Belfort, a young French gentleman, and his valet, Frontin, are lost at night in a violent storm. Frontin mistakes a nearby building for an inn; it is, in fact, the convent of the Visitation Sisters, as the travelers soon learn from the indignant Portress who answers their insistent doorbell ringing. The two men are discussing their predicament when they hear a voice from within the convent singing a mournful air. Belfort is astonished and disconcerted when he recognizes the singer as his beloved Euphémie. She has become a novice during his two-year absence and is about to take her final vows. Despite his many amorous adventures, Belfort assures Frontin, Euphémie is the only woman he truly loves, and he determines to free her from her cloistered life. Grégoire, the convents drunken gardener, and an equally tipsy Coachman, afford the two travelers the means to enter the convent: Belfort will be disguised as a nun, while Frontin will assume the role of a priest from an adjacent monastery. The four men go off to a cabaret to seal their friendship in the pleasures of the bottle.
Euphémie is alone in the convent parlor, mourning the loss of her beloved Belfort. Ushered in by the gardener Grégoire, Belfort (now "Sister Séraphine") meets the Portress (Sister Bonaventure, who recounts her youthful misadventures), the Mother Superior, and the other Sisters, all of whom are captivated by "Séraphine." Euphémie faints as she recognizes the new "Sister," who claims that Belfort is her brother. Frontin (now "Father Hilarion") arrives, delighted to be surrounded by so many "pretty lambs." Suddenly Sister Joséphine rushes in to announce that the Mother Superiors parrot has escaped. All the Sisters run off to try to capture the bird, while Belfort, Grégoire, and Frontin spirit Euphémie away. The Sisters return to find Euphémie, "Séraphine," and "Father Hilarion" gone; they are convinced that the devil is at work in their home and hurry off to toll the convent bells.
The scene is divided by a common wall separating the convent cellar from the chapter room of the adjacent monastery. The runaways have been apprehended; "Father Hilarion" (Frontin) and Grégoire are locked into the monastery chapter room by three indignant monks, while Euphémie and "Séraphine" (Belfort) are confined to the convent cellar by the Portress. Frontin finds a set of keys in the pocket of the priests robe that he has taken; they not only open a larder where the monks provisions are kept, but also provide access to a secret door that opens into the convent, an illicit addition to the monasterys architecture that one of the priests has had built. The two pairs use the secret door to trade places. When the Visitations find "Hilarion" and Grégoire in their cellar and the monks discover Euphémie and "Séraphine" in their basement, all are scandalized. Belfort reveals his true identity and gains permission from the nuns to take Euphémie away so they may be married. With the promise that the secret door will be walled up, each character comments in turn on the events that have transpired, as the opera ends in a typical, sprightly vaudeville.