This is the season where Eren finally discovers his father’s basement and the truth about the Titans is finally revealed.
Battle at Wall Maria
The season opens with the scouts traveling by night to avoid the sleeping titans on moonless nights. Armin discovers that Bertolt and Reiner are at Wall Maria when they arrive. Eren quickly seals the outer hole without any opposition, arousing suspicion. Erwin assigns Armin to investigate the situation, and they find Reiner hidden in a wall cavity. However, they nearly get decapitated by Levi before the Beastly Titan arrives.
Erwin deduces the Beast Titan’s plan to trap them and counters by using Eren as bait to keep the Armored Titan from attacking from the rear. The titans engage in hand-to-hand combat, while the scouts use the new powerful weapon, the thunder spears.
Reiner flashes back to killing Marco and the events leading up to the scout’s arrival. Just before Reiner is finished, Bertolt arrives, and after Armin’s failed negotiation and Mikasa‘s failed assassination, he transforms, causing Hange‘s squad to be annihilated in the chaos.
Irwin’s group battles an attack of boulders from the Beast Titan, while Eren’s group tries in vain to stop the Colossal Titan from reaching the wall.
Armin, the new Titan
In a last-ditch effort, Irwin leads the remaining forces in a charge at the Beast Titan in an attempt to distract him and allow Levi to use the surrounding titans to kill the Beast. Just as they start the charge, Irwin is taken down by another barrage of boulders.
The dwindling forces continue the charge, firing smoke guns, but the Beast kills them off one by one. Suddenly, Levi emerges from the smoke and slices the Beast to pieces, but the Beast’s companion titan takes him away.
A single soldier from the charge survives unscathed. Armin realizes that the Colossal Titan is running low on stamina and sacrifices himself in his plan, while Mikasa’s team successfully removes Reiner from his titan form with help from Hange, who shows up alive and well.
Berthold, thinking he has victory in hand, realizes Armin’s plan, and Eren, who has exited his titan form, removes Bertolt from his. Eren grieves over Armin’s charred body, but the Beast arrives and cryptically tells Eren that they are both victims of Eren’s father.
Levi arrives and drives the Beast away, leaving Berthold behind. Armin shows signs of life. Meanwhile, Hange tries to kill Reiner, but Jean stops her, stating that they may need the power of the Armored Titan.
Upon arriving, Mikasa realizes that they can use the titan serum on Armin to take Berthold’s power. Just before Levi hands over the serum, the surviving soldier arrives with Erwin still alive.
A debate ensues between the four of them over who should receive the serum, but Levi ultimately decides to give it to Armin. Armin then eats Bertolt, acquiring the Colossal titan’s power.
After Armin wakes up and recovered, they head to Eren’s father’s basement. Inside the seemingly normal study, they find books filled with knowledge hidden in the desk, along with a photograph of Eren’s father and two other people. The scouts return home with news of their successful takeover.
Later, Eren recounts the memories he acquired from his father to Armin for documentation.
Grisha Yeager, Aaron’s father, recounts his past as an Eldian citizen living in an internment zone in Marley. As a child, he and his sister escaped to explore a blimp but were caught. His sister was brutally murdered by the authorities.
Grisha’s father then recounts the history of Ymir Fritz, the first Titan, and the subsequent Nine Titans who went on to establish the Eldian Empire. However, Marley gained control of seven of the Nine Titans, leading the Titan King to flee to Paradise Island protected by the Three Great Walls. Grisha’s ancestors were left behind in Marley and forced to live in internment zones.
Grisha later joins a rebellion where he meets Dina Fritz, a descendant of the royal family, and they have a son named Zeke. Despite their efforts to raise Zeke as a hero for their cause, he ultimately betrays them and they are transformed into mindless Titans, including Dina who later would eat Grisha’s wife, Eren’s mother.
During this time, Grisha speaks with a guard who reveals himself to be the one who killed Grisha’s sister. The Owl, an informant within the rebellion, kills the murderer and reveals that he too can transform into a Titan at will.
Eren Krueger, the Owl’s real name, explains to Grisha that he too is a follower of Ymir and possesses one of the Nine Titan’s power. He wants Grisha to take his place as the Attack Titan and take possession of the Founding Titan’s power, as those who take the power of a Nine Titan will die within 13 years. He also reveals that the Nine Titans are all linked in some way to the Founding Titan, Ymir.
Beyond the walls
Following their discovery in Eren’s father’s basement, Eren and Mikasa reunite with Historia who, after reading a letter from Ymir, experiences vivid visions of the Founding Titan. The Scout Regiment and its leaders convene to decide what to do with the information they’ve gathered.
Eren, in the past, reveals his mission to obtain the power of the Founding Titan from the King, who couldn’t be trusted to save the Eldians and had misled the populace of the Walls into believing there was no one other than them.
Eren realizes he could use the power of the Founding Titan by touching someone of royal blood, but chooses to keep this to himself. The decision is made to reveal the truth to the people, and after a time skip, the inside of Wall Maria is safe to live in again. The Scouts embark on an expedition beyond the Wall, encountering only one Titan and eventually reaching the sea, fulfilling Armin’s dream to see it.
Eren speaks of the potential enemies beyond the sea, bringing season 3 of Attack on Titan to a close.
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Attack on Titan Season 3 Part 1 Recap