In the kingdom of Elvastia, the nation is protected by a majestic black-crimson dragon named Gramwind, who resides in a giant cage within the castle. The woman tasked with caring for this dragon is given the title of Dragon Saint (Ryū Seijo) and is also designated as the official fiancée of the kingdom's prince. The current Dragon Saint is a young count's daughter named Lyudia, who has diligently devoted herself to tending to Gramwind in his cage since childhood while being engaged to the first prince, Edic.

Lyudia's life is upended when she is suddenly summoned by Prince Edic, who informs her that she is being relieved of her duties as Dragon Saint and that their engagement is annulled. The position of Dragon Saint is to be given to her elder stepsister, Leila. Without any say in the matter, Lyudia is told that her next destination is a marriage to the Black-Clad General, Wilfried, though General Wilfried personally refuses the match, stating coldly that Lyudia does not befit him.

Returning to her family home, Lyudia finds no comfort, only the contempt of her father and her victorious stepsister Leila. Her situation takes an unexpected turn when General Wilfried appears to take her back to his own mansion, offering her a place to stay as a live-in guest. There, Wilfried makes amends for his earlier refusal and formally proposes a new engagement to Lyudia. However, her stepsister Leila, now the official Dragon Saint, soon arrives to demand Wilfried serve as her guard, a request he flatly denies.

Though Lyudia forms a bond with a small white dragon named Shiro-ryū-chan while starting a new life with Wilfried, her deep connection to Gramwind proves impossible to ignore. When Gramwind awakens from slumber in a fury over the disruption of his bond with Lyudia and the neglect from Leila, the dragon rampages, destroying the castle and endangering the kingdom. Leila and the prince are left in a desperate state, unable to perform the sacred duties of caring for the dragon. Lyudia, with her genuine connection to the creature, must make a perilous journey back to calm Gramwind's wrath, even as she finds herself torn between her destined role as the dragon's bride and her newfound happiness with Wilfried. The narrative unfolds along the tensions of court politics, as Leila and her allies scheme to undermine Lyudia, leading to crises that force Lyudia to confront whether her future lies with the dragon god she has always served or the man who has come to cherish her.