Francette, the optimistic daughter of a ducal family, loses everything overnight after her sister's engagement to the crown prince is broken off. The family's wealth and status are seized, and Francette is left to fend for herself. Rather than despair, she moves to the city's working-class district and begins selling homemade pastries, using the baking skills she once learned for charity work. She lives a quiet, self-sufficient life with only a feisty duck for company.

One day, Francette rescues a small, weak slime on the roadside. To her surprise, the slime can talk, introducing itself as Wibble (or Pururun in the original). Wibble belongs to Gabriel, one of the kingdom's powerful Monster Dukes known as the Slime Duke. Gabriel is a stoic and capable young man who, despite his high station, is often misunderstood by those around him. After learning of Francette's kindness to Wibble, Gabriel proposes marriage, and Francette accepts.

The story follows Francette as she moves to Gabriel's domain, the lake district of Triste (or Spring), to live with him as his fiancée during their engagement period. There, she must adapt to the eccentric people of the region, including Gabriel's intimidating mother, a dragon duke who is the kingdom's second prince and worries about her, and various relatives who cause trouble out of jealousy. What begins as a convenient arrangement gradually deepens into genuine affection, as Francette's forward-looking nature and talent for finding the good in people help her improve the lives of those around her. Her baking skills prove invaluable, leading to the creation of a popular local treat called slime buns, which generate income and assist with slime pest control. The narrative arcs involve Francette adjusting to her new life, confronting interpersonal challenges, and contributing to the revitalization of Gabriel's domain, all while surrounded by a host of helpful and mischievous slimes.

Main characters include Francette de Blanche, the resilient and cheerful fallen noble lady; Gabriel Griette de Slime, the misunderstood Slime Duke; Wibble, the talkative and talented slime who can clean, heat baths, and serve as a guard; and Axel de Adalard Dragon, the second prince and Dragon Duke who remains concerned for Francette's welfare.