Lost in the Cruel World (進撃の巨人 Lost in the Cruel World Shingeki no Kyojin Lost in the Cruel World?) is a visual novel by Hiroshi Seko. It was first released on September 18th, 2013 in Japan bundled with the third anime Blu-ray. The story was also included in the novel Attack on Titan: Lost Girls which was later adapted into a manga of the same name.
During the battle of Trost District, Armin Arlert informs Mikasa Ackerman that Eren Yeager has been killed in battle alongside his comrades. With a lost sense of purpose, Mikasa speeds through the city killing Titans until her vertical maneuvering equipment runs out of gas, sending her falling to the ground below.
As she lies in a daze, having fallen onto a shop awning, Mikasa is filled with regret at her inability to save Eren and distaste at the biting cold brought by a light rainfall. Looking up at the gray sky, she sees a white butterfly fluttering across the battlefield. She grows upset, blaming the rain for the tragedy of that day since hardships in her life have always been accompanied by rain. Before her grief can consume her, she stops herself from thinking.
In the year 844, Dr. Grisha Yeager and his son Eren visit the Ackerman family. While Dr. Yeager checks on Mikasa's pregnant mother, Eren and Mikasa play indoors. Eren expresses his desire to join the Survey Corps and explore the Outside World, and together they go outside. In the woods, Eren criticizes Mikasa for her lifestyle, comparing her to a chicken kept in its coop, content to live until she is killed. Their conversation is cut short when they hear the barking of wild dogs. The dogs retreat shortly after they arrive, leaving several corpses behind. Eren and Mikasa return home and inform their parents of the bodies, learning shortly afterward that they were robbers, causing Mikasa's parents to grow worried.
Eren continues to accompany his father during checkups on Mikasa's mother every ten days, spending time with Mikasa each time. He tells Mikasa of his life in Shiganshina District with his close friend Armin Arlert. Mikasa grows fond of Eren and hopes for continued visits. Eventually, her mother catches a fever and the Yeager family's visits increase to every five days. Grisha and Eren continue to visit the Ackerman family for two seasons. One day Eren arrives with wounds from a fight, and he tells Mikasa of his plan to see the Outside World.
Some time later, Mikasa is sent to Shiganshina to return a hat Dr. Yeager had forgotten at the Ackerman residence. In Shiganshina, Eren takes her to the Heroes' Parade where the Survey Corps will return. When the Corps returns, it is in shambles, and Mikasa watches as a soldier delivers the dismembered hand of another soldier to his grieving mother. Eren hits a citizen speaking against the Survey Corps, causing him to get beaten by the man while Mikasa watches helplessly. Another citizen keeps Mikasa from interfering, assuring her that Eren deserves his punishment. Eren is taken care of by his father, but Eren makes Mikasa promise not to tell anyone about why he was beaten.
Grisha takes Mikasa back to her home, and he understands why Eren must have gotten himself into the situation. Grisha informs Mikasa that he believes she is the only one who can protect Eren from his own reckless dreams. During Grisha's subsequent visits to the Ackerman household, Eren is not present. Mikasa writes a letter to Eren, but she receives no response. During one of Grisha's visits, it is discovered that Mikasa's mother's condition has worsened. Grisha advises them to move to Shiganshina so that they can receive his immediate care in case of an emergency.
In Shiganshina, Grisha informs Mikasa that the Garrison had reported Eren's attackers as deceased, killed in a carriage accident. Grisha assures her that the men had it coming, and his words linger in Mikasa's mind. As she ponders this, Eren finally reunites with her. Eren informs her that the Survey Corps has been disbanded permanently and the Walls have been sealed. Because of this, he and Armin built an "Airplane" out of a prototype left behind by Armin's deceased parents to go to the Outside World alone. Mikasa worries for Eren's life, and Eren gives her a caged butterfly and his scarf before he leaves.
When the time for Mikasa's mother to give birth to a new child draws near, she stays with the Yeager family. As Mikasa worries over Eren, she sees another reality where her parents are killed. When she snaps back into focus, she finds herself amidst a festival in Shiganshina. In the midst of the festivities, she is picked up by a hypnotist calling himself the "Mirror Man," claiming before his audience that he will make her kill him. When Mikasa shows hesitation, the Mirror Man tells her that she must kill him if she wants to see Eren again and go back to where she belongs. Suddenly, an explosion echoes through the district. When Mikasa returns her focus to the Mirror Man, she finds him lying motionless. He gets up, announcing his success and revealing his death to be an elaborate stunt.
Mikasa leaves the festival, returning home to see her newly born baby brother. Outside of the house, Mikasa is met by Armin, who informs her that the Airplane did not fly. He tells her that Eren died in his place in the crash, and he apologizes through his tears.
Back in the chaos of Trost, Mikasa opens her eyes to the gray sky. The light rain ceases, and the white butterfly disappears with it. Feeling as if she had awoken from a long dream, she realizes that it is in Eren's nature to fall into death's grasp despite her efforts. Her grief sets in, and she laments having lost her family again. Setting herself on the ground and kneeling in the street, she decides to surrender her life to the Titans. But as a Titan reaches for her, she defends herself and wards off the Titan despite not knowing why.
As another Titan appears behind Mikasa, she remembers Eren's words to her when his life was nearly taken by one of her kidnappers. Vowing to fight in memory of Eren, she lets out a battle cry.
As Mikasa prepares to fight, the Titan behind her swings its fist at the one in front of her. Staring in awe at the sight before her, an embodiment of humanity's fury, Mikasa wishes on the rogue Titan for her to see Eren again.