Elite assassin Mantis returns from vacation and learns that Cha Min Gyu, who was the head of MK Enterprise, is dead. He decides to leave MK Enterprise and seek a new opportunity with other contract killers. He has a friend named J that he has known for many years. They trained together to become assassins. J is lethal as a contract killer, but she was kicked out of MK Enterprise due to falling out of favor with her superiors. She holds complex feelings towards Mantis.Meanwhile, Dok Go was one of the founders of MK Enterprise, and he was Mantis’s teacher. He retired from the contract killing world, but, after Cha Min Gyu’s death, he returns to take over MK Enterprise. Dok Go tries to lure Mantis back to the company.(Source: AsianWiki; edited by MyDramaList)