Kim Sun-woo (Lee Byung-hun) is a high-ranking 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 Dae-sik (Hwang Jung-min), a son from a rival family, and whose men had been beaten up by Sun-woo for overstaying their welcome at their nightclub. Kang, preparing to leave on a business trip, assigns Sun-woo to shadow his young mistress Hee-soo (Shin Min-ah), whom he fears is having an "affair" with another man. As he performs his duty—following Hee-soo, and escorting her to a music recital one day—he becomes quietly enthralled by the girl's beauty as glimpses into his lonely, empty personal life become prevalent. When he does come to discover Hee-soo and her lover at her home, he fiercely beats him and prepares to inform Kang, but his attraction to her causes him to hesitate. He thus spares the two on the condition that they no longer see each other again, earning him Hee-soo's enmity.