One night, a son and daughter accidentally witness their parents' intimate moment. Afterward, the daughter, Mari, asks her father, "Dad, is sex really enjoyable?" This marks the beginning of the Shoda family's twisted sex education. In the bathroom, the mother teaches the children how to give blowjobs in disturbing lessons. In the kids' bedroom, the mother privately gives sex lessons to her son, while the father does the same with his daughter. Under the watchful eyes of their parents, the brother and sister then practice what they've learned. The scene becomes a disturbing symbol of both family bonds and dysfunction.