After the events of the Wall Maria's destruction, the government's desperate attempt to reclaim the lands using the refugees leads into nothing but more casualties. This prompts Mikasa, Eren and Armin enroll into the army and they become members of the 104th Trainees Squad. This is when Mikasa meets Jean Kirschtein, who awkwardly compliments her for her beautiful hair. However, much to Jean's dismay, Mikasa ends up cutting her in order to avoid any incidents with the 3D Maneuver Gear.
During their training, Mikasa helps Eren as much as she can as he initially fails to grasp the basics of the 3D Maneuver Gear. However, they manage to get through and Mikasa quickly proves her skills, being praised as one of the best trainees in the history. At one point, Mikasa asks Annie to teach her a move that she was teaching Eren, which eventually calls for a bet with the 104th Trainees Squad to see who is the stronger fighter. After the 104th Trainees Squad's graduation, she ends up ranked first.