The Beast Titan appears outside Wall Rose, suggesting that humanity's territory has been penetrated far deeper than anyone realized. Reiner Braun and Bertholdt Hoover, two of Eren's closest comrades, reveal themselves as the Armored Titan and Colossal Titan in a shocking confession on top of Wall Rose. The Survey Corps pursues them into the Titan Forest, where a chaotic multi-Titan battle unfolds. Eren learns that Ymir, another Titan shifter, has chosen to side with Reiner and Bertholdt. The battle concludes with the enemy escaping and the Survey Corps returning with heavy losses but critical intelligence. The revelation shatters the bonds of trust within the 104th Training Corps forever.
The Beast Titan hurls massive boulders with devastating accuracy, forcing the Survey Corps to acknowledge a new and even more dangerous threat beyond the walls. Reiner's casual confession that he is the Armored Titan while sitting on the wall with Eren is one of the most startling reveals in anime history. Bertholdt transforms into the Colossal Titan and unleashes a blast of steam that burns everything within a massive radius, covering their escape. The Survey Corps leads a Titan-shifter battle in the forest where Eren fights Reiner in a brutal hand-to-hand struggle between their Titan forms. Ymir transforms to protect Historia and fights against Reiner, but ultimately chooses to return to Reiner and Bertholdt. The arc ends with Eren developing the ability to harden his skin through focused willpower, a breakthrough in controlling his Titan power.
This arc permanently transforms Attack on Titan from a humanity-versus-monsters story into a conflict between rival human factions. Reiner and Bertholdt's reveal creates an irreconcilable fracture in the protagonist group and forces Eren to confront that his enemies are people he once called friends. The arc deepens the mystery of the Titans by revealing that Ymir possesses knowledge of the world beyond the walls and has lived for centuries. The discovery that Titan shifters appear human most of the time raises profound questions about identity, loyalty, and whether the Survey Corps is fighting monsters or people.