As a dedicated officer, Nestor Patou finds his peaceful life in the Bois de Boulogne abruptly disrupted when he is thrust into the gritty atmosphere of the Les Halles district in Paris. His intentions are noble as he attempts a raid on the Hotel Casanova, a haunt for sex workers, but the operation backfires dramatically, leading to his dismissal and public disgrace as he is wrongfully deemed corrupt.
Now with his reputation demolished, Patou seeks refuge in the shadowy corners of Les Halles, specifically at Chez Moustache, a notorious tavern frequented by the city's underworld. Here, he encounters Irma la Douce, a bewitching courtesan who, recognizing his plight, offers him compassion and understanding. As Patou grapples with his new reality, he becomes entwined in the curious tales narrated by Moustache, the charismatic owner of the tavern, who captivates patrons with his whimsical anecdotes, invariably concluding with the line: > "but that's another story."
Patou's life takes a darker turn when he courageously intervenes to rescue Douce from her violent pimp, Hippolyte. With his affection for her growing stronger, Patou hatches a bold scheme to liberate Douce from her circumstances. Transforming himself into the persona of Lord X, a fictional wealthy English gentleman with a questionable history, he becomes her sole client. This elaborate deception is funded by Patou's nighttime work at the marketplace, forcing him to forsake his days in order to provide Douce with the luxuries she deserves.
As the indulgences of Lord X increase, suspicions begin to swirl around Patou and his hidden identity. Douce's feelings start to blur, leading her to wonder if Lord X is involved with another woman, further complicating their relationship. The tension escalates, culminating in an inevitable showdown as Patou resolves to put an end to this pretense. Little does he know, Hippolyte has been tracking him, mistakenly believing that Patou has committed murder against Lord X.
The mix-up leads to dire consequences; Patou finds himself arrested and imprisoned. However, the news of Douce's pregnancy sparks a daring escape plan. Disguised in his former police uniform, he effortlessly mingles with a search team at an apartment complex, narrowly evading capture once again.
Meanwhile, as the clever Hippolyte aids Patou in a covert search along the charming banks of the Seine, Patou’s true identity begins to surface, now enhanced by the sophisticated persona of Lord X. Ultimately, with his innocence established and reputation restored, Nestor is free to envision a future filled with happiness alongside Irma. Their engagement takes a fantastical twist when Irma, brimming with emotion, gives birth to their child right within the hallowed walls of a church. In an entertaining twist, Moustache, initially clueless about the entire ruse, unexpectedly encounters the genuine Lord X amidst the celebration, his face a canvas of bewilderment as he turns to the audience, inviting them into his state of confusion. As Lord X exits, Moustache's expression remains a humorous blend of shock and astonishment.
