After dying of an illness, a young woman awakens to find herself reincarnated as Celine Eldred, the selfish and overweight villainess duchess from a popular novel she once read. In the original story, Celine is the cruel older sister to the beautiful heroine, Adele, and her fate is a grim one: she is destined to be executed by the hero prince after a series of spiteful plots. Armed with the knowledge of this terrible future, the reincarnated woman vows to change everything. She commits to a strict self-improvement regimen, determined to shed her excess weight, abandon her arrogant personality, and live a quiet, peaceful life without disturbing the heroine's path to a happy ending.

The setting is a classic fantasy royal court, where social status and noble families dictate power. As Celine works on her transformation, she expects to be ignored, especially by the story's primary antagonist, Prince Wilfred. He is known as the final boss, a cold and ruthless figure who is supposed to be the greatest threat to the kingdom and the hero. However, Celine’s genuine efforts do not push him away. Instead, they capture his intense and dangerous attention. Rather than despising her, Prince Wilfred develops a deep, obsessive love for her, showering her with a suffocating yet passionate affection that completely derails the original plot.

The main characters include Celine, a protagonist who is defined by her earnest desire to be a better person through real effort, focusing on diet, exercise, and kindness. Prince Wilfred is a yandere male lead; he appears calm and composed on the outside, but harbors a possessive and insane love for Celine that borders on terrifying. Other key figures include Adele, the sweet-natured heroine and Celine's sister, and the first prince, Lewis, who is the supposed love interest in the original story. As Celine's life changes, these characters also find their fates altered by her actions.

Several notable narrative arcs define the series. The first major arc focuses on Celine's survival and reformation, as she desperately tries to avoid her death flags by losing weight and studying to become a proper noble. This arc subverts the typical villainess story by emphasizing genuine, difficult self-improvement over scheming. Next is the unexpected obsession arc, where Celine is bewildered to find that instead of hating her, Prince Wilfred is utterly devoted to her. His constant doting and clingy behavior, which she never anticipated, become a new and complicated kind of threat to her plans for a quiet life. Subsequent storylines often involve the political intrigues of the court, where Celine’s new confidence and the prince’s overwhelming power force other nobles to reevaluate her. The narrative also explores the original heroic narrative falling apart as the prince ignores the heroine to pursue the villainess, creating a romantic tension that is both thrilling and humorous. Throughout these arcs, Celine struggles to find what the story calls peaceful happiness, caught between the fear of her original fate and the growing, undeniable pull of the yandere prince's love.