Gerald Hardcastle [David Niven], a wealthy London banker, and his wife Prudence [Deborah Kerr] live a strained, listless life in which they share a bed only out of habit and rarely speak beyond necessity. The film threads together five couples who each want to use a birth control pill called Thenol, yet none will admit it openly. The line-up of pairs includes Gerald and his French mistress Elizabeth “Liz” Brett [Irina Demick], Prudence and her doctor Dr. Alan Huart [Keith Michell], the Hardcastle household’s maid Rose [Vickery Turner] and their chauffeur Ted [Hugh Armstrong], Gerald’s brother Henry [Robert Coote] and his wife Grace [Joyce Redman], and Henry’s daughter Geraldine [Judy Geeson] and her boyfriend Tony Bates [David Dundas].
All five couples discreetly pursue access to Thenol. Grace obtains a prescription written by Huart, while Ted gets pills from a local chemist who is a friend of his. Yet the fragile plans begin to unravel when Geraldine starts stealing Grace’s pills and replacing them with aspirin, triggering an unexpected pregnancy in Grace. After Geraldine confesses the pill-switching scheme to Grace, Grace tells Gerald about it, and Gerald then uses the same ruse on Prudence to manufacture incriminating evidence of her affair with Huart.
Meanwhile, Ted tries a ploy of his own: he tells Rose she needs the pills for her health and hides them in a vitamin bottle. Rose, anxious about pregnancy herself, switches the pills in Prudence’s Thenol bottle with those in Geraldine’s vitamin bottle, only to discover that she has swapped Ted’s pills for Gerald’s aspirin. The result is that Rose ends up pregnant, and Ted informs Gerald of Rose’s pills—without disclosing that they were vitamins. When Gerald asks how Rose’s Thenol failed, she assumes he already knows about her own guilt, and the truth between the couples begins to collide in a chain reaction.
With Prudence still not pregnant, Gerald buys more aspirin to undermine Huart’s reputation, hoping to expose the doctor’s affair. The various schemes—Grace’s efforts to keep her pills away from Geraldine, Prudence’s exposure scenario, and the complexities of Liz’s entanglements—start to converge. Gradually, the plan backfires on Gerald: both Geraldine and Prudence become pregnant, completing a messy web of infidelities and evasions as the year progresses.
The tension among Liz, Prudence, and Gerald intensifies. Liz grows restless and eventually leaves Gerald, while Prudence discovers Huart’s Thenol prescription. At first neither Gerald nor Prudence wants a divorce, but Prudence offers to take the blame for the pregnancy if Gerald will spare Huart’s professional reputation. Gerald accepts this arrangement for a time, yet a chance encounter changes the course: he spots Liz in town, who openly tells him she is going to have his baby. With the prospect of a future shaped by new love and impending parenthood, Gerald agrees to an amicable divorce.
In the months that follow, the situation culminates in a surprising, bustling end: six babies are born, Rose having twins, underscoring how the tangled schemes and stubborn secrecy ultimately yield more offspring than anyone anticipated. The film closes on a note of comic realization, where the couples face their new responsibilities and the reality that love, connection, and the stubborn desire to control life can outpace even the best-laid plans.
