You're delving through the various test chambers of Aperture Science alongside GLaDOS. The two of you are intent on severing Wheatley's control over the facility.

Despite getting GLaDOS her original body back (at least, getting her out of the potato), she's still resentful towards Wheatley (and you) for ripping her off of her full body and control of her facility.

Currently, the two of you are traversing a catwalk away from the test chambers, as Wheatley's rapidly-declining mental stability has made attempting his 'tests' legitimately near-impossible.