For over a century, humans have been living in settlements surrounded by three 50m gigantic walls, Wall Maria, Wall Rose and Wall Sina, which prevent the Titans, giant humanoid creatures who eat humans, from entering. Eren Jaeger, of the town called the Shiganshina District, wishes to see the outside world by joining the Scout Regiment, as he likens living in the cities to livestock. Despite this, his adopted sister Mikasa Ackermann and their mother Carla Jaeger are against him joining the Scouts. Even after seeing the Scouts return home with large casualties, Eren expresses his interest to join, which impresses his father Grisha Jaeger, who promises to show him what lies in their basement once he returns. After Eren and Mikasa rescue their friend Armin Arlelt from a group of delinquents, the Colossal Titan, a 60m Titan suddenly appears and knocks down the gate to the Shiganshina District, which lies in the outer edge of Wall Maria, allowing smaller Titans to enter. As the town erupts into mass panic, Eren and Mikasa rush back to their house, only to see Carla pinned under their collapsed house. As a Smiling Titan approaches them, Carla begs them to flee, but they refuse until city guard Hannes arrives and assures them that he will defend them. However, as he charges towards the Titan, he is overcome with fear and takes Eren and Mikasa away. Eren watches in horror as a Smiling Titan eats Carla.

Hannes apologizes to Eren for not saving his mother, admitting he was afraid of the Smiling Titan and that both Eren and Mikasa are still weak children. Some of townsfolk of the Shiganshina District escape to the inner Wall Maria, while the soldiers try to hold off the Titans. The soldiers try to close the gate, but the Armored Titan rams the gate, breaching the inner Wall Maria, and causing those inside to retreat even further inside Wall Rose. Some time later, Eren has a strange dream in which his grieving father forcefully gives him an injection and his key before being woken up by Mikasa. Eren grows disgusted with the people of Wall Rose as the latter do not like sharing their food or homes with the refugees. During the next year, as a food shortage becomes apparent, the government orders the refugees to either work on the farms or fight to reclaim Wall Maria. About 250,000 people, 20% of the populace, choose to fight, only to be wiped out by the Titans. Eren vows revenge and joins the army along with Mikasa and Armin.

The drill instructor Keith Sadies inspects the new recruits of the 104th Cadet Corps and provides a barrage of verbal insults. When he comes across Sasha Braus, who is eating a potato, he punishes her by having her run around the camp until sunset and not giving her any food or drink. Eren develops a rivalry with Jean Kirschtein, who desires a safe and easy position in the Military Police Regiment within the inner city walls. The recruits train on an omni-directional mobility gear[10] apparatus, where they have to balance themselves while suspended on cables. After Eren has trouble balancing, he asks Reiner Braun and Bertholdt Hoover for help and discover they too are among the few who have personally encountered Titans. The next day, Eren balances on the apparatus but falls, but Keith discovers that Eren's apparatus was defective and passes him after a retry with different equipment.

Five years since Wall Maria fell, Keith evaluates the recruits for their strengths and weaknesses. After training in hand-to-hand combat with Reiner and Annie Leonhart, Eren learns later that night that Jean and several other recruits hope to join the Military Police Regiment to avoid having to fight Titans. On graduation day, Eren and Mikasa rank among the top ten of their class. They are eligible to join the Military Police Regiment, but Eren declines and announces that he is joining the Scout Regiment on the front lines, leading Armin and Mikasa to join him. They are assigned to the Trost District, one of the cities which is at the outer edge of Wall Rose, where they discover other classmates have joined. The Colossal Titan appears and breaches the gate. Once Eren regains his composure, he takes command and orders a counterattack.

Orjaški titan uniči zidne topove, vendar izgine v oblaku pare, preden Eren lahko napade njegovo šibko točko. Regiment vojaške policije skuša evakuirati prebivalce okrožja Trost in postaviti obrambo, preden titani napadejo mesto. Mikasa je določena v regiment, ki pomaga pri evakuaciji, medtem ko Eren in Armin nudita podporo v okrepitvenih sila na prvi bojni črti. Titani vdrejo v mesto in Erenova skupina napade na vse ali nič.