Mantis, an ace assassin, returns to the contract killer industry after a hiatus, encountering his trainee friend Jae-yi and a retired legendary killer Dok-go, who now runs the organization.

The fates of a man with a hidden past, a wise and just woman, and an upright man intertwine.

In the Joseon Dynasty, two friends who grew up together — one the master and one the servant — reunite post-war as enemies on opposing sides.