数年ぶりに田舎の実家へ帰省した奏。兄嫁・さららは昔と変わらず地味で優しく兄を献身的に支える理想の妻だった。だが真夏の夜、奏はクーラーの壊れた部屋で汗だくになりながらオナニーで快感に溺れるさららの姿を目撃してしまう。実は真面目な兄嫁の裏の顔は、欲望を抑えきれない変態痴女だった。過激なオナニー現場を見られたさららは、旦那が眠るすぐ傍で義弟を誘惑し若い肉棒にむさぼりつく。そこから、2人は誰にも言えない背徳関係が始まる…。

Kanade returns to his hometown in the countryside for the first time in several years. His sister-in-law, Sarara, is still the same unassuming, kind, and devoted wife he knew, the ideal wife. But one summer night, Kanade witnesses Sarara drenched in sweat, masturbating in a room with a broken air conditioner, and indulging in pleasure. The seemingly serious sister-in-law's true nature is that of a perverted nymphomaniac who cannot control her desires. Having been caught in the act of extreme masturbation, Sarara seduces her brother-in-law right beside her sleeping husband and devours his young penis. From there, a forbidden relationship begins between the two, a secret they must keep from everyone…