Yurine Tsukino, widowed at a young age, lives with her daughter and son-in-law, Junki. Leading a quiet, ordinary life, Yurine cherishes the close bond they share as a family, happy to see her daughter and Junki getting along well. However, one day, she accidentally witnesses an argument between them about sex. Realizing her daughter feels uncomfortable with intimacy and cannot satisfy Junki's desires, Yurine grows deeply concerned. When her attempts to talk with her daughter fail, Yurine stumbles upon Junki masturbating alone. Overcome by shock and emotion, she takes an unexpected action—feeling responsible as both a mother and mother-in-law. What begins with a handjob soon escalates to a blowjob, and ultimately, full intercourse. Though Junki cannot resist, Yurine struggles desperately against her own desires. In the end, their family ties are irrevocably transformed.