Directed by Lim Kyeong-taek, No Mercy follows In-ae, who was falsely charged, is released from prison. Eun-hye, her younger sister, is an 18-year-old high school student with the mental age of a ten-year-old. She adores her sister, who means the world to her. They get together for a small reunion party, and hope for nothing but happiness for them in the future.
Next day, however, Eun-hye doesn’t return from school. In-ae reports her missing to the police and asks her teacher for help, but no one seems to care about Eun-hye’s whereabouts. Devastated by this tragedy, and by the society that doesn’t care for the weak, In-ae embarks on an investigation all by herself.
She finds out that her sister suffered violence and sexual abuse from school bullies. Not only that, Eun-hye has been passed on from a loan shark, to a massage parlor, then to someone else. In-ae is hot on the trail and will not give up her quest to bring back Eun-hye, but the path leads her to even more shocking secrets from her past: the owners of a convenience store, photo studio, and car repair shop all raped her a few years ago.
Now, it’s payback time! To console her sister, to find her, and to survive… In-ae’s brutal revenge begins.