During the aerial period of the Republic of China, there was a metropolis called Decheng. Contrary to its name, it is a veritable "Sin City" with moral decay and frequent crimes. Rogue prosecutor Bao, noble detective Gong and bookworm favorite Zhan Chao () of Decheng Criminal Investigation Department formed an "iron triangle" to fight crime, and another playboy detective and the most handsome policewoman came to help. Detectives joined forces to form an alliance and cracked a series of bizarre cases, which became the light of hope that pierced the darkness.