Nagisa Himawari, the younger sister, devises a plan to claim her older brother Hiroyoshi's virginity. Hiroyoshi, who has never been involved with a woman, gradually becomes aware of Himawari's advances. Always going braless, Himawari constantly tempts him until Hiroyoshi can no longer resist—sneaking a touch of her breasts while she pretends to sleep. But this is actually a trap set by Himawari. The next day, she feigns anger over the incident, yet offers forgiveness on one condition: she blindfolds him and guides his hand to unzip her pants. This delicate, entangled relationship between brother and sister spirals into an intense, emotionally charged episode.