Mansei hatsujô furin-zuma.

Saeki Shingo and Yuri are siblings who lost their parents at an early age and live together. However, Shingo gets engaged to his girlfriend Keiko, and the three of them start living together. Although this new life should be full of happiness, Keiko has some worries. She is having an affair with her boss, Tsuchiya. Moreover, Keiko, who had been turned into a masochist by Tsuchiya, worries that she may no longer be able to have normal sex. Meanwhile, Yuri is completely addicted to lesbian play with her girlfriend Miyuki...