Spoilers!!

What is inside the wall?
Titans.

Who is the beast titan?
Zeke Yeager Eerens half brother

How Eren can coordinate?
His father ate the royalistic family because they did not want to help him.

Erwin is hot.
He will die.

What are Titans?
Humans.

The revelation that Titans themselves were in the very walls that protected human civilization. Without going too far into the history of the walls themselves, which go back a long, long way before Eren’s story, we know a few things from this season of the anime: Certain Titans (the “Shifters” like the Female Titan, Colossus Titan, Armored Titan and Beast Titan) can damage the walls which are otherwise impervious, and that the Church of the Walls, a sect of radicals who see the walls as religious objects, will go to any lengths to preserve the secrets behind their creation. Even under torture, those secrets were not divulged this season; maybe next year. But perhaps Hange Zoë‘s theory about a piece of the Female Titan’s skin being similar to the Walls’ material is already most of the way there …

While the existence of Titans within the walls was certainly a big WTF moment early on which allowed the story to swing briefly into an investigation of the strange phenomenon, more importantly, it gave the Scouts a reason to ride the length of the wall looking for any holes where a Titan invasion might be coming from. This, of course, led to the devastation of the troops out in the wild, especially during the battle of Utgard Castle. During this mini arc, we get the reveal that Krista Lenz is actually Historia Reiss, the last surviving member of the royal family. Coupled with this reveal was the surprise that Krista/Historia’s bodyguard Ymir was actually a Titan Shifter in disguise! “Being a goddess isn’t all that bad.” (She also happened to be the one who surfaced and killed Marcel in Season 1…) While that was bad news for Annie Leonhart, a.k.a. the Female Titan, Ymir’s own transformation comes in quite handy for the Scouts throughout the rest of the season, though it does complicate things once the other named Titans reveal themselves.

BBBAAAASSSSEEEMMMMEEEEENNTTTT

awakens from a dream of the Colossus Titan, and Eren informs him of the Survey Corps' situation. Armin is shocked at the news that he received the Titan serum instead of Erwin and that he had eaten Bertolt. Levi explains the circumstances which led Armin to receive the serum, and Hange informs him of his new responsibilities in taking the place of Erwin's life; as well as her own as the new commander. With the news given, Eren, Mikasa, Levi, and Hange make their way to Eren's old home while the others stand guard. Arriving at the wreckage of their home, they clear a path to the basement but discover that Eren's key is not to the basement door. Levi breaks the door down, and the four find what appears to be a normal doctor's office. At Grisha Yeager's desk, Mikasa discovers a locked drawer. Eren uses the key, and they discover three well-preserved books. At Trost District, Commander Nile Dok recalls making fun of Erwin's questioning of the history books about the no existence of humans beyond the Walls, and Commander-in-Chief Zackly suggests Nile should apologize for his doubts to Erwin in person. They are then told that the Survey Corps has returned as the nine surviving members arrive at Trost. In the basement, the four come across an incredibly lifelike image of Grisha with another family. On the back a note is written, identifying the image as a photograph and revealing that humanity lives beyond the Walls in luxury, having never truly gone extinct.

Summary

Armin sees the Colossus Titan in his dream

As Armin recalls the results of Bertolt's transformation in his sleep, he is suddenly awakened by a vision of Bertolt's Colossus Titan, hearing a voice calling out that it is hurting. Armin is left shaken by the vision but is distracted by an unconscious Sasha, who quietly groans in her sleep that her injuries are hurting her. As Armin tries to check on her, Eren arrives, relieved to see that Armin has finally awakened. Levi, seeing that Armin has awakened, fires a flare to call everyone back, and orders Eren to tell Armin what has happened.

Armin is shocked to learn that only nine soldiers survived the battle, but he is even more horrified at the knowledge that he became a Titan and devoured Bertolt. Armin demands to know why he was chosen to receive the Titan injection and not Erwin, and Levi informs him that Mikasa and Eren were so intent on his survival that they went against direct orders to save him. Hange makes it clear that Eren and Mikasa will be punished for what they did, and Levi admits that he also let his personal feelings affect his decision to let Erwin die. Hange admits that she would have preferred that Erwin lived, but will accept Levi's decision, as he was the one Erwin entrusted the serum too. However, she makes it clear, to Armin's horror, that he will be expected to contribute to humanity even more than before, now that he has been entrusted with both Erwin's life and the Titan power. Levi assures Armin that he is not expected to replace Erwin, but reminds him that he will have to use his new powers so that no one regrets the decision to keep him alive over Erwin. Hange admits that she is in the same position as Armin, having to succeed Erwin as commander of the Survey Corps, before announcing that she, Eren, Mikasa, and Levi will go Eren's basement on their own, while the others keep watch from atop Wall Maria.

Eren, Mikasa, Hange, and Levi enter the basemen

Arriving at Eren's old home, Hange notes with relief that the fire from Bertolt's transformation has not reached it. As he helps clear away debris, Eren recalls the numerous times he asked his parents if he could see the basement, and being denied every time. Finally clearing a way to the basement, Eren recalls the last conversation he had with both of his parents when his father promised he would finally let Eren see the basement. Arriving at the door to the basement, Eren tries his father's key and is horrified to find out that it does not work on the basement's lock. Levi, growing impatient, simply kicks through the door and forces it open, allowing them to enter. Inside, they are greeted by the sight of what appears to be a normal doctor's office. Levi notes that there is nothing in plain sight that would arouse the suspicions of the Military Police Brigade, and orders Eren and Mikasa to find the secrets that are hidden. Mikasa discovers a keyhole in the side of Dr. Yeager's desk, which fits Eren's key. Opening the hidden drawer, Eren is shocked to find it empty, but Levi points out that it has a false bottom which hides three journals. Hange notes the odor of mint oil and charcoal, possibly treatment to repel insects and prevent moisture respectively. Anxious to see what his father was hiding, Eren opens one of the books with Mikasa.

The photograph found in the basement

In Trost District, Nile tells a group of Military officials about a time when he and Erwin were children, when Erwin pointed out to his father in class that, as humans are not able to freely leave the Walls, it should not be possible for anyone to say definitively that there are no humans alive outside of the Walls. Erwin believed that the fact that all of the Walls' history books stated so definitively that no other humans were alive without proof meant that the books' publishers, the Royal Government, wanted to trick the inhabitants of the Walls into believing that there were no other living humans. Nile sadly admits that at the time he had ridiculed Erwin for believing such a far-fetched idea, but Darius Zackly and Dot Pixis both assure him that he can apologize once Erwin makes his triumphant return. Their conversation is interrupted by Anka, Pixies' assistant, who informs them that the Survey Corps has returned.

As the citizens of Wall Rose cheer for the returning Survey Corps, Eren, Mikasa, Levi, and Hange solemnly reflect on what they learned from the basement. Inside of the first notebook they had inspected, they had a stunningly lifelike image of Grisha standing beside a fair-haired woman and young child. On the back of the image had been a message from Grisha claiming that the image was not a drawing, but an image burned onto paper called a photograph. He then went on to write that he came from outside of the Walls where the human race persists in a refined existence and that humanity has not perished.