Five years after the Fall of Wall Maria, the Colossal Titan reappears suddenly and breaches the Trost District gate, flooding the city with Titans once again. The newly graduated 104th Training Corps is thrown into real combat for the first time, suffering heavy casualties among inexperienced soldiers. Eren is eaten by a Titan during the battle but unexpectedly transforms into the Attack Titan, shocking his comrades and commanding officers alike. He manages to seal the breached gate with a massive boulder, saving the district from total annihilation. The battle forces humanity to confront the possibility that some humans can become Titans.
The Colossal Titan kicks in the outer gate of Trost, and Titans pour into the district as the garrison scrambles to mount a defense. Eren sacrifices himself to save Armin from a pursuing Titan but is devoured whole, only to emerge moments later as a fifteen-meter Titan form. Mikasa and Armin protect Eren's Titan body while he fights blindly, struggling to maintain his human consciousness and coordinate his movements. Captain Levi's squad arrives and extracts Eren after he successfully carries the boulder to the gate opening. Commander Pixis devises a bold plan using Eren's Titan power to seal the breach, a gambit that succeeds but reveals Eren's existence to the military hierarchy.
The Battle of Trost is the crucible that forges the 104th Training Corps into true soldiers and reveals Eren's Titan-shifting ability to the world. It shifts the story from a survival horror premise into a political and strategic drama about how humanity can weaponize Titan power. The battle also introduces the internal conflict between the military police, the garrison, and the Survey Corps over how to handle Eren. Eren's transformation is the first concrete clue that Titans are connected to human beings, setting up the series' central mystery of Titan origins.