Set in London just prior to World War II, the film follows a day in the life of Guinevere Pettigrew, a middle-aged, straitlaced vicar's daughter and governess who has just been fired from her fourth job. When Miss Holt, the head of the employment agency, tells her she is not able to find her another post, the destitute Miss Pettigrew leaves the office with an assignment intended for a colleague, unaware that the potential employer, flamboyant American singer-actress Delysia Lafosse, is seeking a social secretary rather than a governess.

Arriving at the luxurious apartment where Delysia is staying, Miss Pettigrew discovers that the younger woman is involved with three men: the devoted but penniless pianist Michael Pardue who has just been released from prison, the controlling but wealthy Nick Calderelli who owns the nightclub where she is performing, and the young theatre impresario Phil Goldman who is in a position to cast her in the lead role in a West End play. As she tries to help Delysia sort through her various affairs, Miss Pettigrew finds herself swept up into the world of high society. After being given a makeover by her new employer, she attends a fashion show where she meets and is strongly attracted to top lingerie designer Joe Blomfield who is currently involved in a tempestuous relationship with the show's host, Edythe Dubarry.

Over the course of twenty-four hours, Guinevere and Delysia become friends and help each other achieve their romantic destinies. After a series of complications, Delysia and Michael sail for New York aboard the Queen Mary, while Miss Pettigrew is found in Victoria Station by Joe who, now convinced that she is the woman for him, has been looking for her all night. They leave the station together, arm in arm.