​An abandoned mansion known as the Jailhouse sits on the outskirts of town, named for the giant walls surrounding it, making it look like a prison. Many rumors surround the mysterious building, including those of monsters living inside, people who venture in and never return, and even ghosts. But such tales are surely fiction. Or are they?

Shun, a shy transfer student at the local middle school, is a lonely kid who spends his time making a freeware game. He ends up at the Jailhouse after being bullied by a popular and seemingly nice student named Takurou whose father owns the building. As he enters the mansion with Takurou, Shun is joined by his friends, the brainy Hiroshi and Anna, the kindhearted class president. Takurou's lackey, Takeshi, and Takurou's girlfriend Mika also accompany them. But once inside, they are unable to leave, and they come face to face with the blue monsters inhabiting the Jailhouse. But even stranger is the fact that their experiences in the mansion play out just like Shun's game. Armed with this knowledge, can they survive their ordeal, or will they end up with a "game over" in real life?

[Written by MAL Rewrite]