Kim Sun-woo (Lee Byung-hun) is a high ranking mobster and enforcer for Kang (Kim Yeong-cheol), a cold and calculating crime boss to whom he is unquestionably loyal. The two share concerns over business tensions with Baek Jr. (Hwang Jung-min), a son from a rival family, which is when Kang assigns Sun-Woo what is perceived to be a simple errand while he is away on a business trip — to shadow his young mistress, Heesoo (Shin Min-ah), whom he fears is having an "affair" with another man, giving Sun-Woo the mandate to kill her (and her paramour) if he manages to discover it. As he performs his duty — following Heesoo, and escorting her to a music recital one day — he becomes quietly enthralled by the girl's beauty and innocence as glimpses into his lonely, empty personal life become more prevalent. When he does come to discover Heesoo's lover directly in her home, he fiercely beats him and prepares to inform Kang, but his attraction for her causes him to hesitate. He thus spares the two on the condition that they no longer see each other again, earning him Heesoo's enmity.