It appears that portal-packing protagonist Chell -- much like the acerbic AI antagonist of the first Portal game -- is still alive. After having her near-lifeless body dragged away from the ruins of Aperture Science, she awakens in something being called a "rehabilitation" center. To keep her mind sharp and her heart at ease, a computer voice will tell her to perform a few exercises: look up and look down (to set your invert preference, obviously). She'll be told to find "art" and stare at it. And then, it's time to sleep. Fade to black.
Chell awakens to find that she's been in stasis for years, evidenced by the peeling wallpaper and the body-shaped recess formed in her mattress. Outside her door, there's a banging noise and the panicked voice of Wheatley, a robotic employee of Aperture who is trying to escape from the nearly-destroyed facility. An on-screen indicator tells you that you can speak to him by pressing the A button. Does Portal 2 introduce new context sensitive actions? The answer is no: your character jumps, instead, proving to Wheatley that you may not be intelligent, but you are at the very least "a good jumper." That should come in handy for the task at hand: finding a gun that shoots portals -- it may be the only way out of the doomed laboratory.
Wheatley is able to move your housing unit along a rail system, not unlike the ones seen in Half-Life 2's Citadel. As it flies through the facility, the walls start to crumble. As you peek out, you'll see that Aperture is a much larger place than anyone could have imagined. There's only one way to break back into the portal testing facility: use your former home to literally crash-land into GLaDOS' home turf.
You'll tumble back into the holding chamber from Portal's now-iconic introduction. As you look through the glass walls, you'll see that you've made quite a mess: the walls are falling apart, and vegetation seems to be "taking back" the chambers that you've ruined. GLaDOS is absent for now. You did murder her, after all. Instead, a male robotic announcer tells you over the radio that since your last visit, a potential world-ending catastrophe has occurred. (Combine, perhaps?) Still, for the sake of science, you must resume testing!
The first few chambers you visit will quickly bring back memories of the first game. How can you not feel nostalgic as you place your first box on a red button switch? Hearing the side-effects of the Material Emancipation Grid once again brought a quick grin to my face. The familiarity is painted with a twinge of horror: the decaying facility's crumbling walls, and the knowledge that GLaDOS will want revenge, make this return trip immediately stomach-wrenching.