In the opening scene, an unnamed highly skilled and trained assassin enters a hallway and kills numerous people with her gun and knives before being surrounded by cops and smiling a grim smile. The assassin is shown in a room in an unnamed facility. She is drugged and is given plastic surgery. While recuperating, she has a flashback showing an unnamed man with a mustache trying to resuscitate her after a breath holding exercise. He is calling her Sook-hee.
The facility turns out to be part of South Korea’s intelligence agency and is run by a woman named Kwon-sook. Kwon-sook tells Sook-hee that to give her a new start the agency has faked her death. Kwon-sook tells her they have given her a new identity and name, Chae Yeon-soo. Yeon-soo says she doesn’t care and wants to die. Kwon-sook tells Yeon-soo she is pregnant. Kwon-sook offers her a deal: train with her as an agent, work as an agent, and she will have freedom after 10 years of service. Yeon-soo accepts and while in training, gives birth to a daughter, Eun-hye.
Still at the facility, Yeon-soo is having a procedure to remove a tattoo from her shoulder and while this is happening, she has a flashback to the unnamed man with a mustache giving her the shoulder tattoo when she was 20-year-old Sook-hee. Another flashback occurs, and we again see 20-year-old Sook-hee as a highly skilled and trained assassin trying to kill an unnamed man with yellow teeth. Sook-hee is captured and, while beating her, the unnamed man with yellow teeth tells her he did not kill her father. Eventually, the unnamed man with a mustache arrives, shoots the man with yellow teeth, and frees Sook-hee. The flashback continues, and we see Sook-hee as an 8-year-old being trained to load a pistol by the unnamed man with a mustache. The man tells the young Sook-hee to devote her life as an assassin to him.