Detective S

S became a detective to avenge his father "R". He was a police killed by a serial killer 10 years ago. In the end, neither could the police uncover the case. "S" feels the incompetence of the police. S became a famous detective later, and while she was investigating on a case, she discovers a “wooden button” with a snake carved on it that looked like what was found in R's belongings. S hears unexpected news while investigating on a case centered on the “wooden button,” a clue that she discovered in five years. For five years, 'tree buttons' still remain in many places in unsolved murders... Mr. S goes out with the police to find the escaped criminal.