S became a detective to answer for what happened to his father R—a police officer killed by a serial killer ten years ago, a case the police were never able to close. Working a new investigation, S discovers a carved wooden button with a snake on it that matches something found in R's belongings. Following it leads somewhere unexpected: across five years of similar unsolved murders, the same buttons keep appearing. Now S and the police are hunting an escaped criminal, and what began as a personal quest to settle the past is converging with something very much in the present. Detective S is a mystery visual novel about inherited unfinished business.