At ten years old, Pia Rockwell recovers from a high fever with memories of her past life as a Japanese doctorate student specializing in paleontology. This realization comes with a second, more troubling shock: she has been reincarnated as a minor villainess in an otome game called Caroline and the Rainbow Magic Sweets. In the game's original storyline, Pia serves as the meek and forgettable fiancée of Rufus Stan, the prime minister's son, only to be publicly banished from the kingdom after a petty outburst. Determined to change this fate, the timid Pia abandons any ambition of romance or noble intrigue, dreaming instead of becoming a commoner, excavating fossils, building a museum, and working as a curator.

To achieve her peaceful future, Pia must first dissolve her engagement to Rufus. Summoning her courage, she asks him to break off their betrothal. However, instead of agreeing, the cunning and sharp-tongued Rufus becomes intrigued by her unusual request and her nervous yet earnest demeanor. Rather than release her, he proposes a bet that will span the next seven years. He wagers that he will never fall for the game's heroine, Caroline, nor banish Pia. If he wins, Pia remains his fiancée; if he loses and does fall for Caroline, he promises to grant Pia a large sum of money and a territory of her own, effectively funding her paleontological dreams.

Trapped by her own MAX-level timidity, Pia has no choice but to accept the bet. As the years pass, she focuses entirely on her fossil studies, showing no interest in noble society. Her genuine obsession with bones and her obliviousness to her own charm only serve to deepen Rufus's unexpected affection. Misinterpreting her constant fear and trembling as worry over losing him rather than his status or wealth, Rufus becomes determined to protect her. The story follows Pia as she navigates the misunderstandings of noble life, evades the original plot's traps, and gradually realizes that her terrifying fiancé might be her greatest ally, all while trying to keep her promise and secure her future as a paleontologist.