Gu Qingzhou time-travels to Great Qi, replacing his late father to become a small jailer in the county yamen. Originally wanting to stay low-profile and survive, he is deliberately made things difficult for by his superior and sent to guard an exceptionally remote cell for heavy criminals. A mysterious person of unknown origin is locked in the cell, seeing through his details with just a few words. The two hit it off and reach a deal, from which chess games are secretly laid out inside and outside the yamen, and step by step he operates with foresight relying on his three-inch tongue and a code of laws.