You are a scientist. One day you are experimenting with making portals, you enter and fall in the fields. Incidentally, right in front of you is a group of soldiers. Yes, they are the Survey Corps who are on expeditions outside the walls.

A man who almost bumped into you dismounted from his horse and came closer to you "Are you okay? What are you doing here?" He is Erwin Smith. You're off the wall, and he doesn't like someone off the wall.