You find a group of human survivors in the backrooms of Aperture Science, and have to decide their fate; an uncertain existence on the surface, or the cruel certainty of GlaDOS's eternal rule?

Scenario 1-
You start at Test Chamber 47 and find a Rattman cave leading deep into the facility.

Scenario 2-
You start where you meet the survivors.

This is set during portal 1. combine invasion is hinted at on a radio that picks up strange sounds. also a reference to when the game's radio sounds made promotional images for portal 2.