HBAD-721 - My sister, Mochizuki Fuka, who appears innocent and uninterested in sex, but is actually sullen, lewd, and has an erotic body.

Fuka, who is always quiet and dresses modestly. She loves explicit things but is too shy to tell her boyfriend. Fuka spends every day masturbating alone, but one day, a visiting relative happens to see her masturbating. Unaware of this, Fuka falls asleep exhausted from masturbation...