Arthur Prohack, Cecil Parker, a Treasury civil servant renowned for his extreme frugality with public funds, suddenly inherits £250,000 and takes six months' sick leave. This windfall unsettles the already delicate family balance and sets off a series of strategic moves among the Prohack clan, tugging at loyalties and testing the limits of each character’s ambitions.
His children, Mary and Charles, come forward with their own demands: Mary seeks funds for a theatrical project, while Charles asks for money to back an investment venture. Meanwhile Eve Prohack, Hermione Baddeley, purchases a vastly larger house, filling it with an aviary and then an aquarium, signaling a new frontier of display and status for the family.
Charles also buys his father a new car, which on its first drive is involved in an accident with Mimi Warburton, Glynis Johns. Initially frosty, Charles takes Mimi on as his private secretary, drawing her deeper into a web of personal and professional entanglements.
Charles cancels a meeting with Arthur and arranges a board meeting without him; in retaliation Mimi arranges for Arthur to “accidentally” take Charles's place at a meeting with Lady Maslam, Charles's patroness, Lady Maslam, Heather Thatcher. The ruse stirs further complications as power, influence, and affection collide.
Left behind at home, Charles and Mimi lunch together and fall in love, leading Mimi to consider resigning, since when she first joined Arthur she had promised not to fall for Charles. The personal conflict feeds the broader tensions, adding a romantic undercurrent to the financial and social maneuvers.
Arthur's friend Sir Paul Spinner, Campbell Cotts, arranges to invest some of Arthur's money, bringing a new edge of risk and opportunity to the mix. Eve then organizes an elaborate party to celebrate Mary's engagement to Oswald Morfrey, Denholm Elliott a sickly but forthright junior official at the Ministry of Agriculture, whose quiet integrity contrasts with the bustling family drama.
Yet the couple—Mary and Oswald—break free from their theatrical circles and elope, marrying without their parents' knowledge. Arthur hunts them down, but Oswald refuses to help, highlighting the stubborn independence that his role at the Ministry embodies.
Spinner informs Arthur that a run on the stock market could ruin the money invested, escalating the stakes and underscoring how luck, timing, and prudent choice shape the family’s fortunes. Mimi discovers that the investment scheme is on the brink of ruin and argues with Arthur at Lady Maslam's home, where he had gone to seek her influence and support.
Relieved that the immediate burden may be lifting, Arthur retires to bed with a cold and experiences a fever dream inspired by Arthurian legends on the BBC Third Programme. Mimi wakes him to report that Charles has broken up with her and is about to flee to Paris with Lady Maslam, but this turns out to be a misunderstanding—Charles has not boarded the plane and returns home with Arthur to propose to Mimi.
Arthur discovers that Spinner's advice has ruined Charles's scheme, yet Spinner has in fact managed to avoid the crash himself and to secure another £250,000 for Arthur. Reconciled with Eve, Arthur plans to move back to their old house and to help Oswald, Mary, and Charles find more stable and healthy homes and livelihoods. He then returns to work and donates the new £250,000 anonymously to the Treasury, presenting it as a massive repayment of back taxes, restoring balance to the family’s finances and leaving a quiet mark of civic responsibility.
