Yoo Bo Na appears to be an ordinary woman in her thirties, balancing work, marriage, and raising her young daughter. Beneath her unassuming lifestyle, however, she hides an extraordinary secret. Known only as "Kingfisher," she is a legendary sniper who eliminates dangerous criminals who have escaped justice. After spending three years on childcare leave, Bo Na returns to her covert mission, once again juggling the demands of family life with the deadly responsibilities of a vigilante.