Leon (Jean Reno) is a lonely professional killer hired by others. One day, the neighbor's little girl Matilda (Natalie Portman) knocked on the door of his house and asked for shelter from the murder there. It turns out that the owner of the neighbor's house is the eyeliner of the police's drug squad. He is punished by the vicious police (Gary Oldman) for killing his family because he embezzles a small packet of drugs. Matilda was saved by Lyon, but survived and remained there. Leon taught the little girl to use the gun, and she taught Leon French. The relationship between the two became increasingly close and harmonious.The girl wanted to take revenge, but instead was caught. Leon arrived in time to save the girl. The battle between good and evil, mixed with sorrow and hatred, is gradually escalating, and greater conflicts are inevitable

里昂（讓·雷諾飾）是名孤獨的職業殺手，受人雇傭。 一天，鄰居家小姑娘馬蒂爾達（納塔麗·波特曼飾）敲開他的房門，要求在他那裡暫避殺身之禍。 原來鄰居家的主人是警方緝毒組的眼線，只因貪污了一小包毒品而遭惡警（加里·奧德曼飾）殺害全家的懲罰。 馬蒂爾達得到里昂的留救，倖免於難，並留在里昂那裡。 里昂教小女孩使槍，她教里昂法文，兩人關係日趨親密，相處融洽。女孩想著去報仇，反倒被抓，里昂及時趕到，將女孩救回。 混雜著哀怨情仇的正邪之戰漸次陞級，更大的衝突在所難免……