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.