12-year-old Mathilda is reluctantly taken in by Léon, a professional assassin, after her family is murdered. An unusual relationship forms as she becomes his protégée and learns the assassin's trade.
After her father, stepmother, stepsister, and little brother are brutally murdered by her father's employers, a 12-year-old girl finds herself alone and desperate. Seeking refuge, she manages to take shelter in the apartment of a professional hitman.
Determined to avenge her family's deaths, she persuades the hitman to teach her the methods of his trade. Under his tutelage, she learns the skills necessary to confront and seek revenge on the corrupt DEA agent responsible for her brother's death and the destruction of her life.