Prostitution.

Soo-hyeon and Hye-ji are sitting on a pile of debt thanks to their husband and father who ran away in the middle of the night. Tae-san the loan shark demands the money no matter what. He rapes the mother and daughter and requires them to join prostitution. They survive day by day selling themselves until one day, Tae-san suggests a way they can clean slate over one night and that is for the mother and daughter to have the most extreme sex with one man.