The Devil You Know.

Yu-jeong is a star English tutor, popular nationwide for her lessons and beauty. She has the perfect life going on- her education business is blooming, she has a successful doctor husband and a loving daughter. One day, she meets Sun-hee from a volunteer group. He seems like a friendly guy with charms. Gang-su, Yu-Jeong's childhood friend who is like a little brother to her, however warns her about Sun-hee that he has a bad reputation even among gangsters. But what Yu-jeong doesn't realize is that Sun-hee is a narcissistic sociopath- until it's too late. Meanwhile, Sun-hee's gang brothers steal money behind his back, and Sun-hee's old time friend approaches him to start a new drug business. Yu-jeong fights tooth and nails to break free from Sun-hee and enlists Gang-su's help- and everything turns deadly when it all reaches a boiling point.