Chapter five from volume nine of the Horror World of Junji Ito Collection, Hallucinations.
A terrified young woman runs through Oshikiri's house, afraid that he will find her there. Oshikiri recognizes her as his classmate, Mio Fuji. He demands to know why she's there so late, but Mio screams and runs from him. When he grabs her, she disappears into thin air.
The next day Mio is at school as if nothing happened. She is her normal happy self and seems totally different from the girl Oshikiri saw. Oshikiri concludes that his house contains a portal to other dimensions and the Mio he met last night must have been from a parallel universe. The present Mio has noticed him watching her; but Oshikiri denies it. A bully named Aoyama teases Oshikiri about his height, and the two get into a fight although Mio tries to intervene to stop it.
Back at home, Oshikiri wonders again about the alternate Mio he saw in his house and why she was so afraid of him. He suddenly hears Aoyama's voice. When he goes out into the hallway, he sees that Aoyama is a giant monster heading towards him. Aoyama melts into a puddle of slime, proving that what Oshikiri saw was real. However, when Oshikiri goes to school the following day, Aoyama is there as usual.
Oshikiri and Mio become close over time. Mio has heard from classmates that Oshikiri's house is like a magnificent western castle; and she asks him to show her around. Oshikiri takes Mio to his house but won't show her inside, and explains that it contains a portal to another dimension where his and Mio's doubles live. Mio believes that Oshikiri is making this up to keep people from getting too close to him. She encourages him to let her in (to both his heart and the house.) Oshikiri agrees that what he saw might have been hallucinations - but then sees that Mio is missing.
Mio doesn't return to school for three days. Oshikiri believes she has become trapped in the parallel dimension. He hears her voice and goes out into the hallway, where Mio has become a gigantic blob monster. She says that Oshikiri did this to her, and vows to curse him, before dissolving away into slime like Aoyama. Oshikiri realizes that the parallel dimension is in a state of chaos and it's his counterpart that's causing it.
Suddenly, Oshikiri finds himself on a different floor of the house. There, his counterpart has tied up Mio and is experimenting on her with a syringe. He states that he's tried his drugs before on both her and Aoyama, but the experiments ended disastrously. Mio and Oshikiri are now trapped in this other Oshikiri's world. Both versions of Oshikiri fight over the syringe, and one gets injected, causing his bones and muscles to tear apart and extend.
Oshikiri frees Mio and explains that he is the "real" Oshikiri she knew. He had taken the syringe from his counterpart and injected him. The alternate Oshikiri is dying, but cackles to them that there are an infinite number of alternate dimensions to be accessed, all of which contain an Oshikiri even worse than him. He dissolves into slime like all his other "experiments." Oshikiri and Mio wonder how they're going to return to their own world.