On June 6, 2031, a class of sixth graders is invited to participate in the playtesting of Actual Magic, the world's first full-dive virtual reality MMORPG, at a facility called Altea. The protagonist, Yuuma Ashihara, is initially amazed by the game's realistic fantasy world. However, after the testers attempt to log out, the boundary between the virtual world and reality begins to break down, causing the game to fuse with the real world. This catastrophic event plunges the city into chaos, most terrifyingly transforming Yuuma’s once-popular classmate, Sumika Watamaki, into a horrifying monster.

The story follows Yuuma and his twin sister, Sawa Ashihara, as they fight to survive in this new, dangerous reality where the rules of Actual Magic now apply to the physical world. Death within the game's context now means death in real life. Trapped with their classmates in a sealed-off, dimly lit building, the twins discover that Sawa has been possessed by a powerful entity known as the demon Valac. Following Valac's guidance, Yuuma must navigate this blended world to rescue his missing friends, confront rampaging beasts, and uncover the truth behind the catastrophic fusion before it claims everyone he knows.

The main cast includes the logical and level-headed Yuuma Ashihara and his dependable yet sharp-tongued twin sister Sawa, who harbors a secret connected to the game. Their classmates include the kind-hearted Nagi and the transformed Sumika, whose monstrous change serves as the inciting incident for the group's struggle. The narrative explores themes of survival as elementary school children are forced to confront life-or-death stakes far beyond their years, forming a desperate party to fight back against the invasion of their reality by the game world.