逃婚离家出走的月牙儿（金晨 饰）巧遇了神秘的不老不死法师无心（韩东君 饰），由于受了月牙儿半个包子的恩惠，无心决定还月牙儿一个人情——靠着捉妖赚钱，带着月牙儿吃香的喝辣的。无心与月牙的第一单生意来自军阀顾玄武（王彦霖 饰），顾宅遭遇井中妖怪袭击，命案频发。无心一心想着捉妖赚钱，却不想引出了同样不老不死的井中精怪岳绮罗（陈瑶 饰），心狠手辣的岳绮罗却对无心情有独钟，而顾大人手下参谋张显宗（张若昀 饰）对岳绮罗竟暗生情愫，这引发了一连串的恩怨趣事。在乱世中，无心与月牙儿日久生情，与怪力乱神们斗智斗勇，一边恋爱一边结交各色朋友。

Wu Xin is an immortal who doesn't know where he was from, how long he lived, or what he is. He wanders the earth with no purpose in life and no money. During one of his destitute spells, he meets Yue Ya who gives him her last morsel of food. To repay her kindness and get more food in their stomach, he decides to capitalize his only skill set: Killing monsters. They begin a wondrous and often dangerous adventure of fighting supernatural evil, one paying customer at a time.