A Japanese woman who died with deep regrets is reincarnated as Mercedes Grunewald, a young vampire girl in a noble household where she is the unwanted daughter of a nobleman and his shunned concubine. She lives a precarious existence in the countryside with her unskilled mother and elderly maid, fully aware that her father will one day choose a different heir, leaving her family with nothing. To secure their future, Mercedes begins rigorous dungeon crawling training from the age of a kindergartener, discovering that her vampiric heritage grants her tremendous potential for power. Her ultimate goal is to find something within these dungeons that will fill the emptiness in her heart and make her metaphorical moon whole again.

The story follows Mercedes from childhood as she hones her abilities and explores dungeons filled with monsters, treasures, and ancient secrets. Her capabilities quickly prove to be far beyond ordinary, and she tames powerful creatures including an ogre and a dangerous wolf who swear loyalty to her. Her extraordinary strength eventually draws the attention of her estranged father, who recognizes her value and brings her into the main residence where she receives education and a higher standard of living. Mercedes views this arrangement as a transactional opportunity, using her father as a pawn to advance her goals while he does the same with her.

The narrative expands when Mercedes enrolls at Edelrot Academy, where she becomes entangled in a secret struggle for the throne. This school arc transforms into a royal succession drama involving political maneuvering and a coup led by a figure named Bernard. During this conflict, Mercedes discovers that a woman named Hannah, who aids her, is actually her aunt. The succession crisis escalates to the point where the king, who leads a nation that prizes strength above all else, proves to be dangerously incompetent, forcing Mercedes to intervene directly in the power struggle.

In a subsequent volume, the academy students undertake an expedition traveling from Abendrot to Blut with minimal funds. The journey goes wrong when Sieglinde and Felix are kidnapped and taken to the Beatrix Empire, a neighboring nation where women rule and men are treated as inferior. Mercedes pursues them into this empire, ultimately confronting and defeating the Empress. Rather than killing her, Mercedes keeps the Empress imprisoned in her dungeon while a clone rules in her place, a solution that raises unsettling questions about Mercedes's morality.

Throughout the series, Mercedes maintains a stoic and pragmatic demeanor that some characters find sociopathic. Her past life memories influence her behavior, and she frequently recognizes the cliches and oddities of the world around her. A significant revelation emerges when Mercedes conquers a dungeon and learns that her world was artificially created in the past, with records of its origins subsequently erased. This discovery suggests a larger mystery about the nature of reality she inhabits and what she is truly seeking. The dungeons themselves contain not just monsters and treasure but existential truths about the world's creation, and Mercedes continues her journey driven by the desire to find meaning in her second life.