A brother and his younger sister-in-law reside in a house where the parents are frequently absent. Yuki, the younger sister, secretly yearns for Kenji, the elder brother. However, after a certain night, a girl emerges from the closet in Kenji's room. Furthermore, her body is transparent, and she resembles a ghost. Her name is Hiyori. She is a friend from their childhood. Kenji and Yuki are pleased with the reunion, although they are perplexed by the strange manner of Hiyori's appearance. However, Yuki's mind is shaken as Kenji and Hiyori become intimate, and the pain she felt in childhood is recalled. In childhood, Yuki was going to pass the light blue toy ring that Kenji had possessed to Hiyori in exchange for Hiyori's yoyo. However, because Hiyori became sick and moved away, the promise was not fulfilled. Their events from childhood seem to be related to Hiyori's appearance before them. A painful love story involving Kenji and two girls progresses.