Boksoon leads a double life - she is both a mother of a teenage daughter and the legendary professional killer at the top-tier killing agency MK Ent. Caught in between the mission of killing someone and the mission of raising someone, Boksoon refuses to complete an assigned mission and is thrown into an inevitable fight.
Hired by the killing agency MK Ent., Gil Boksoon is both a killer with a 100% success rate and a mother of her teenage daughter, Jae-young. While Boksoon might be a seasoned assassin, she struggles with parenting. When it is about time to renew her contract with her agency, she makes up her mind to retire to repair the relationship with her daughter. While on her final assignment, before notifying the company of her decision regarding the contract, Boksoon discovers a secret about the mission and breaks the rule that killers must try to execute all assignments regardless of the circumstances. Now she becomes the target of her agency and the entire hitman industry.