Robert Blossom is a brassiere manufacturer and workaholic (played by Richard Attenborough) who secretly longs to be a musician, conducting imaginary orchestras in the quiet of his home. His wife Harriet Blossom is an imaginative and romantic-minded painter (played by Shirley MacLaine) who loves and supports her husband, yet often feels left behind by the rhythm of his factory-driven life.
When Harriet’s sewing machine breaks, Ambrose Tuttle is sent to repair it (played by James Booth). In the course of fixing the machine, Ambrose shares a heartfelt life story about being an orphan with no true home or family to love. Harriet is deeply touched by his tale, and a surprising spark kindles between them. Their bond quickly blossoms into a love affair, staged as a melodramatic daydream in which Ambrose appears as a heroic figure who rescues Harriet. When Robert returns unexpectedly, Harriet hides Ambrose in the attic and instructs him to slip away in the night. But Ambrose is entranced by Harriet and resolves to remain, becoming her secret paramour.
The affair settles into a volatile routine: the lovers meet clandestinely during the day while Robert runs his factory, and by night Harriet furnishes Ambrose with Teach Yourself books to educate him and transform the attic into a full, living space—one that mirrors the education and opportunity Ambrose never had. As Ambrose grows bolder and Harriet grows bolder in her deception, the question of loyalty hangs over the household.
Ambrose’s disappearance triggers the arrival of Det. Sgt. Dylan, a sharp Scotland Yard investigator (played by Freddie Jones). Dylan’s instincts drive him to Harriet, who insists Ambrose never arrived at her home, despite the mounting evidence. The detective keeps pressing, sometimes catching Harriet off-guard, while Ambrose remains hidden in disguises—at times as a Mexican “Juan.” Dylan never quite connects the dots, and by the time he thinks Harriet may be involved with her supposed half-brother, the very notion of murder begins to loom in his mind.
Meanwhile, Robert’s mind frays under the attic noises and the persistent sense that another presence lurks in the house. Harriet convinces him that it is all in his imagination, allowing him to doubt his own sanity. Yet Ambrose and Harriet both realize they care for Robert and feel guilty for triggering his breakdown. Ambrose decides to use his new education to help Robert grow the business, urging diversification, which unexpectedly makes Robert wealthy beyond his wildest dreams and frees him to retire in order to pursue his real passion: music.
As retirement nears, Dylan returns with a breakthrough: he believes Harriet’s secret lover “Juan” is real and still lurking in the attic. Robert and Harriet confront the attic together, only to discover that Ambrose is very much alive and well. The couple proceeds to divorce, with Robert gifting his businesses to Harriet and Ambrose so they can continue their life together while he dedicates himself to composing and performing music.
Time passes, and in a final, quiet moment of revelation, Ambrose—now sober and suited for the office—has breakfast with Harriet, then leaves for a day of work. Harriet stamps the floor, prompting Robert to rise from the basement wearing a mod-inspired outfit that echoes a rock star’s flair. The two share a tender, renewed embrace, signaling a complex, unconventional harmony that lingers long after the curtain falls.
