Reincarnated as a vampire child in a noble household, Mercedes Grunewald is the daughter of a powerful aristocrat and his shunned concubine. She has always known she will never be chosen as her father's heir, and when that day comes, her mother, her aging maid, and Mercedes herself will be cast out with no resources or skills to survive. Determined to prevent this fate, Mercedes makes a decision at an age when most children are just beginning their education: she will become a dungeon crawler.

Mercedes lives in a nation of vampires, giving her the natural potential to grow strong, but she is still a small child facing deadly underground labyrinths. Rather than simply slaughtering the monsters she encounters, she discovers a unique talent for taming them, building a loyal army of creatures that she can command as she delves deeper into increasingly dangerous dungeon layers. Her past life ended with deep regrets, and she is determined that this time will be different. Her ultimate goal is to find something within the dungeons that will make her moon full, a personal quest for fulfillment that drives her forward.

The story begins in the rural countryside where Mercedes lives with her mother in a precarious existence on the margins of vampire society. After coming face to face with her father, her circumstances change dramatically. She is brought into the main Grunewald residence, where she receives education and a higher standard of living. Mercedes proves to be a pragmatic and calculating protagonist, willing to use her father as a means to achieve her own goals even as he attempts to use her in turn. Her enrollment at Edelrot Academy draws her into a secret struggle for the throne, entangling her in a royal succession drama that threatens to destabilize the entire kingdom.

As the series progresses, Mercedes travels to neighboring nations such as the Beatrix Empire, a land ruled by an empress where men are treated as inferior. She becomes embroiled in political intrigue, coup attempts, and conflicts involving rival kingdoms, all while continuing her dungeon exploration and monster taming. Supporting characters include Bernard, a figure whose actions suggest a quick move toward seizing power, and Hannah, a mysterious aunt who appears unexpectedly. Mercedes is accompanied by her tamed monsters, including Benkei and Kuro, who fight alongside her when called upon. Her journey takes her from a discarded child fighting for survival to a player in the highest circles of vampire politics, all while she searches for something that will give meaning to her second chance at life.