The brother is captivated by his tsundere younger sister's accidental panty shots and secretly watches her having sex with her boyfriend. He discovers her hidden masochistic side as she gets choked by her boyfriend during intercourse. Later, she pleads with him in the bathroom, "Onii-chan, wash me," leading to a secret rendezvous in his room to avoid getting caught by the family. Memories resurface of their childhood habit of touching each other, reigniting their forbidden intimacy as they indulge in sex once again.