"Villainess Level 99" is an anime series set in a fantasy world where the protagonist, Yumiella Dolkness, is reincarnated as the villainess of an otome game she once played in her previous life. Aware of her destined role as a minor antagonist destined for defeat, Yumiella decides to avoid the game's main storyline entirely. Instead, she focuses on leveling up her abilities to ensure her survival, inadvertently becoming one of the most powerful individuals in the world. Her overwhelming strength, however, leads to misunderstandings and unintended consequences, as others perceive her as a potential threat or a mysterious figure.

The narrative explores Yumiella's attempts to navigate her new life while avoiding the game's original plot, which revolves around the protagonist, Eleanor, and her romantic pursuits with various male characters. Yumiella's efforts to remain inconspicuous are complicated by her extraordinary power, which draws attention from both allies and adversaries. Her interactions with other characters, including the game's original love interests, often deviate from the expected outcomes, creating a dynamic and unpredictable storyline.

Key characters include Patrick Ashbatten, a nobleman who becomes intrigued by Yumiella's strength and enigmatic nature, and Eleanor, the game's heroine, who struggles to reconcile Yumiella's presence with the predetermined events of the game. The series balances elements of comedy, action, and drama as Yumiella's actions reshape the world around her, challenging the boundaries of fate and free will. The story delves into themes of self-determination, the consequences of power, and the complexities of interpersonal relationships within a structured narrative framework.