Léon (Jean Reno) is a professional hit man, with no family and no friends. However, he has casually befriended Mathilda, a 12 year-old girl whose entire family, including her adored 4-year-old brother, is wiped out by a group of corrupted cops. The girl pleads with Léon to teach her how to avenge her little brother's death. Once she learns a few skills, Mathilda embarks on the mission to kill the psychotic cop who masterminded her family's execution. However, her plans don't go as expected, and soon Léon must step in to help.