In the heart of New York City, a solitary and efficient hitman named Léon lives a monotonous life as a "cleaner" for a mob boss. Despite his profession, Léon follows a strict code of conduct, maintaining a distance from people and emotional attachments, allowing his life to revolve solely around his contracts and the meticulous care of his houseplants. His existence takes an unexpected turn when he encounters Mathilda, a spirited 12-year-old girl with a troublesome family life living in the same apartment building. When Mathilda's family is brutally murdered by corrupt DEA agents over a drug deal gone wrong, she manages to escape the carnage by seeking refuge with Léon.

Léon reluctantly takes Mathilda under his wing, and an unusual bond forms between them. As Mathilda becomes aware of Léon's occupation, she pleads with him to teach her his skills so that she can avenge her younger brother's death. Torn between his solitary lifestyle and the desire to protect Mathilda, Léon carefully begins training her in the art of assassination. Their relationship deepens, blurring the lines between a makeshift family dynamic and accomplices, while the threat of the corrupt DEA agents, led by the unhinged and relentless agent Stansfield, looms ever closer.