The story begins with the protagonist, a commoner appointed as a Saint, attending a party to announce her engagement to the Crown Prince. To her shock, the Crown Prince publicly breaks off their engagement, accusing her of being a "lustful Saint" who incites sexual arousal in others. He claims her Saintly ability to heal and regenerate has a side effect of increasing libido, and that she has been unchaste with the knights she served with. The protagonist refutes this, stating she maintained her purity with the knights, but the Crown Prince dismisses her claims, influenced by Count Streletz, a resentful nobleman who had previously made unwanted advances towards her. The nobles present, prioritizing their own status, remain silent, viewing her as a commoner unfit for the royal family. The protagonist accepts the annulment and the implied exile, though the Crown Prince declares she will continue to serve the country as a "commoner's lustful Saint" protecting it from monsters.
A week later, the protagonist, now residing in a dedicated Saint's room in the royal palace, contemplates her situation. While unaffected by the public humiliation, she worries about her future. Her previous protection stemmed from the Crown Prince's favor, and now, without it, she fears for her safety and ability to continue her work. She reflects on the Kingdom of Koffe, a nation bordering the formidable Victorious Empire, and her role in defending it from powerful monsters. She finds fulfillment in her Saintly duties, particularly in how her abilities indirectly boost the economy by stimulating the red-light district, providing a safety net for the less fortunate and employment for knights.
Upon returning to the monster-hunting front lines, she is met by the people from the red-light district and knights for a farewell party. They express their support and acknowledge her purity, but then reveal a grim reality: they are planning to leave the area. They explain that the monster threat has become unmanageable without her, and if she were to fall ill or be unable to work, the front lines and the district would be overrun. They urge her to find a life free from such constant peril.
During this gathering, a red-haired knight, who had been a comrade at the front lines, invites her to his country, the Empire. He reveals he is not from Koffe and that his "brother and sister-in-law" need her help. He explains they are the Emperor and Empress, and they are struggling to conceive due to stress. The protagonist, intrigued by the prospect of using her abilities to help, agrees.
Upon leaving Koffe, the protagonist discovers that Count Streletz's accusations have led to a strong sentiment for her expulsion from the country, making her departure smoother than anticipated. She travels with the red-haired knight, Richard, towards the Empire. During their journey, Richard reveals his true identity as Prince Richard, the Emperor's younger brother.
Arriving in the Imperial capital, they are met with an overwhelming welcome. The protagonist, Monica Regulus, is astonished by the grandeur and the fervent reception, a stark contrast to her previous life. She is housed in a Saint's temple and begins assisting the Emperor and Empress with their infertility issues and various monster-related problems plaguing the Empire. Monica's exceptional abilities prove invaluable, solving issues that have troubled the Empire for years, including a mysterious monster outbreak in the southern region caused by the felling of a sacred tree that served as an ancient barrier. Her work not only aids the Emperor's reign but also elevates the Empress's standing by associating her with these benevolent acts.
Throughout their work, Richard remains by Monica's side, acting as her protector and confidant. He consistently shields her from potential harm and the side effects of her abilities, particularly the arousal-inducing aspect. Monica observes Richard's unwavering dedication and kindness, finding him a trustworthy and peculiar individual, especially given his high status as a Prince. She begins to suspect he might be asexual, as he has never shown any signs of being affected by her abilities.
As Monica successfully resolves various issues, the Emperor and Empress's stress levels decrease, and their health improves. The Empress's menstrual cycle returns, and the Emperor's physical well-being is restored, suggesting their infertility problem is nearing a resolution. With her primary objective nearing completion, Monica finds herself increasingly involved in the Empire's affairs, with Richard always present to support her.
