Levi Ackermann, Hange Zoë, and their squad members battle against Kenny Ackermann and the Anti-Personnel Control Squad in the crystal caverns of the Underground Chapel. Hange is grievously injured by Caven, but overall the Scouts are successful enough to push their enemies back to their final defensive post.
Meanwhile, Rod Reiss pressures his daughter Historia into using a syringe to transform into a Titan so she can eat Eren Jaeger and reclaim the Founding Titan his father had stolen from her sister Frieda. However, Historia realizes that inheriting the Founding Titan will not save anyone because even if her sister had the power to eradicate all the Titans, there is something about inheriting the power that prevents people from using it. She chooses to shatter the syringe and works to free Eren, but her father laps up the remaining fluid and transforms into a Titan.
Rod Reiss returns with a satchel in hand. He is relieved that there is finally no one here but him, Historia Reiss, and Eren Jaeger. Rod tells his daughter that her sister, Frieda Reiss, is not entirely dead and asks if she would like to meet her. When she agrees, he kneels and takes a syringe out of his satchel. Eren recognizes it as the kind his father used on him.
Elsewhere in the Underground Chapel, the Anti-Personnel Control Squad hunkers down and prepares for an attack. The doors leading above ground burst open and barrels of gunpowder, augmented with bags of oil, roll down the stairs to the floor below. Levi Ackermann, Hange Zoë, and their squad members charge in after them. Sasha Braus fires a flaming arrow from her bow and into one of the barrels, causing it to explode. Other squad members fire signal flares into the air.
This creates a smoke screen for Levi and Mikasa Ackermann to begin their attack. Levi counts out a total of thirty-five Anti-Personnel Control Squad members and calls out their location to the other Scouts. Meanwhile, Caven shouts for her squad to spread out and surround their enemies one at a time, but the smoke makes it difficult for the Anti-Personnel Squad to fight. Their Anti-Personnel ODM requires that they fire in the same direction they intend to shoot their anchors, so they are unable to attack someone directly behind them.
Jean Kirschtein and Conny Springer both take out enemy soldiers as Sasha and Armin Arlelt provide covering fire. Caven realizes that when the Scouts are able to get close to them, their swords give them the advantage. The Anti-Personnal Squad is at risk of being wiped out when Kenny Ackermann arrives and shoots at Levi to draw his attention.
Kenny tells Levi that even if he gets past here, it will all be for nothing, and the two engage in combat. He manages to wound Levi, who throws a bag of oil at him. Kenny shoots it, causing a fireball that Levi leaps through to attack him. He takes a slice to the gut, which is enough to wound but not incapacitate him.
Seeing Hange approach, Caven fires wildly off target to feign a weakness and create an opening. Hange falls for the bait and chases after her. Caven lands on a wall and shoots one of the anchors from her Anti-Personnel ODM into Hange's shoulder. This allows her to swing Hange across the field until momentum tears her free and throws her against one of the crystal cavern's pillars. Hange falls motionless to the ground. Taking advantage of the Scouts' collective shock, Caven calls for the Anti-Personnel Squad to fall back to their final defensive post. When they pass through a mouth of the cavern, one of their squad members cuts down a large net to prevent the Scouts from immediately following. Kenny goes on ahead to guard the ritual.
Hearing the ruckus from all the combat, Rod tells Historia that they need to hurry. Eren tries frantically to communicate with her through his gag since he knows what the syringe means for her, but she does not understand why he is so upset. Rod tells her that it is because Eren knows that they intend to return the power his father stole to its rightful place, inside her.
He explains that a hundred years ago, this cavern was created by the power of a particular Titan. This power also built the three Walls and altered the people's memories. Some bloodlines were excluded from this, but their descendants and that of the rest of humanity no longer retained any memory of the world prior to a century ago, with one exception. Eight years ago, when Frieda was fifteen she ate her uncle and inherited the Founding Titan and the memories of the old world. This process had been done for a hundred years, passed down through generations of the Reiss family.
Rod tells Historia that with Frieda's power there would not have been any problems. She probably could have eliminated the Titans if she chose, but Grisha had stolen that power away and now it resides in Eren. Since Eren does not have the blood of the Reiss family, Frieda's power cannot be properly used while it is in him.
Hearing this, Kenny interrupts and asks for confirmation that Reiss blood is a requirement. On receiving it, Kenny is upset to learn that it will mean nothing if he eats Eren himself. He grabs Rod and points his gun at his face. Kenny dislikes that Rod has been using him this whole time while knowing what he was after and that he would not be able to get it. Undaunted, Rod tells Kenny that he is still grateful for his service and calls him a stray dog that his younger brother took in. Kenny defends Uri and threatens to blow off half of Rod's head, but Historia interrupts and yanks away Kenny's gun.
She tells Kenny to let her father go and Kenny regards her with exasperation. He calls her pitiful and spells out how her father intends to turn her into a monster so she can eat Eren. Historia does not back down and says she will eat Eren and inherit her sister's power and memories. Kenny asks if she remembers everything Rod has done to her. The only thing Rod cares about is her blood because he does not want to become a Titan himself. However, Rod defends himself, and finally struggles against Kenny, saying that he must not become a Titan and asks Historia not to believe anyone else.
Kenny drops him and Rod frees him from his service. He tells Kenny to go find another purpose and live out the rest of his years. Unmoved, Kenny says that would bore him to death. He climbs the stairs up to the platform where Eren is restrained and says he will no longer interfere, but he will not give the Reiss family any advantages either. Kenny removes Eren's gag and cuts his forehead so Eren is injured. This way he and Historia can fight as Titans. If Historia wins, they will have peace. If Eren wins, things will stay as they are.
Rod panics and puts the syringe into Historia's hands. He explains that it will turn her into a powerful Titan and then she must bite Eren's spine and consume his spinal fluid. Kenny pulls back to watch the show and Rod steps back as well, as he urges Historia to hurry. Historia's expression is fierce as she points the syringe at her arm, her hand shaking, but then she looks up and realizes that Eren has yet to transform. She asks him why and he tearfully explains that this situation should not have happened. They would not be in this position if his father had not come here five years ago.
Eren remembers everyone who had died because Frieda was unable to act; Armin's grandfather, several of their classmates from the Cadet Corps, the original Squad Levi, many people in Stohess District, and the soldiers who died saving him from Reiner Braun and Bertholdt Hoover, including Hannes. He can never atone for it all. Eren tells Historia that he should not have happened, causing her to remember her own relationship with her mother, and he asks Historia to eat him so she can save humanity.
Historia lets him know about the time he called her a normal person and how it made her happy. She prepares to inject herself, but she recalls how Ymir would badger her whenever she tried to do something "nice" for the sake of other people rather than herself. She asks her father why the Reiss family did not do anything about the Titans in the last hundred years. Rod explains that it was the wish of the first Reiss King that humanity be controlled by the Titans, believing that it was the only path to peace. He does not know the reason itself because it is only known to those who inherit the world's memories.
Years ago, Rod and his brother Uri tried to convince their father to free humanity from the Titans, but he would not. When Uri in turn inherited the Founding Titan, Rod understood. His brother had become a higher being, a god, which is necessary in order to govern the foundation of their world. He calls it his mission to bring that god back into their world and takes hold of the syringe to guide it to Historia's arm.
Historia remembers Ymir's words on top of Utgard Castle and her wish for Historia to live a life she is proud of. This causes her to slap away the syringe and it shatters on the crystal floor as Rod looks on in horror. He lunges at Historia, but she throws him over her shoulder and he lands on the floor with a sickening crack. Historia calls him out for being manipulative and refuses to let him kill her.
She grabs his satchel and runs up the stairs to Eren. Historia takes the keys out of her father's bag and begins to free him to his dismay. He urges her to hurry up and eat him so everyone will stop suffering, but she punches him and tells him to shut up. She pulls the first set of chains free and tells him that she wants nothing to do with exterminating the Titans. Let humanity get wiped out. Historia calls herself humanity's enemy and the worst girl who ever lived. She wants Eren to escape and then she will destroy everything else.
Down below, a crippled Rod pulls himself across the floor to the remains of the syringe and laps at the spilled injection fluid, causing him to transform into a Titan.
Currently Publicly Available Information
A device Rod Reiss wants Historia Reiss to use and transform into a Titan. It is believed to use spinal fluid, and by injecting the transformation serum, one is granted the inherent abilities of a Titan.