Aperture Science is in ruins after the events of Portal and Portal 2 (And the Co-op story), and the entire facility becomes destroyed. glaDOS is left fixing the place left, right and centre. Unluckily, or lucky for the player, a human falls into Aperture Science.
A sneak peek into the Ruins. I'm not fantastic at Hammer, but this is the type of style I want to have, but obviously, with everything looking a lot nicer.
The main gist is, that everything will be quite dark, as the facility hasn't been used in years.