Risa's friend Shuichi Makita brings her home to her parents. He claims to have found her wandering in the street, having lost all her memories. The doctor thinks that she must have suffered from some kind of serious psychological trauma. When she is in bed, a giant caterpillar-like monster attacks her; but it's gone by the time her parents arrive. She isn't sure whether what she saw was real, and worries that the caterpillar will come back for her.
Shuichi takes her to his house, where she meets his very ill father. She discovers that she had actually promised to marry Shuichi, and his father is very adamant that the two must get married. Risa's growing sense of dread becomes worse after her visit to Shuichi's house.
Shuichi comes over to see Risa, saying that his father is dying and has been asking for her. They run over to Shuichi's house. She is horrified to see that Mr. Makita has a long line of skulls coming out of his head. Shuichi explains that the shock of seeing this for the first time had given Risa amnesia, and her mind had interpreted the image as a caterpillar, causing her to have nightmares. She didn't see the skulls on her previous visit because Mr. Makita wheeled himself in on a trolley, with the skulls hidden behind the door.
The skulls are the heads of Mr. Makita's ancestors attached to his own. Through him, they are able to continue thinking and feeling; but the strain of keeping them alive is killing him. Shuichi now wants Risa to have a baby with him so that he can have a child to continue the family line and thus, his ancestors’ knowledge. Risa suddenly regains her memories, but refuses to marry Shuichi and tries to run.
Shuichi pursues her but stops in pain and starts clutching his head. Risa watches as all the skulls, now including his father's, attach themselves to him. They encourage him to grab her because there is no other way he can ever find a wife. Risa runs but the shock becomes too much for her and she completely loses her mind. As she reverts to a catatonic state, the ancestors think that she and Shuichi married, and the family is safe for another generation.