Dangerous Cohabitation.

Woo-seong, a freelance translator, relieves stress by having sex with his sex partner, Ji-yeong, whenever things get stuck at work. Ji-yeong provides Woo-seong with a new type of sex every time. One day, Woo-seong meets his college senior, Cheol-min, and is invited to come to his house. He learns there that his former lover, Hye-ran, is now his senior's wife. Woo-seong awkwardly calls Hye-ran her sister-in-law, and together with his current girlfriend, Mi-jin, the two couples drink and have a good time. The following day, Cheol-min leaves for work. That evening, Woo-seong and Hye-ran resolved a misunderstanding from their past and ended up having sex. However, they are caught by Mi-jin, so she decides to seduce Cheol-min as revenge. From that day on, the four met occasionally and enjoyed swapping partners.