Sayuri lives with her daughter and son-in-law in a two-family home. Alongside her work as a full-time housewife, she occasionally models swimsuits, introduced by an acquaintance. One day, she finds her daughter-in-law Kunio masturbating with the swimsuit her girlfriend used... Apparently, their marriage is not going well because he has a huge penis. However, Sayuri is captivated by his enormous penis, and tells herself that it's for her daughter's sake, and relentlessly seduces him day and night. Kunio, who has been put off for days by her lewd behavior, cannot resist her seductive behavior...