In this story, a small handful of the girls from Gunslinger Girl are matured and skilled enough to go on mission by themselves. Triela, Henrietta, Claes, and Rico are the only girls who are able to do this, but they don’t work alone. They work as a team. Triela is the field leader. Claes is the sniper. Henrietta and Rico are the assault squad. Sometimes, they will bring along Angelica and her handler, Marco Toni. Marco needs to be with Angelica due to certain medical issues with her. On one of their missions an incident happens between the target and Triela. An incident that could kill her or change the lives of her and her squad forever. So she makes an agreement to meet with him in her dorm room with the other girls in her squad. A dark secret is revealed to them and changes their views of the agency.