When Light is kicked out of the Concord of the Tribes, his former comrades instantly turn on him. Light escapes this diabolical act of betrayal by the skin of his teeth...only to find himself in the deepest part of the Abyss, the most dangerous dungeon in the realm! To avoid being eaten by carnivorous monsters, he uses the Unlimited Gacha, his sole magical skill. But where it previously only produced junk items, this time Mei—a gorgeous Level 9999 fighter in a maid outfit—springs forth! Fast forward three years and Light has carved out his own kingdom in this backwater dungeon, summoning more beautiful Level 9999 warriors who swear absolute fealty to him. Now a powerful Level 9999 Overlord himself, Light plans to ascend to the surface and take revenge on his betrayers one by one!

Individuals with Momotarou and Oni blood coursing through their veins maintain a longstanding rivalry. The Oni are careless, rash, and disorderly, while the Momotarou protect the human world from the uncontrollable Oni. Shiki Ichinose is a reckless and irresponsible teenager, aware of his defiance and even proud of his expulsion from school. Despite Shiki's rebellious attitude, his foster father, Tsuyoshi, cares about him deeply. Tsuyoshi's selfless love is soon shown in an explosive incident when a Momotarou barges in looking for Shiki. As a Momotarou himself, Tsuyoshi has been hiding Shiki for years and keeping his Oni origins a secret even from him. As he gives up his life to save Shiki and confesses everything in his dying breath, Shiki's Oni blood awakens for the first time, transforming him into an unruly monster overflowing with rage and a thirst for revenge. Shiki's momentary outburst attracts the attention of Naito Mudano, an Oni teacher at the Rakshasa Academy—an institution for young Oni in need of understanding and perfecting their powers. Before Shiki can avenge his father, he must learn to control his headstrong and impulsive nature and, above all, fight against his own demonic blood.

In the middle of a ball, Scarlet's fiancé, Kyle, suddenly calls off their engagement. She's falsely accused of being a bully and people unfairly call her a "Villainess." The aristocrats and noble families all denounce her. For years, she had to put up with his abuse and idiocy, but she can't take anymore of it! At her wit's end, she asks for one last favor; to give him a good fist in the face. So begins Scarlet's story of revenge against Kyle and his cronies! A fantasy about an elegant yet rebellious fighter, who doesn't let anyone take advantage of her!!

The story starts with Sanda, a second year middle school student, getting attacked with a kitchen knife by his classmate Fuyumura. While Sanda couldn't make sense of the situation at first, he gradually came to believe that Fuyumura's actions were just mean acts stemming off from the latter having a peculiar puberty phase. But that was all actually a misunderstanding. Fuyumura is seriously questioning if Sanda is actually the descendant of a certain family...