Years ago, the gods fought against each other before being sealed away. However their artifacts remained in the souls of the HuaXia clan as part of a contract and hope that someday in the future, they will awaken once more using those artifacts. (Source: Baka-Updates Manga)

After the climactic battle to decide the fate of humanity, the hero Max, alongside his party, emerges victorious against the formidable Demon Lord. As he draws his last breath, the defeated Maou vows to return more powerful than ever to take his revenge. Ten years later, although Maou has resurrected, the process is incomplete, resulting in him taking on the form of a child. Nevertheless, not wanting to waste another second, Maou pays his nemesis a visit—but the hero who once gallantly protected humankind from the demon's clutches is now a pitiful slob bearing no resemblance to a man of his former caliber. Despite his disappointment, Maou moves in with Max in hope of building him back up into a worthy adversary. But as the demon learns more about the events that transpired in the decade of his absence, he realizes that there is a story behind Max's downfall that he knows nothing of.

Three months have passed since the Demon King was defeated by a hero. Mankind rejoices over their victory, but dwellers of the demon world do not share the same sentiment. With the loss of their king, they must now hold a tournament to decide who will become his successor. Monitoring the competition is the hot-headed yet powerful Vamirio—one of the Four Heavenly Kings. Among all the participants, she is extremely distrustful of one: Helck, a human hero who claims to hate humanity. As much as she wants to get rid of him, it is impossible due to Helck's incredible strength and popularity. Amidst the competition, Vamirio discovers that humans have vanished from various towns. As a result of this puzzling turn of events, Vamirio grows desperate to reveal Helck's true intentions by carefully crafting a plan to make Helck lose. If she proves to be successful, she might uncover where his loyalty truly lies before the demon world is endangered.

After his high school graduation, Yuuki Wakura realizes it has been five years since his sister's disappearance during a "Mato disaster." Throughout Japan, mysterious entrances have opened to a dimension called "Mato"—a demonic metropolis filled with disastrous monsters known as the "Shuuki" that threaten the safety of humans. However, a new hope arose, as the discovery of a Mato fruit that grants supernatural abilities exclusively to women allowed them to fight against the Shuuki. Due to this, a new matriarchal government was formed, establishing a female organization called the Anti-Demon Corps, which caused men to fall to the bottom of society. When Yuuki suddenly finds himself in Mato, he is unable to escape on his own. But just as he anticipates a gruesome end by the Shuuki, a girl in a militia outfit appears and protects him. As the chief of the Seventh Unit of the Anti-Demon Corps, Kyouka Uzen is ambitious yet devalued by her own peers due to her power being deemed unusual. Now in a life-or-death situation, Kyouka takes the opportunity to find out the true potential of her superpower by using Yuuki: in order to defeat the Shuuki, he has to become her slave.