A four-year timeskip shifts the narrative perspective to Marley, revealing the nation that has been the hidden antagonist throughout the series. Eren has infiltrated Marley under a false identity, posing as a wounded soldier from the internment zone to gather intelligence on enemy operations. Reiner is shown suffering from severe post-traumatic stress disorder, struggling with the guilt of his actions as the Armored Titan. The Tybur family, Marley's most powerful noble house, declares war on Paradis at a public festival, calling upon the world to unite against the island devils. Eren transforms at the moment of the declaration and launches a devastating attack on the Marleyan military leadership. The assault kills thousands of civilians and military personnel, including Willy Tybur and the War Hammer Titan.
Eren infiltrates Liberio, the Marleyan internment zone for Eldians, living as a wounded soldier named Kruger and building connections with Falco Grice. Reiner's deteriorating mental state is explored in depth, showing his suicidal ideation and split personality as a coping mechanism for his wartime trauma. The Warrior candidate program is introduced, revealing how Marley selects and trains children to inherit Titan powers through brutal competition. Willy Tybur delivers a masterful speech declaring war on Paradis, claiming the Tybur family will lead the global military coalition to eliminate the island. Eren transforms at the theater where Tybur speaks, decapitating the War Hammer Titan and consuming its power before engaging the Marleyan military. The Attack Titan proceeds to devastate Liberio's port district while the Survey Corps arrives to extract Eren, culminating in a massive battle against Reiner's Armored Titan. The episode ends with the Survey Corps fleeing Marley in a flying boat, carrying Porco Galliard's Jaw Titan captive.
The Marley Arc recontextualizes the entire story by presenting the perspective of the enemy, revealing that Marley has its own legitimate grievances and that Paradis Eldians are not purely innocent. Eren's transformation from righteous protagonist to ruthless attacker who kills civilians forces the audience to question who the real villain is. The arc establishes the global geopolitical situation and the existential threat facing Paradis: the entire world has united against them and possesses overwhelming military and technological superiority. A profound moral symmetry is established: Eren has become exactly what Reiner was in the Battle of Trost, forcing Reiner to confront the consequences of his earlier actions.