Do-young, a student in the security studies department, receives a gun along with a mission from a mysterious man at a place he visited through an introduction from a senior at school. The mission is to personally protect Cheong-yu until three months later when she returns to the United States after coming back to Korea. Do-young becomes intrigued by the look in Cheong-yu's eyes in the photo shown by the man. After much contemplation, he decides to take on the job, risking the possibility of being involved in a dark world. During this time, Do-young learns that Cheong-yu came to Korea to uncover the secrets surrounding the suspicious deaths of his mother and brother. He senses that he is stepping into increasingly dangerous territory, yet he finds himself unable to leave Cheong-yu's side.