The elder brother Kenji and his younger sister-in-law Yuki live alone in their parents' often empty house. Yuki has a secret crush on Kenji, but their relationship becomes complicated when a ghostly figure named Hiyori appears in Kenji's room one night. Despite her transparent appearance, Hiyori is a childhood friend of Kenji and Yuki, and they are happy to see her again. However, as Kenji and Hiyori become closer, Yuki's painful memories of their childhood resurface. Specifically, she remembers the promise she made to exchange her yoyo for Kenji's toy ring, which was never fulfilled due to Hiyori's illness and subsequent move. As the three navigate their complicated relationships, a painful love story unfolds.