Episode 03: The Door of Hope
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Credit for awesome summary Attack on Titan Season 4 Wikipedia.
Reiner thinks about his past and a conversation he had with his mother on the curse of being an Eldian. He entered the military training program where selected candidates were chosen to become Titans. Porco Galliard was devastated when Reiner was chosen to be the Armored Titan.
Commander Magath sent Annie Leonhart, Bertholdt Hoover, Marcel Galliard and Reiner on a mission to Paradis to reclaim the Founding Titan, but Marcel, saving Reiner, is devoured by Ymir's Pure Titan.
Following the assault on Wall Maria, they sneaked into Wall Rose and joined the Training Corps two years later.
Emerging from recollections of his past, Reiner attempts suicide, but stops after hearing Falco outside. He then tells himself that he needs to live because of his role as a mentor to Falco and the other Candidates.
Falco stops by the hospital and has a conversation with an Eldian soldier who admits to faking his trauma-induced amnesia to avoid being sent back home.
NOTE: This episode is from Volume 24 of the manga.

