An abandoned mansion called the Jailhouse sits on the town's outskirts, named for the massive walls that enclose it, giving it the appearance of a prison. Numerous rumors surround this mysterious structure, including stories of monsters dwelling within, people who enter and never come back, and even ghosts. Yet such tales are surely fiction. Or are they? Shun, a shy transfer student at the local middle school, is a lonely boy who spends his time developing a freeware game. He ends up at the Jailhouse after being bullied by Takurou, a popular student who appears nice but whose father owns the building. As Shun enters the mansion with Takurou, he is joined by his friends: the intelligent Hiroshi and Anna, the kindhearted class president. Takurou's lackey, Takeshi, and Takurou's girlfriend, Mika, also accompany them. However, once inside, they find themselves unable to leave and come face to face with the blue monsters inhabiting the Jailhouse. Even stranger is the fact that their experiences in the mansion unfold exactly 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?