Hikaru, the older sister, devises a plan to take her younger brother Masaru's virginity. Masaru, in the throes of puberty, finds himself increasingly drawn to her. Hikaru teases him by going braless, and unable to resist, Masaru attempts to touch her breasts while she pretends to sleep. But this is exactly what Hikaru anticipated—it's all part of her trap. The next day, she pretends to be angry about the incident, but offers to forgive him under one condition: she blindfolds him and guides his hand to unzip her pants, escalating the forbidden tension between them.