Leon, played by Jean Reno, is a professional assassin living in New York. His young neighbor, Mathilda (Natalie Portman), faces family issues, particularly with her father being a small-time criminal. One day, her entire family is murdered by a drug lord, Stansfield (Gary Oldman), because of her father's actions. Mathilda turns to Leon for assistance and vengeance.
Léon is a reclusive and quiet hitman who specializes in his deadly profession. His life changes dramatically when a corrupt DEA agent kills Mathilda's family. With no one else to help her, Mathilda infiltrates Léon's isolated existence and proposes an unusual arrangement: she will handle household chores in exchange for his knowledge of killing. Driven by her intense desire to avenge her brother, Mathilda struggles to find out if she has what it takes to pursue revenge.
Léon (Reno) leads a solitary existence in New York, earning his living as a hitman for the mafia. He is mostly silent and extremely cold-hearted, appearing devoid of any compassion. However, when his 12-year-old neighbor, Mathilda (Portman), comes home to find her whole family murdered by a drug dealer (Oldman), Léón decides to help her. At her request, he starts teaching her the skills needed to seek revenge on the man who killed her family.