Léon, played by Jean Reno, is a solitary hitman who lives a regimented existence in a small apartment in New York City. He spends his days cleaning his rifle, exercising, and tending to his beloved houseplant. His only contact with the outside world is through his handler, Tony, played by Danny Aiello, who sends him on jobs for various criminal clients.
One day, Léon's quiet life is disrupted when his next-door neighbors are killed by corrupt DEA agent Norman Stansfield, played by Gary Oldman. The only survivor is 12-year-old Mathilda, played by a young Natalie Portman in her breakthrough role. Mathilda was out running an errand when the hit took place, and upon returning home, she finds her entire family dead.
Fearing for her life, Mathilda turns to Léon for help. At first, he is hesitant to get involved, but he eventually takes her in and becomes her protector. As the two spend more time together, Léon begins to see Mathilda as the family he's never had. He teaches her how to defend herself and becomes her surrogate father figure.
However, Mathilda's grief turns to anger and a desire for vengeance against Stansfield. She begs Léon to teach her how to become a "cleaner" so she can exact revenge. Léon is reluctant but eventually agrees to take her under his wing.
As Mathilda trains and prepares for her mission, Léon begins to see the consequences of his violent lifestyle through her eyes. He starts to question his own choices and whether he wants to continue living as a hitman.
The climax of the film sees Mathilda and Léon attempting to take down Stansfield, culminating in a violent shootout at a hotel. The film ends with a heartbreaking finale that cemented it as a classic of '90s cinema.