Akasaka is returning to Japan after spending a year working overseas with his company. His brother, Seiji, has been looking after his house in his absence, but since Akasaka returned earlier than expected, he is not there to let him in. To Akasaka's dismay, Seiji had been pressured to rent out his lovely new house while he was away. He gets angry on finding that the tenants' keys have been left in the keyhole, his dog has run away, and the electricity is cut off. When he goes into the house, the kitchen and bathroom are infested so badly with mold that Akasaka vomits. Seiji denies all knowledge and Akasaka is too tired to argue, so he cleans up the mold and goes to bed.
While Akasaka was away, the house had been rented out to Mr. Rogi, his former high school science teacher and an employee of Akasaka's father, who was the vice principal. Rogi had conducted experiments with mold and at one point punished Akasaka for breaking the shelf that held his specimens, even though Akasaka didn't do it. He had insisted on renting the house while Akasaka was away, claiming that his own family's home had been destroyed in a fire. Akasaka believes that Rogi deliberately trashed his house because he holds a grudge against him. He remembers that Rogi's family were very strange: his dog growled at the sight of them, their baby constantly screamed, and even their young children looked old and decaying.
When Akasaka awakens the next morning, he sees that the entire structure of the house has been overtaken by mold, despite being less than two years old. Seiji says that the Rogis just disappeared one day; he tried to look for them, but they haven't returned to their former home. The mold continues to worsen, completely consuming the house. Having never seen anything like it, Akasaka believes that it's a new breed of fungus developed by Mr. Rogi. Seiji begs Akasaka to move out at once, causing Akasaka to suspect that Seiji knows something. Under pressure, Seiji admits that he came over to the house one day and saw that both the Rogis' children were completely covered in the same mold that is infesting the house. It appeared to be growing on them and trailed after them wherever they went. He again urges Akasaka to get out of the house as soon as possible, but quickly leaves when Akasaka asks to stay with him; Akasaka guesses that his brother doesn't want to risk being infected.
As Akasaka's house continues to decay, the mold develops strange, tube-like structures that leak black liquid. Akasaka himself becomes increasingly lethargic, with no energy to leave the house or do anything else. He tries to find out where the tubes are coming from, tracing the source to a door that he couldn't open because of the mold. When he forces the door open, he finds the hideously mutated corpses of the Rogi family; the mold and tubes are growing from their remains. Akasaka idly scratches his face and tears the skin, unwittingly allowing traces of the mold left on him to enter into his body.
Eventually Akasaka falls victim to the mold's infection, causing his body to slowly disintegrate like the Rogi family's. He sits alone in his crumbling house, repeating "itchy ... itchy ... itchy ..." to himself as he scratches the skin from his face.