In 1957 London, Mrs. Ada Harris, a widowed cleaning lady, becomes obsessed with one client's haute couture Dior dress; it inspires her to buy her own Dior dress. After suddenly receiving a war-widow's pension, she travels to Paris to do so. She stumbles into a showing of Dior's 10th anniversary collection and is befriended by André, the Dior accountant, and Natasha, a Dior model. However, the Dior director, Claudine, resents Ada's intrusion into the exclusive world of haute couture.

Dior has fallen on hard financial times, and, because Ada will pay in cash, they reluctantly agree to make her a dress. While in Paris for fittings, she stays with André and encourages him to express his affection for Natasha—who shares his interest in existential philosophy.

When Claudine is forced to fire several Dior's workers for financial reasons, Ada organises a strike and forces Claudine and Christian Dior to hear André's ideas to modernise and make the business profitable.

Ada returns to London with her dress. She loans it to her client, Pamela, a struggling actress, who wears it to an event. It catches fire and is ruined. Ada's Dior friends read about the disaster in the newspaper and send her another dress, one she initially coveted more than the purchased one.