Erwin Smith: The 57th Exterior Scouting Mission, Part 4 (エルヴィン・スミス ―第57回壁外調査④― Eruvin Sumisu -Dai Gojū-Nana Kai Hekigai Chōsa (4)?) is the 20th episode of the 1st season and the 20th episode overall of the Attack on Titan anime, produced by Wit Studio and Production I.G.
With the Female Titan restrained, Erwin Smith and Levi try to figure out its identity with no success as it is able to harden its skin to prevent them from cutting out the human inside. Jean, Armin and the rest of Levi's squad realize it was Erwin's plan to use Eren as bait to draw out the mole within the Scout Regiment who joined them after the Walls fell and only told his real plan to those who were part of the Legion before the fall. Meanwhile, the others wonder about what is happening inside the forest until the Female Titan gives a loud scream that draws all Titans in the vicinity to her. Despite the Scout Regiment's attempt to stop them, the Titans ignore the humans and eat the Female Titan's body in a frenzy, which makes Erwin call off the expedition. However, knowing that there is no confirmation that the mole has also been eaten by the Titans, Erwin orders Levi to resupply while the mole, disguised as a cloaked Scout Regiment member, attacks Levi's Squad, killing Gunther Schultz.
Following the capture of the Female Titan, Eren questions its identity, wondering if it is an abnormal or something like the Colossal Titan. Oruo and Petra interrupt his thinking and tell him to focus on getting farther in and standing by. Meanwhile, Levi readies himself to strike the Female Titan, however, Erwin is not satisfied with just trapping the Female Titan and orders the soldiers to fire the rest of the wires.At the outskirts of the forest, the soldiers continue making sure that no Titans get inside. Christa notices one of the Titans attempting to climb, but Nanaba does not seem to be worried at all. She also states that it should not be long until Commander Erwin issues a withdrawal order. While Sasha and Mikasa wonder if it is the sound of cannons firing inside the forest, Ymir asks Bertholdt about Christa's whereabouts, but Bertholdt does not know. Jean and Armin are forced to shift positions as a Titan is beginning to climb farther into their position, noting that it is scary how they are able to learn. Afterwards, Jean shares his idea about what is going on inside the forest: He thinks that they lured the Female Titan into the forest in order to capture it.
Inside the forest, Eren also reaches a similar conclusion, but wonders why is it that not even the elite soldiers knew about the plan. This angers Petra and Oruo as Eren implies that Erwin does not trust them. However, Eld agrees with Eren and concludes that it is because Erwin knew of a spy among the soldiers. He then assumes that the only soldiers aware of the plan were those who survived 5 years ago. They wonder if that spy was the one to kill Sawney and Beane, remembering the question Erwin asked everyone at that time, finally realizing its meaning. Eren ponders about Erwin's decision, knowing that too many soldiers died because of it. Eld asks him if he thinks that Erwin was wrong. At the same time, discussing the same issue, Armin tells Jean that Erwin simply made his choice - whether to sacrifice hundreds of soldiers or all the people inside the Walls. He states that only those who do not hesitate to make sacrifices are able to change something.
Meanwhile, Hange continues taking advantage of the Female Titan's regenerative abilities in order to immobilize it. However, in spite of the Titan's restricted movements, Levi and Miche are unable to break through its hardened skin. Erwin notes that it is an ability similar to that of the Armored Titan. He then decides to use explosives to blow up the target's wrists and orders the soldiers to ready them. At the same time, Levi lands on top of the Female Titan's head and sarcastically remarks that the person inside should just come out as they have no time to waste. He then begins threatening the Titan, stating that there is no way of escape and also asking whether it is okay to cut off the hands and legs of the person inside. Hearing this, the Female Titan lets out a loud screech that can be heard all the way to the forest outskirts, much to everyone's surprise.Still taken aback, the soldiers wonder if it was the Titan's dying scream, when Miche tells Erwin that he can smell Titans coming from every direction and the soldiers at the outskirts find themselves to be ignored by the Titans running into the forest. Sasha, however, tells Mikasa that she recognizes the noise - it is the scream of a cornered beast that has nothing to lose. She warns Mikasa to be extra cautious and Mikasa, knowing that Sasha's instincts are usually right, wonders about Eren's whereabouts.
Inside the forest, Erwin orders the soldiers to ready the explosives now, but they have no time as the first group of Titans reaches the Female Titan. Realizing that they're to devour the Female Titan, Erwin orders everyone to engage in combat with them. However, upon seeing the upper hand of the Titans, Erwin signals a retreat. Knowing that he underestimated the enemy, Erwin then orders everyone to return into the formation. However, he orders Levi to refill his gas and blades beforehand.
The soldiers at the forest outskirts then notice the retreat signal, happy to finally be able to return home. Gunther Schultz also sees it, letting the rest of the squad know that they're going back. Wondering if they can finally see the person inside the Female Titan, Eren is told by Petra that it is all thanks to him as he decided to have faith in them back then. Oruo then complains about Eren whining the whole time, but Eld reveals to Eren that both Petra and Oruo peed their pants during their first expedition, much to their anger and Eren's surprise.
The group then notices a signal flare, thinking it was fired by Levi. However, upon witnessing their own signal flare, the mysterious person readies their blades and heads their direction. Meanwhile, Hange asks Erwin about his decision to have Levi resupply, claiming there is no time for it. Erwin explains that while they saw the Female Titan being eaten, the person inside it was not seen. Deducing based on Hange's previous hypothesis, Erwin states that there is a possibility that the culprit managed to escape using the attire of the Scout Regiment. At the same time, the person nears Eren and others. Just as Gunther notices that it is not Levi, he gets attacked and Eren witnesses Gunther's corpse hung on a tree.