Grandfather Arlelt kept a secret book which contained forbidden knowledge of the world beyond the Walls, with descriptions of fiery waters and sandy snowfields. Eventually, the man gave this book to his grandson Armin Arlelt. When he was nine years old, Armin showed this book to his friend Eren Jaeger, inspiring him to see the outside world.
When Shiganshina District is attacked by the Colossal Titan in the year 845, Armin Arlelt and his grandfather escaped. When Eren and Mikasa run to their house in the carnage to see if their mother is safe, Armin warns Garrison soldier Hannes of their peril, saving them from being eaten by Titans.
One year after Shiganshina District is overrun by the Titans, the royal government issues a culling disguised as a reclamation expedition. In this expedition, Armin's grandfather is killed, and Armin grieves for him. Armin would spend the next year with his friends Eren and Mikasa, working the fields to provide food for the people of the Walls.
During training, Armin shows little physical strength but excels in classroom with intellectual skills.
After graduation, on the same day Armin is nearly eaten when Trost District is breached by the Colossal Titan and many Titans enter the city. He is saved by Eren, who is thought to have died in the process. However, it is later discovered that Eren survived due to his Titan ability. With this knowledge, Armin formulates a plan to save Trost with Eren's powers, and with the help of the cadets and the Garrison, the plan succeeds.
Armin discovers the identity of the Female Titan after it invades the 57th expedition, to be in fact his former cadet comrade, Annie Leonhart. After bringing this information to Commander Erwin Smith, Annie is successfully captured in Stohess District.