Gunslinger Girl (ガンスリンガー·ガール Gansuringa Garu) is an ongoing manga by Yu Aida, which was later adapted as a 13-episode anime television series, produced by Madhouse and Bandai Visual, which premiered in Japan on Animax and Fuji TV between October 8, 2003 and February 19, 2004.

Set in modern-day Italy, the story of Gunslinger Girl revolves around a corporation called the Social Welfare Agency (SWA, or the Agency, as it's often called), sponsored by the Italian government. Publicly, the mission of the SWA is to save and rehabilitate human lives, but in reality, the SWA is a public front for Section Two of the Political Warfare Section, a subdivision of the Italian intelligence responsible for counter-intelligence, counter-terrorism and a series of experiments designed to create the perfect government-commissioned assassin. While Section One conducts their operations with normal human operatives, Section Two employs the use of young cyborgs on the fringe of technology.

A male agent hired by the SWA chooses a young girl from a near-death experience or who is in critical condition. Section Two then redefines her body in a surgical operation to be refitted with artificial body parts, such as synthesized muscles and carbon fiber frames (80% of their bodies are carbon frame, fiber, and muscle). As a result, the girl gains incredible strength (at least peak human strength), quick reflexes, and a high resilience to damage and pain.

After the operation, the male agent is assigned as the girl's handler, forming a pair called a "fratello" (Italian for 'brother'). The first task of the handler is to choose a name for the girl. While refining the young girl, she undergoes brainwashing and becomes subject to conditioning: a watery drug injection that allows the girl to use her newly-enhanced body with maximum efficiency, as well as heighten her perceptions (such as sight, hearing and overall instinct) of the world around her. These enhanced senses would generally be used to anticipate danger, whether to the handler or to the girl herself, almost by a sixth sense. However, extensive use of the conditioning also shortens the girl's long-term memory, as well as her life span, since the conditioning strains the girl's mental and physical endurance.

The handler is directly responsible for his girl's training, conditioning, field performance, development and overall welfare. Once the handler takes charge of his assigned girl, he is free to use the teaching methods he considers best suited for his character. These methods tend to vary with each handler, although they have the option of using greater amounts of drugs and brainwashing. Some handlers prefer to treat their girls more humanely with minimal conditioning (as Giuseppe and Hilshire did) while others prefer to rely more heavily on conditioning their girls, treating them like machines (as Jean and Lauro did).

The end result is usually the same: a deadly assassin who is unquestioningly loyal to her assigned agent. Once they are officially sanctioned by the Italian government, these fratello are sent on controversial government missions, such as assassination and infiltration. For those who would oppose the Italian government, a young girl is often the least considered threat, much to the sorrow of those who encounter a fratello.

Currently, the Political Warfare Section is working to take down the Padania Republic Faction (PRF), a group of nationalistic radicals dedicated to the independence of northern Italy, as well as opposing globalization.

The story focuses primarily on the various relationships of the girls and their handlers, whether loving, abusive or neglectful, and their reactions. Gunslinger Girl also seeks to paint a contrasting portrait of its characters — the girls are cybernetically enhanced assassins, yet they are still just young adolescent girls at heart.

The young girls that are synthetically rebuilt and conditioned to kill instinctively without remorse are often coldly referred to as "machinery" or "dolls" by the SWA and their handlers (Jean refers to his as a tool). Yet, even though the bodies are reconstructed and the memories of a past life erased, the young girls have a soul, a sense of humanity towards each other, and still enjoy some simple things in their sheltered life.

From Wikipedia