柴崎はるは結婚から数か月後、義祖母の体調不良を機に夫の故郷である寒村へ移住する。独特なルールに戸惑いつつも村に馴染もうと努力するはるだったが、夫婦の引越し祝いの宴会の夜、夫の先輩夫婦におそわれてしまう。それをきっかけに、はるは村に代々伝わる恐ろしい「淫習」の存在を知ることになり――。

A few months after her marriage, Haru Shibasaki moves to her husband's hometown, a remote village, due to her grandmother-in-law's ill health. While bewildered by the village's unique rules, Haru tries to adapt to the village life. However, on the night of the housewarming party, she is attacked by her husband's senior couple. This incident leads Haru to discover a terrifying "sexual custom" that has been passed down through generations in the village.