kills Louis by luring him down a ladder by the bridge and dislodging it, causing Louis to fall into a wagon. Before falling into the wagon, 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 arm, and his body drops into another wagon. 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 poor artist. ==Cast==