After separating from her husband, mother Ryo raised her only son Makoto on her own. Taking her ex-husband's past financial struggles as a cautionary lesson, she took Makoto's future seriously and imposed an education regime so strict it could be called excessive. However, as Makoto reaches adolescence, his focus on studies begins to wane, and he becomes increasingly driven by sexual desire. Worried about her son's future, Ryo—knowing full well it's taboo—decides to relieve his urges through oral sex. But this act awakens her own long-suppressed sexual desires. What begins as sex education gradually escalates, progressing from lessons about female anatomy and the feel of a woman's genitals to full intercourse, giving rise to a new, forbidden relationship between mother and son.