A boy named Yuya Tenjo has known nothing but abuse and contempt for his entire life. Brutally bullied by his classmates for his appearance and weight, and neglected by his own parents, his only source of kindness was his grandfather. When his grandfather dies, Yuya is left completely alone, forced to live in the old man's house after being cast out by his family. At his lowest point, he discovers a mysterious door in his late grandfather's home. The door leads to a fantastical other world, an abandoned house that once belonged to a great sage. Inside, Yuya finds a treasure trove of legendary weapons, magical artifacts, and crops with extraordinary effects.

Stepping into this new world triggers a transformation. As he defeats monsters in the forest of Argena, he gains levels and skills as if in a video game, growing taller, stronger, and more handsome. This cheat-like ability works both ways; any skill or item he acquires in the fantasy realm can be brought back to his original world. As Yuya grows more powerful and confident, his life on Earth changes just as dramatically. His new physique and quiet confidence attract attention, and he is soon scouted by the prestigious Ousei Academy, where he befriends the kind and gentle Kaori Hojo, the school's "princess" and the daughter of the academy's chairman.

Yuya's life becomes a balancing act between two worlds. In the other world, he saves a princess named Lexia von Arselia from a monster, earning the lifelong affection of the determined half-elf royal. He also encounters Luna, a silver-haired assassin originally sent to kill Princess Lexia but who ultimately joins their group as a bodyguard after being defeated and forgiven. Yuya adopts two powerful familiars: a Black Fenrir puppy named Night and a healing boar named Akatsuki. While he navigates political intrigue in the Arselian Kingdom and grows closer to its princess, back on Earth he finally confronts the bullies who once tormented him, becoming the most popular and admired student at his new school.

The story follows Yuya as he unlocks the secrets of the sage's legacy, defeats powerful divine beasts, and eventually draws the attention of gods and otherworldly invaders. Notable arcs include his enrollment at Ousei Academy, where his social standing is completely reversed; his rescue of Princess Lexia and the subsequent recruitment of Luna; and the revelation of a grander cosmic conflict involving the Sage, the Genesis Dragon Oma, and the battle against the Evils that threaten both Earth and the fantasy world. As Yuya struggles to maintain his humble nature while wielding godlike power, he ultimately learns to accept the love and friendship he was denied for so long, becoming unrivaled in both the real world and the other.

The anime adaptation, produced by Millepensee, aired 13 episodes from April to June 2023. A new TV special has been announced, scheduled to broadcast at the end of March 2026.