Set in Italy, Gunslinger Girl follows the exploits of the Social Welfare Agency (often referred to as simply "the Agency"), ostensibly a charitable institution sponsored by the Italian government. While the Agency professes to aid the rehabilitation of the physically injured, it is actually a military organization specializing in counter-intelligence and counter-terrorism.
The girls of the Social Welfare Agency are no ordinary children. They are the grizzly remains of human wreckage pieced back together with cybernetic implants and trained to kill by the government. The oldest, Triela, pursues her targets with a ferocious enthusiasm – unwilling to settle for less than total annihilation. Her mirror in this bloody stalemate is Pinocchio – a shell of a boy raised as an assassin by the FRF – a terrorist faction at war with the SWA. Cold and cruelly efficient, he wields sharpened steel as though it were his own hand. Once human, these shattered souls have become murderous machines with only vague recollections of what it meant to be real – and a brutal compulsion to be the last killer standing.