The Great Jing Dynasty enacted the Peaceful Demons Law. Demons who have gained intelligence and do not kill indiscriminately can register as citizens, pay taxes, and live alongside humans in the same cities. Su Mo, a clerk in Qinghe County, has reached the third layer of Qi Guidance but cannot defeat a cultivator. His job is to register demons and settle disputes between humans and demons. He carries a yellow census book listing 372 demon households, a copy of the Peaceful Demons Law that even the immortal sects must bow to, and an ancient mirror hidden between his brows that reveals the attachments and obsessions of the heart. He has always thought the strange visions that began after he nearly drowned at twelve were just "seeing things." Only later does he learn the mirror is called the Heart-Illuminating Mirror. When the immortal sects come to seize a demon's core, he slaps the law down on the table. When demons break the law, he punishes them as the Peaceful Demons Law requires. Why does the rat demon pretend to be dead? Why does the spider spirit weave until its oil runs dry? And who is the old turtle guarding the ferry waiting to hear from? While others draw their swords at the sight of demons, he turns to the law. From one county to the Nine Provinces, he judges right and wrong and investigates the unfinished half of his late father's bloodstained note. Afraid of heights and pain, and often wearing his official hat backward, he nevertheless relies on his wits, his connections, and a single law to hammer the truth that "demons are people too" into the world, one inch at a time.