Six months after her younger sister, Rika, and her husband moved into the apartment managed by her uncle, her older sister, Itsuki, and her husband moved in next door. Rika had no complaints about her life with Itsuki, who she got along well with, but she had one worry. She felt the presence of a different woman in her husband's life, and because he had been by her side for so long, she knew he was betraying her. However, a strong stimulation was waiting for Rika, which would erase her gradually growing anxiety... The relationship between her husband and her older sister that her brother-in-law told her about... And then, Rika's body was embraced by him, and then...