Kill Boksoon is about a single mother who is also a legendary contract killer for the top-tier agency MK. When she refuses to complete a mission, she is thrown into a conflict with her company, which forces her into an inevitable fight for survival. The film follows her struggle to balance her dangerous profession and her relationship with her teenage daughter.

The Double Life: Gil Bok-soon is a single mother to her teenage daughter, Jae-young, but is also a renowned and highly successful assassin at M.K. Ent.. She struggles to balance the two lives, especially as her daughter goes through difficult teenage years.

The Mission Refusal: Just before renewing her contract, Bok-soon is tasked with a mission. However, she is faced with a moral dilemma and ultimately refuses to carry out the hit, making her a target.

The Company and Colleagues: M.K. Ent is run by Cha Min-kyu, who trained her and holds a deep respect for her, though he is also a dangerous figure. When she refuses the mission, her colleagues, including fellow assassin Han Hee-sung who is manipulated by Min-kyu’s sister, turn against her.

The Consequences: Her refusal and subsequent actions put her in an inevitable fight for survival. She is forced to fight against her fellow assassins and the company she once worked for. The plot also involves her daughter, who has her own secrets, adding another layer to Bok-soon's challenges.