Misaki Sonoka, a younger sister who awakens to pleasure as her breasts are used as a masturbation device by her otaku brother (FNS-229).

"He used to be so gentle..." Sonoka had always supported her decadent, reclusive older brother. However, one day while delivering food, she was forcibly dragged into his room, her G-cup breasts becoming a sacrifice to his desires. Ignoring her screams, he frantically kneaded her breasts, ejaculating on her chest. Despite this, to protect her brother's only refuge, she chose to live as his "breast masturbation aid." In the days of repeated internal ejaculation and breast sex, her brother, craving her breasts, began to leave the room. Although the situation was distorted, it was her brother's hope for survival. She gave up resisting, surrendering herself to his sexual demands. This is a sad yet lewd story of sibling symbiotic dependence.