After her older sister's engagement to the crown prince is suddenly broken off, Lady Francette and her family lose all their wealth and status. Left with nothing but her determination, Francette moves to the slums and ekes out a living by selling homemade pastries. It is there she meets a talking slime named Wibble, who introduces her to Gabriel, the stoic and eccentric Duke of Triste, a man who also happens to be a slime. To her great surprise, Gabriel proposes a marriage of convenience, and Francette accepts, beginning her new life in the strange backwater territory known for its abundant slime population.

The series follows Francette as she adapts to her unusual circumstances and breathes new life into Triste. What begins as a practical arrangement slowly develops into a genuine romance. Francette joins forces with Gabriel and the slimes to revitalize the territory, including working at the Lakeside Duck Bakery to develop new products. Her integration into this eccentric world is not without challenges, as she must also navigate the politics of the monster dukes, including a visit from Magritte, the alluring Siren Duke, whose close relationship with Gabriel stirs feelings of anxiety in Francette.

As their wedding approaches, Francette and Gabriel face final hurdles. Preparations hit a snag when they discover the aurora pearls needed for her wedding dress are no longer available. With the help of the Ogre Duke, Emilie, they decide to cultivate their own pearls in Triste. The lead-up to the wedding also brings a reunion with Francette's mother and sister, who are confronted with the impoverished life she had been living. The conclusion of the series follows the couple as they overcome these final obstacles, including a kidnapping, to finally achieve the wedding of Francette's dreams.