Former professional gamer Himemori Aori has spent countless hours playing online with his gaming team, the Jaguars, alongside three close friends he has never met in person. The fourth member of their group, a player known only as Bell, is the one Aori feels the strongest connection to. When the Jaguars finally arrange their first offline meetup, only three people show up. Bell is not from this world. She is a princess from a fantasy realm, and she has opened a one-way passage to her world for Aori alone.

Aori steps through to a land fractured by conflict between three races: humans, the魔族 (demon race), and the獣人族 (beastmen). Bell, who can generate powerful weapons from thin air, is being hunted by forces that want to use her power. Aori decides that the only way to protect her is to claim the title of Demon King himself, using his gaming instincts and reflexes to survive in a world where death is permanent. He wields the weapons Bell creates for him and fights against demon generals, beastman armies, and ultimately the only other human reincarnated from Earth: the hero.

The story unfolds across three main arcs. In the first, Aori arrives in the other world, reunites with Bell, and establishes himself as the Demon King by defeating the demon army's leadership. The second arc sends Aori and his allies to the Beast King's castle to negotiate or fight for the hero's location. The negotiations fail, the castle becomes a battlefield, and the demon army's commander Lupus, burning for revenge over his father's death, executes a plan that deepens the war. In the final arc, the group learns the hero's location from the Beast King Jilba and marches to the southern edge of beastman territory, a place called Saimi. There they face the hero, whose overwhelming power nearly destroys them one by one. Aori must find the one sliver of hope that can turn the tide.

The central relationship is between Aori and Bell. Bell is small enough to fit into a pocket, and her bond with Aori grows from online friendship into something deeper as they fight side by side. The other members of the Jaguars also play critical roles, and the trust built through years of gaming together proves as strong as any blood oath when they finally meet face to face in a world at war.