The add-on brings a new six-map campaign with a fully voiced GLaDOS as well. Companion elements and custom in-game media as well.
From the description:
You are a test subject in the Aperture Science lab who winds up in another "puzzling" situation. You are also in a race against an ever wicked "test dungeon master" trying to make life hard and success a relative term.
What's up with GLaDOS anyway ? Has she gone mad ? She had the cooperative testing initiative yet choses to let you break your heart and head over an inanimate companion testing element time and again.