While testing a new VRMMO, a young man named Reito Kanzaki finds himself trapped inside the game world for three harrowing years. He fights endlessly, eventually defeats the final boss, and finally manages to log out, expecting to return to his normal life. However, he immediately realizes something is terribly wrong. The reality he returns to is not the one he remembers. This world is subtly twisted by supernatural forces, where monsters now roam and a mysterious girl claiming to be his little sister suddenly lives with him. Most shockingly, Reito discovers that his character status and overwhelming abilities from the death game have carried over into the real world, granting him powers far beyond any normal human.

Now possessing the title of Demon Defeater, Reito enrolls in a school with an adventure department, where students are trained to combat the demons that have begun appearing in reality. He is joined by new partners including Yukiori and Koi Wing as he investigates the mystery of why his game world has bled into reality. His primary motivation is to find a way to rescue the friends he left behind, those who could not escape from the Astral Border game world when he did.

The story follows several key narrative arcs. Reito joins his schools inter-school exchange competition, but a routine match inside a special zone turns deadly when a real demon suddenly appears. This demon has a singular goal: to defeat Reito Kanzaki. The situation escalates dramatically during another inter-school exchange game when a demonic being manifests in the singular area. Reito is horrified to recognize the attacker as Iori, a companion he once partied with inside the death game. He is forced to fight against his will to subdue her, believing she is being manipulated by an unknown enemy. This confrontation reveals the existence of a mysterious factor, a specific trait apparently shared by Reito, Iori, and possibly others like Souma and Mia, which makes them targets for these attacks. This investigation forces Reito and his friends to confront a new enemy in a world where the relationship with the old Astral Border is slowly coming into focus.