A psychologist for inmates has knowledge of a secret, experimental brain implant, designed to change convicts' behavior so that they will not remember or repeat their crimes and, therefore, be able to return productively to society rather than remain in an increasing prison population. The procedure, however, suffers some imperfections: Three killers are at it again.

Léon, the top hit man in New York, has earned a rep as an effective "cleaner". But when his next-door neighbors are wiped out by a loose-cannon DEA agent, he becomes the unwilling custodian of 12-year-old Mathilda. Before long, Mathilda's thoughts turn to revenge, and she considers following in Léon's footsteps.