Attained in "Mission 4: Into the Abyss" at the end of "Chapter 2: Recovering BT". Story Related and can not be missed.
*** Spoiler - click to reveal ***A little after the part where you ride the platforms, which become houses, you will need to jump off. Just continue to follow waypoint to the objective. Once you've reached the far catwalk, you'll get an achievement for leaving the sideways town AKA "Incepted".