The story begins with Christina Ashburton, a Duke's daughter, being transported in a luxurious carriage, heavily guarded by Imperial soldiers. She reflects on her impending marriage to Emperor Maximilian of the Sovelt Empire, a union mandated by a treaty between her home country, Physalia, and the Empire. This treaty dictates that Physalia must provide a "Saintess" to marry the Emperor to maintain its independence. Christina, who recently regained memories of a past life as Natsuki, feels like a "sold calf" being sent away. She possesses latent magical abilities, capable of warming her surroundings, but is unsure of their full extent, as historical records of Saintesses show no use of their powers. She laments the fate of previous Saintesses, including Seraphina, who was confined to a villa, and her own half-sister, Angelica, who was supposed to be the Saintess but refused to marry the Emperor.
Two months prior, Christina, then unaware of her past life, was living a miserable existence as the neglected daughter of Duke Gary Ashburton. Her father had married her stepmother, Emerine, and treated Christina with indifference, while doting on Angelica. On a snowy day, a day often associated with misfortune for Christina, she was summoned to the castle. There, the King declared her engagement to Crown Prince Gerald dissolved, and instead recognized her as the Saintess, ordering her to marry Emperor Maximilian. This decision was made because Angelica, the true Saintess, refused to marry the Emperor and instead wished to marry Gerald and become Queen of Physalia. Christina, feeling she had no place left in Physalia, accepted her fate.
Christina arrives at a dilapidated castle, far from the capital, which is to be her new residence. She is terrified of ghosts and finds the castle's appearance deeply unsettling. Sebastian, the butler, explains that Emperor Maximilian dislikes Saintesses and intends for Christina to live in isolation in this castle, essentially as an ornamental wife. Despite her fear, Christina, now embracing her past life's practicality, decides to make the best of the situation. She immediately focuses on renovating the castle, prioritizing her room to make it less frightening. She also requests a budget for these renovations and begins planning to make the castle habitable and profitable, including establishing a farm and a silkworm farm.
Meanwhile, Emperor Maximilian is frustrated by the marriage contract issue. He dislikes Saintesses, particularly Seraphina, his grandfather's former Saintess, whom he views as manipulative and dangerous. Seraphina, confined to a villa, is secretly plotting. Maximilian's primary concern is the marriage contract, which requires a formal ceremony and signing in the presence of a priest. He decides to visit Christina at the remote castle to fulfill this obligation, hoping to minimize interaction. Upon arrival, he is shocked to find the castle grounds transformed into a vibrant vegetable garden and the castle itself thoroughly cleaned and renovated. Sebastian explains that Christina, using her magic, has completely altered the landscape and the castle's interior, even creating a massive reservoir. Maximilian is stunned by the extent of her power and the dramatic changes, realizing that Christina is far more capable than he anticipated.
Later, Christina falls ill with a cold after a magical mishap while creating the reservoir. While recovering, she learns from her maids, Ann and Mina, about the severe water shortage in the region and the villagers' reliance on the newly created reservoir. She also discovers that the castle was built centuries ago for a Saintess by a former Emperor. She decides to sell some of her expensive dresses to fund further improvements and contemplates the possibility of using her magic more openly. She also learns that the Emperor is expected to visit soon to finalize the marriage contract.
