Girls of the Thirteenth Month.

Kazuho Anamori, ​​who had difficulty fitting in with her class, meets Miko Karakida, who suffers from heart disease, and becomes close friends with her, however, six months later, Miko dies while hospitalized. Kazuho receives a letter saying that there is something to discuss about Miko. When she goes to the address she has been called to, she finds a mysterious transfer student, Rio Ukima, waiting for her.