Leon (Jean Renault) is a lonely professional killer who is hired. One day, a neighbor's little girl, Matilda (Natalie Portman), knocked on his door and asked to avoid being killed with him. It turns out that the owner of the neighbor's house was an informant of the police anti-drug team, and was punished by the evil police officer (Gary Oldman) who killed the whole family for embezzling a small packet of drugs. Matilda was rescued by Lyon, survived and stayed with Lyon. Leon taught the little girl to use guns, and she taught Lyon French. The two became closer and get along well. The girl wanted to take revenge, but was caught instead. Leon arrived in time to save the girl. The battle between good and evil mixed with sorrow and hatred gradually escalates, and greater conflicts are inevitable...