Leone "Léon" Montana (Jean Reno) is a hitman (or "cleaner," as he refers to himself) living a solitary life in New York City's Little Italy. His work comes from a mafioso named Tony (Danny Aiello), who operates from the "Supreme Macaroni Company" restaurant. Léon spends his idle time engaging in calisthenics, nurturing a houseplant that early on he describes as his "best friend," and (in one scene) watching old Gene Kelly musicals.
On a particular day, he meets Mathilda Lando (Natalie Portman), a twelve-year-old girl with a black eye and smoking a cigarette, living with her dysfunctional family in an apartment down the hallway. Her abusive father and self-absorbed mother have not even noticed that Mathilda has stopped attending class at her upscale school. Mathilda's father (Michael Badalucco) attracts the ire of corrupt DEA agents, who have been paying him to store cocaine in his residence, after they discover that he has been stealing some of the drugs for himself. A cadre of DEA agents storm the building, led by a sharp-suited and drug-addicted Norman "Stan" Stansfield (Gary Oldman), who murders Mathilda's entire family, missing her only because she was out shopping when they arrived. When she returns and notices the carnage, she calmly continues down the hallway and receives sanctuary from a reluctant Léon. Wikipedia