The story begins with the protagonist, a woman of considerable girth, discovering a shocking newspaper headline: the Second Prince Guilford's crest has been revealed. Upon checking her own body, she finds an identical crest on her abdomen, leading her to the realization that she is the Prince's destined partner. This revelation throws her into a panic, as she feels utterly inadequate for such a role, being significantly overweight and lacking confidence. Despite her initial despair, she resolves to lose weight before meeting the Prince.
However, her ambitious diet plan quickly falters. Six months pass with no progress, her servants inadvertently sabotaging her efforts with excessive food, and her own lack of discipline. Her doting father, who spoils her with treats, is unaware of her predicament. He mentions that Prince Guilford's destined partner has yet to appear, a topic of much public speculation. The Prince has been actively searching, even venturing into commoner districts, and his appearance has reportedly worsened due to the prolonged search. The news that Prince Guilford will visit their home within a week sends the protagonist into further panic. Faced with this imminent threat of exposure, she impulsively declares her desire to study abroad. Her father, though surprised, agrees to arrange the best environment for her studies, and she insists on leaving as soon as possible, using her newfound motivation as justification.
Meanwhile, Prince Guilford is also grappling with the appearance of his crest on his collarbone. He finds the concept of a destined partner unnecessary, preferring to focus on his duties. His elder brother, the Crown Prince, tries to encourage him, sharing his own positive experience with his partner, Ada. Guilford, however, remains cynical, viewing his destined partner as a potential nuisance. He begins the official process of revealing his crest and preparing for his partner's arrival, though he harbors no particular hope for a pleasant encounter.
As the search for Guilford's partner continues fruitlessly, rumors begin to circulate about her whereabouts, including being married, enslaved, or even deceased. Guilford's frustration grows, leading him to write a heartfelt message published in the newspaper, attempting to reassure his unseen partner. He learns that his partner's absence might be due to fear of his "Ice Prince" reputation. He also reflects on the emotional void within royalty, which is only filled by the presence of a destined partner, who brings essential human emotions like joy and sorrow, allowing them to connect with their people. This realization sparks a personal interest in finding his partner, and he begins actively searching noble households.
The protagonist, now in the neighboring country of Restoria, enrolls in the First Royal Academy. She immediately faces prejudice due to her weight, experiencing ostracization and mockery. During a class, she is falsely accused of stealing food by Fourth Prince Radnik, who bases his accusation on her appearance. She is saved by Rosemary, a striking red-haired student who intervenes and exposes the true culprit. Rosemary, also a foreign student, quickly befriends the protagonist, who adopts the name Furin.
Furin and Rosemary become close, and Furin confides in Rosemary about Radnik's alleged "curse" of personality change, a claim made by Radnik's sister, Princess Melodia. Rosemary, a skilled investigator, begins to research Radnik's past and discovers he was once a model student who abruptly changed his personality and broke off an engagement. They also learn about the historical parallels between Radnik's behavior and the villagers of Inas Village, who also underwent strange personality changes before being wiped out by monsters.
Meanwhile, Prince Guilford, driven by the growing threat of monsters and the need to find his partner, travels to Restoria. He investigates the forest where monsters have appeared and discovers a connection to the legendary "demons" that precede monster outbreaks. He also agrees to attend a student exchange event at the Second Royal Academy, hoping to gather information.
At the exchange event, Furin encounters Guilford, whose voice and presence deeply affect her, causing her to weep. She flees to the restroom, overwhelmed. Later, while discussing Radnik's condition with Rosemary, they learn from two brothers, who are from a village adjacent to the destroyed Inas Village, that the villagers also experienced drastic personality changes before their demise. They connect this to Radnik's transformation and the strict, exclusionary nature of the First Royal Academy, which mirrors the Inas Village's culture.
Guilford, attending the event, is intrigued by the information gathered and plans to investigate the lost village of Inas. He also reflects on his own growing obsession with his destined partner, his appetite diminishing as his thoughts consume him.
Furin's father visits her, concerned about the monster incident. He offers to bring her back to Iljua, but she refuses, determined to grow and find her own path. During a market outing with her father, they witness a purse-snatching. Furin's father intervenes, showcasing his combat skills. This event reignites Furin's motivation to lose weight, realizing that physical fitness opens up many possibilities.
Furin's father, Ulrich Tuniche, a renowned merchant, is recognized by a beautiful young woman who offers to repay his kindness. However, upon learning Furin's name, the woman's demeanor changes drastically, and she abruptly leaves. Furin is confused by this encounter. Later, she sees Rosemary in the crowd, but Rosemary seems to avoid her.
Furin also learns that Noah, an acquaintance from her past, is an information broker known as a "walking weapon" due to his vast knowledge and memory. He warns her to be cautious of him, as he might be interested in her family's information.
The story culminates with the discovery of multiple monsters in the forest near Restoria. Guilford, as a Holy Knight, is dispatched to investigate. He encounters the monsters and, with the help of Rosemary and the protagonist's childhood friend Leo, who has become a powerful Archmage, they defeat them. The incident raises concerns about the potential emergence of "demons," ancient beings that cause monsters to appear. Guilford decides to investigate the legendary Inas Village for clues. He also agrees to attend the student exchange event, hoping to gather more information and perhaps find his destined partner.