Roughly a century before the beginning of the series, a mysterious race of giant, man-eating humanoids, known as "Titans", suddenly appeared and nearly exterminated humanity. In order to protect themselves from this threat, the few survivors built three concentric Walls, called Maria, Rose, and Sheena, and encased themselves in this limited territory, forgetting everything about the outside world and the history before the building of the Walls.

In the present, a boy named Eren Yeager and his childhood friends Mikasa Ackerman and Armin Arlert dream about seeing the world outside the Walls some day; but their peace is abruptly interrupted when an unusual 60-meter tall Colossus Titan and an Armored Titan breach the outermost Wall, and Eren sees his mother dying in the resulting devastation. Eren vows revenge against the Titans and later enlists in the military branch Survey Corps, accompanied by both Mikasa and Armin.