While in a coma, Kara (Supergirl) is stuck in a neural construct, of her making, to help her deal with the trauma of losing badly to Reign. Brainiac-5 shows up as a pathfinder to help lead the way back into the real world.
Watch the full episode here http://www.cwtv.com/shows/supergirl/legion-of-superheroes/?play=c302961e-74cb-4c78-b608-49fbcf8dc491
