At the fictitious St Swithin's Hospital, Dr Gaston Grimsdyke, John Fraser, is an accident-prone doctor with more interest in the nurses than the patients. Having been sacked from his job as a medical officer at a men's prison for misbehavior with the Governor's daughter, Grimsdyke is taken on by his old medical tutor Sir Lancelot Spratt, James Robertson Justice, who is determined to turn him into a capable surgeon.
Grimsdyke soon discovers that a plum senior medical post is about to become vacant, and he begins scheming to be considered, even after his cousin Miles, John Fraser, tells him that the job has already been unofficially offered to someone else. Spratt and the hospital matron, Matron Sweet, Joan Sims, clash frequently, and Spratt orders Grimsdyke to romance the matron and “soften her up.” The plan backfires when she misreads his supposed anonymous overtures and comes to believe that Spratt is her secret admirer, setting off a tangle of misunderstandings and comic complications.
Meanwhile, Grimsdyke has fallen for the much younger physiotherapist, the determined Elizabeth Ercy as Jeannine, the hospital’s capable therapist whom he hopes to impress. Yet Jeannine regards him as too old for her, and Grimsdyke experiments with various schemes to appear younger and more appealing. His bold attempt to win her heart backfires several times, and she reveals that she has just become engaged to a man named Lambert Symington, Jeremy Lloyd, a development Grimsdyke had tried to derail.
Complications escalate when a rejuvenation serum, which Grimsdyke had planned to use on himself, is accidentally injected into Sir Lancelot, prompting a cascade of chaotic consequences. Spratt then persuades Grimsdyke to release an experimental mood-enhancing gas at a hospital dance, hoping to gain an edge. The gas sends Spratt into a frenzy, and he ends up romancing the matron at the party, though the next day she resigns, telling him she is married to her career and must move on to a different post.
With the senior medical post ultimately going to his cousin Miles, Grimsdyke’s mood brightens, even as he learns that his own prospects were blocked in part because he was thought to be too young-looking. The hospital’s leadership reshuffles leave Spratt and Grimsdyke in a familiar pattern of rivalry and reluctant collaboration, and the new matron proves to be as resistant as ever to Spratt’s ideas about how the hospital should be run. In the end, Grimsdyke finds a sense of momentum in the ongoing dynamics between ambition, romance, and professional ego, as the film settles into its quietly comic rhythm of misadventures and misunderstandings.
