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For five years, Eren Jaeger lived his life unaware of his possession of the power of the Titans until the year 850, during the struggle for Trost. Upon being caught and eaten by a bearded Titan, Eren's injuries and burning desire to slaughter the Titans trigger his first transformation into the his Titan form. In a nearly mindless state, Eren wanders Trost as a Titan, slaughtering many Titans throughout the day all while avoiding bringing harm to the human cadets around him. His first Titan eventually exhausts itself, and Eren emerges with healed wounds.
Completely clueless as to the nature of Eren's Titan powers, the Trost Garrison takes Eren into their custody and questions him about his Titan nature. When it becomes clear that the Garrison soldiers present cannot be reasoned with, cannonfire is used against Eren, but he successfully accomplishes a partial transformation of his Titan and protects himself and his friends from the blast. Upon Commander Dot Pyxis' arrival in Trost, the military decides to make use of Eren's Titan to seal the breached gate of Trost.
Upon reaching the boulder needed to seal the breach in Wall Rose, Eren's next transformation goes awry when his Titan falls into a feral state and attacks Mikasa Ackermann before incapacitating itself. Eren's Titan lies dormant for a short time before his friend Armin Arlelt manages to communicate with Eren himself through the nape of his Titan, and Eren regains control. Carrying a massive boulder through the streets as members of the Garrison Regiment and Cadet Corps defend him, Eren's Titan seals the hole in the Wall, securing humanity's first victory against the Titans.
Some time afterwards, Eren joins the Scout Regiment after his change of custody is made official in a military tribunal. During his first month in the Scouts, Eren's Titan is subjected to a variety of studies by Section Commander Hange Zoë. At this time, the Scout Regiment discovers the dependency of the Titan's power upon a fixed goal in mind before transformation in addition to the drawing of blood. After the month's passing, the Scout Regiment sets out on the 57th Exterior Scouting Mission. However, the scouting mission is sabotaged by the arrival of the Female Titan. Within a Forest of Giant Trees, Eren's Titan battles the Female Titan, but he is defeated. While Eren is later rescued from the Female, the scouting mission is cut short and is considered a failure.
A few days later, the Scout Regiment corners Annie Leonhart, holder of the Female Titan, in Stohess District, and she transforms into the Female Titan once more as a last resort. After some hesitation, Eren battles her a second time with his Titan. When he is overpowered late in the battle, his Titan undergoes a sudden change and catches fire in a berserker-like fit of rage. Eren's Titan gains the upper hand over Annie, though the assault comes to a close when Annie uses her hardening ability to encase herself in a crystal-like substance to avoid capture.
The next day, Eren joins the Scout Regiment's efforts in dealing with the sudden Wall Rose invasion. At this time, Reiner Braun and Bertholdt Hoover reveal their respective identities as the Armored Titan and Colossal Titan. When the two transform into their Titan forms, Eren battles the Armored Titan with his own Titan. Despite initial difficulties, Eren's Titan gains the upper hand and nearly defeats the Armored Titan until Bertholdt's Colossal Titan cuts the battle short in a last-resort effort to capture Eren.