Akasaka is returning to Japan after spending a year overseas with his company. He has returned earlier than expected so his brother, Seiji, who has been looking after Akasaka's house in his absence is not there to let him in. Akasaka is upset because he 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. But 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; 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 Rogi's specimens, even though Akasaka didn't do it. Akasaka believes that Rogi holds a grudge against him and decided to trash the house. Rogi had insisted on renting the house while Akasaka was away, and claimed that his own family's home had been destroyed in a fire. Akasaka remembers that Rogi's family were very strange: Akasaka's dog growled at the sight of them, their baby constantly screamed and they all, including the Rogis' young children, looked old and decaying.
When Akasaka wakes the next morning, he sees that the entire structure of the house has been overtaken by mold even though it was a new house that was less than two years old. Seiji says that the Rogis just disappeared one day and even though he's tried to look for them, they haven't returned to their former home. The mold continues to get worse and worse and looks like nothing Akasaka has ever seen before. It has completely consumed the house. Akasaka believes that it's a parasitic new breed of fungus that was developed by Mr. Rogi. Seiji pleads Akasaka to move out at once and Akasaka instantly suspects 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's house continues to decay and the mold develops into strange, tube-like structures. Akasaka becomes lethargic and has no energy to leave the house or do anything else. Black liquid drips from the tubes. He tries to find out where the tubes are coming from and traces the source to one of the rooms in the house that he couldn't open because of the mold. When he forces the door, he finds the hideously mutated corpses of the Rogi family; the mold and tubes are growing from their remains. Akasaka scratches his face and tears the skin, unwittingly allowing traces of the mold left on him to enter into his body.
Akasaka is eventually taken over by the mold and sits alone as his body disintegrates like the Rogis'; repeating "itchy ... itchy ... itchy ..." to himself as he tears the skin from his face.