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.