|Manga||"To You, 2,000 Years from Now"|
|Anime||"To You, in 2000 Years -The Fall of Shiganshina, Part 1-"|
At the beginning of the story, Dr. Yeager is living in Shiganshina District with his wife, Carla Yeager; his son, Eren Yaeger; and his adoptive daughter, Mikasa. He keeps the basement of his house locked and doesn't allow anyone else inside. It may hold the secret to the origin of the titans and some humans' ability to transform into titans.
After the fall of Wall Maria, a tearful Dr. Yeager injects a serum into Eren's left arm, which Eren believes is responsible for giving him the ability to shift into a Titan. Dr. Yeager tells Eren that the serum will cause memory loss and gives him the key to the basement saying that all of the answers can be found there.
It was thought that Dr. Yeager was missing for five years. But recently Eren had a flashback and remembered turning into a titan, after Dr. Yeager injected him with an unknown substance, and then eating Dr. Yeager. It is thought that Dr. Yeager was a titan shifter and when Eren ate him he gained his ability.
Though it is unknown the origin of titans it is be lived that doctor Yeager was the one who created such thing,this theory lacks any evidence.
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