A man was killed in his apartment. During the investigation, the police found that the deceased was a wealthy man in the sports center. The only clue at the scene was the hair of an AB type man. During the investigation, the first suspect was a bar girl because she had called the deceased before the incident. Police officer Zhao Jingyin (Kim Kang-woo Kim) went to investigate and take samples. However, Jingyin learned from his colleagues that the situation had changed. Everyone began to suspect that the deceased and the swimming center coach were gay. But then, the suspect was also killed. The police began to suspect that the serial murders were related to the deceased's experience in military service. Police officer Park Eun-joo (Min-sun Kim), who investigated with Jingyin, was his classmate in the police academy. Although he had a tomboy style, he longed to be cared for in his heart, and his secret crush was Jingyin. However, Jingyin faced a more complicated emotional choice. He had a fixed girlfriend and a lover who was still connected. As he unraveled the mystery, Jingyin discovered that this case hid a childhood memory...