Kun (Jang Dong-gun) is a skilled cold-blooded killer who works for a gang and is trusted by his superiors. However, a past that he cannot bear to look back on makes him live in guilt and doubts and shakes the industry he has been engaged in. At this time, the organization assigns him a very important task and orders him to go to South Korea to assassinate a woman named Choi Mok-kyung (Kim Min-hee). Choi Mok-kyung is a tragic woman. An accident took the lives of her husband and children. She lost the love of her life. This sad wife and mother is immersed in despair all day long and can only rely on alcohol and drugs to get by. Not only that, she also has to take care of her mother who suffers from Alzheimer's disease. When Kun appears in front of Choi Mok-kyung, his identity is not just a killer who wants to take her life. The death of her daughter makes Choi Mok-kyung's heart full of questions, and the answer is in Kun's hands.