前缉毒警严谨在卧底贩毒集团调查陈年疑案时，与“似水流年”咖啡店老板季晓鸥相爱。一桩离奇的杀人案却将两人的爱情拖入深渊。真相迷雾重重，而严谨仿佛坠入无尽的黑暗，他和季晓鸥的爱情又将何去何从……Yan Jin, a businessman and former anti-narcotics policeman, decides to investigate a drug-trafficking case that caused the death of his close friend. He infiltrates into the organization as an undercover cop, and gets to know Ji Xiao Ou. This causes Zhan Yu, who has a one-sided crush on her, to be jealous. However, Zhan Yu was murdered and Yan Jin becomes primary suspect.