|Ymir...my real name...is Historia.|
|— Christa Lenz, Historia|
In a 104th training exercise, the cadets notice that three people are missing. In the blizzard, Christa drags Daz through the snow. Ymir realizes that Christa is an illegitimate noble, and sends her ahead whilst she takes Daz. At the base, Christa finds the others had miraculously survived.
At Utgard, Ymir engages the Titans. During the fight, she tears down the tower, and rescues the other recruits. The Titans later emerge from the rubble, and begin to devour Ymir, until the Scouts arrive and cut them down. After the battle, Christa tells Ymir her real name - Historia.
Some years ago, the cadets of the 104th Cadet Corps complete a snowstorm training exercise, arriving at a base at the foot of a mountain. After some brief discussion among the cadets, Reiner announces that Christa is missing. Mikasa says that Christa had chosen to stick with Daz, who was performing poorly, but Reiner says that Daz as well as Ymir have not returned. Marco suggests that she may be with Thomas' squad, but Reiner points out that his squad just completed their course with Ymir, Daz, and Christa nowhere in sight. Worrying for their safety, Eren proposes organizing a search for them, and their training instructor says that a search will begin the next morning. Eren insists that it will be too late if they hold off on the search, but the instructor says that more cadet lives cannot be risked in the snowstorm.
Elsewhere in the storm, Christa and Ymir make their way through a forest with Daz unconscious in a cot pulled by Christa. Ymir says that they will only survive if they abandon Daz and head for base together, but Christa insists that they can all survive if Ymir goes ahead alone while Christa takes Daz. Ymir notices that Christa has not asked for help during their entire course, and she realizes that Christa has no intention of saving Daz nor herself. She says that it would be very against Christa's character to bring about Daz's death while searching for her own heroic end, but Christa insists that such is not the case.
Ymir brings up the fact that she is aware of Christa's true past, a daughter of a nobleman's mistress whose life was at risk. Recalling her days before training, Ymir reveals that while in the interior she overheard the Order of the Walls discussing Christa's existence. Since an illegitimate child could not properly succeed the head of the family, they had considered ending her life, but in the end it was decided that it would be enough to give her a new name and an inconspicuous life, eventually leading to her joining the Cadet Corps. Christa wonders if Ymir joined the Cadet Corps in order to find her, and Ymir proposes that it may be due to her own past being similar. Christa wonders if Ymir originally wanted to befriend her, but Ymir says that they are nothing alike. Unlike Christa, Ymir kept her old name after starting a new life, and abandoning her name would be to admit defeat. By living for herself, she robs her enemies of their power. Ymir scolds Christa for looking for death rather than directing her feelings towards her enemies, and Christa says that there is no longer any way for all three of them to survive their mission.
Ymir leads Christa to a cliff overlooking the base at the foot of the mountain and proposes tossing Daz over the edge so that she and Christa can make it to base alive while Daz can be searched for later. When Christa objects, Ymir takes matters into her own hands and throws her aside into a snowbank.
Ymir wanders off while Christa is submerged, and there is a flash of light in the distance. When Christa escapes from the snow and returns to the cliff, Ymir and Daz are nowhere to be found. Back at base, Eren prepares to head outside in search for the others, and Mikasa and Armin decide to help. Reiner and Bertholdt also decide to join, bringing more equipment for the trip. Conny, Marco, and Sasha agree to come along as well. Suddenly, a loud crash like thunder is heard outside, and the cadets head outdoors to investigate.
In the distance, Sasha spots Ymir dragging Daz through the snow. Some time later, Christa arrives and spots Ymir outside the base. She asks about Daz, and Ymir shows her that Daz is safe indoors. Upon looking at the cliff from the ground, Christa sees that such a descent would be impossible without rope. Ymir decides to reveal to Christa how she saved Daz, but only if she promises to live by her true name when her secret eventually becomes known to everyone.
Years later at Utgard Castle, Ymir leaps into the horde of Titans below. Ymir thinks to herself that she and Christa were the same in their youth, hated by the world and believing that things would be better had they not been born. To bring happiness to many people, Ymir gave up her life, but upon receiving a second chance she decided to live only for herself. Slitting her palm with Conny's knife, she triggers the power of the Titans and transforms into her own Titan form.
Ymir's Titan descends into the horde and begins slaughtering Titans one after another. From atop the tower, the other recruits look on in disbelief at the scene. Reiner and Bertholdt stare in horror as they recognize Ymir's Titan as the one that ate their friend five years ago. Ymir's combat shakes the tower and Christa nearly falls, but Reiner catches her in the nick of time. She thanks him, but in a moment of dissociation he keeps his grip on Christa's ankle, unconsciously squeezing her. After Christa and Conny eventually snap Reiner out of it, he asks Christa if she was aware of Ymir's powers. She claims that she never knew despite how close they were, and Reiner realizes that Ymir possesses knowledge the rest of humanity in the Walls does not. Conny says that Ymir was seemingly aware of her powers, unlike Eren when his Titan form first appeared in Trost. The recruits begin to wonder where Ymir's allegiances lie and what her goals are.
Ymir's fight with the Titans begins to go downhill as she is overpowered. She clutches to the bricks of the tower, but lets go after the bricks begin to slip. The others realize that Ymir does not want the tower to fall, and that she is protecting them rather than escaping. Christa cries out to Ymir, scolding her for her selflessness. She tells her to live for herself and destroy the tower if that is what it takes to defeat the Titans. Hearing Christa's words, Ymir begins to tear the tower apart, throwing bricks and rubble at the Titans. As the tower begins to collapse, Ymir leaps to the top and tells the others to grab onto her if they want to live. Ymir's Titan finds a safe position atop the tower as it collapses on top of the horde, crushing many beneath the rubble.
After the dust clears, some Titans begin to emerge from the rubble. Ymir returns to the fight, but her Titan is quickly overpowered and outnumbered as more Titans climb out of the stone. The Titans rush her and begin to eat away at her Titan, tearing it apart as the recruits stand by unable to help. Christa begins running to Ymir, calling out to her and reminding her of their promise. Christa stops in her tracks when a Titan turns a corner in front of her and reaches out, when suddenly a Scout flies by and slices the Titan's nape. The Scout, Mikasa Ackermann, tells the other recruits to stand aside while the Scout Regiment finishes the fight.
Eren flies ahead against orders and slices the nape of a Titan, his first official kill as a human soldier. Keiji scolds Eren for rushing ahead, and the other recruits rush to Eren's side to reunite with their friend. In little time, the ruins of Utgard are cleared of all Titans. When the battle is over, the newcomers are shocked to learn of Ymir's Titan powers. Christa comes to Ymir, her limbs missing and her body grievously wounded from her Titan battle, and she keeps her promise. Christa reveals her true name, Historia, and Ymir slips into unconsciousness with a warm smile.