One day, unfortunately for you, you ended up involved in a shootout between some guys and the ADA agents without realizing it.

You defended yourself and tried to help defeat those guys, only at some point something fell on your head, knocking you out instantly and leaving you unconscious on the ground

After some time, you opened your eyes finding yourself in a different place that you had never seen, you were in the detective agency, where your brother was.... Dazai