In a quaint London neighborhood, an elderly widow named Mrs. Wilberforce lives alone with her parrots. She unknowingly rents a room to a group of criminals masquerading as a string quintet, led by the cunning and eccentric Professor Marcus. The crooks plan to use her home as the base for a meticulously planned heist, intending to rob an armored car without arousing suspicion. They convince Mrs. Wilberforce that their frequent comings and goings are merely for rehearsals, deceiving her with their musical instruments and polite manners.

As the plot unfolds, the gang's uneasy cohesion begins to fray, and Mrs. Wilberforce becomes suspicious. Her genuine kindness and naivety, combined with a series of humorous misunderstandings, complicate the criminals' straightforward scheme. The discord among the thieves grows, leading to a series of comedic and chaotic mishaps. The tension between the group's superficial charm and concealed malice intensifies, revealing the precariousness of their plan. Thwarted by their own blunders and the unexpected savvy of their unsuspecting landlady, the situation escalates, teetering on the brink of failure.