Her Affair (2018)

Original title: 그녀의 정사

Young-mi is having a hard time because of her boyfriend Tae-sung who always tries to sleep with her when they meet. One day, Tae-sung suggests an interesting game and sleeps with a woman who approaches him and keeps it a secret from Young-mi. When they are together, Young-mi happens to see a text message from 'this woman' to Tae-sung. Then, she starts doing something unexpected...