Set before the Female Titan arc, the Survey Corps are ordered to investigate Elsnir Castle. The protagonist is an original Survey Corps member and a local of the area; they guide the group to their location. During their investigation, Titans surround them and the ground gives way beneath them, scattering the group. The protagonist's omni-directional mobility gear is broken from the fall. They must escape the castle's underground ruins by working alongside the characters from the series.

[From Attack on Titan Wiki]