An office worker named Konoha Satou dies in a traffic accident and reincarnates into the world of The Dark History, a self-insert fantasy novel she wrote as a teenager. To her horror, she is not reborn as the beautiful, talented heroine of her story, but as Iana Magnolia, the villainess older sister who is fated to be executed in the prologue after attempting to murder the protagonist. Now forced to live within the narrative of her own cringeworthy teenage creation, the adult Konoha must use her intimate knowledge of the plot to survive. She becomes determined to change her character's tragic destiny, protect her new elder sister—the story's original heroine, Konoha Magnolia—and prevent the many dark events she wrote from coming to pass.

The story is set in a fantasy noble society where magic exists. The protagonist navigates her new life as Iana, a girl despised by most of the nobility for her past wicked deeds. Her primary goal is to avoid her designated death scene while dealing with the embarrassment of confronting her old, self-indulgent writing made manifest. She must convince others she has changed, starting with her sister Konoha, a kind-hearted but seemingly powerless young woman who is unaware of her own potential, and her sister's hostile fiancé, Ginoford Dandelion. Key figures from her novel's plot include Sol Nemophila, a stoic butler originally meant to be Iana's killer who instead becomes devoted to protecting her, and Yomi Blacksarena, a charming villain with a dark past who was Iana's original partner in crime and remains confused by her sudden personality shift.

Notable narrative arcs follow Iana as she uses her future knowledge to avert catastrophic events, such as preventing an attack that would put her sister in a coma and earning the trust of those who initially loathe her. As she successfully changes the story, her actions attract the dangerous attention of the Knights of Schwartz, a mysterious organization determined to correct deviations and force the story back onto its original, darker path, even if it means eliminating innocent people who interfere with the plot. Subsequent arcs involve confronting figures from her novel like the psychopathic Duchess Suselina Amaryllis, who kidnaps young women, and battling a powerful Vampire Lord. The growing tension with the Knights of Schwartz leads to tragic outcomes, as characters who were not fated to die, such as Isaac, are murdered to realign the narrative, pushing Iana into deeper guilt and desperation as she fights against the very story she wrote.