Single mother Kill Bok Soon is one of the world's deadliest assassins thanks to her 100% success rate on contract killings. Bok Soon works for M.K Ent, which is run by the man that taught her, Cha Min Kyo. The pair hold mutual respect for each other, but Kill Bok Soon understands that at a moment's notice could take everything away from her. Just before Kill Bok Soon was due to renew her contract, loyalties are put to the test when Bok Soon is involved in a kill or be killed incident.