Long story short, Eren, Mikasa, and Armin made it into the Survey Corps. After being eaten by a Titan and turning into a Titan, Eren is recruited into a special operations force led by Levi Ackerman. Even in the face of various difficulties, Eren's ambition to finish off the Titans remains unshaken.

After Eren finds Grisha's basement, he realizes the fact that he is an Eldia, the original Titan. Marley's modern civilization moved the Eldians to the island of Paradis and brainwashed them. The Titan that Eren and his friends have been dealing with is the "villain" Eldia who was injected with Titan serum from Marley!