Quick synopsis of the video while it was available:
It starts with some shadows of mechanical parts obviously being assembled and/or welded together. We quickly recognize the voice that begins to speak as obviously being that of GlaDOS from the Portal series. Scenes from the machine's assembly (quickly becoming apparent that it is of some kind of larger robot) continues while GlaDOS talks.
GlaDOS:
"I guess it was inevitable. After all, we all evolve in one way or another. Some, obviously more than others. This was just the next logical and necessary step. Besides, they say that the true test in life is the test of one's self. That, and if you want something done right, you have to do it yourself."
GlaDOS continues talking while the scene quickly changes to focus (slowly zooming out) on mostly the upper portion of GlaDOS ... AS A LARGE ROBOT (what seems to be about twice the size of P-Body and Atlas):
"It's time to venture out into the real world."
After a short moment, a surprise voice talks from off screen.
Unknown Voice:
"Let's do this!"
The scene's view quickly snaps out to completely show GlaDOS and, standing next to her (obviously, what was the unknown voice), Chell!
GlaDOS turns her head down at an angle toward Chell giving her a look of sort of like "WTF."
Chell first turns her eyes, and then head, toward GlaDOS and says: "What?"
The scene quickly changes to a slightly more zoomed out view and from off of each of the left and right sides of the screen both P-Body and Atlas lean inward at start to stare at Chell for a moment.
P-Body and Atlas both turn to look at each other while GlaDOS and Chell both look toward P-Body and Atlas. Then, they all look out toward the viewer of the video.
What happens next is a compilation of video clips being what appears to be cutscenes and/or gameplay footage from the game.
Quick rundown:
A scene during a clear night showing a Combine Soldier getting sucked through an extra large portal large enough for GlaDOS to easily go through if she wanted.
A scene showing a four-legged turret "walking/patrolling" an area.
A scene showing a large metal beam falling (after apparently hanging vertical from a chain) straight down a portal and shooting out sideways through the other portal and smashing into a vehicle.
A scene showing GlaDOS spraying an area with propelsion gel and saying, "Well, that's the last of it for now."
A scene showing GladDOS and Chell talking while P-Body and Atlas are waving trying to get their attention to look at the extra large portal (from earlier) in the background since something appears to be climbing through. We hear "Wheatley" (from Portal 2) mumbling as he climbs through and GlaDOS taking notice and saying, "You have got to be kidding me." Wheatley appears as what can sort of be called a robot form of his own as what appears to be made from different parts of previous moon-landing vehicles. Wheatley says, "Oh my goodness, am I glad to see all of you!
A quick scene being what appears to be high-energy pellet being fired from GlaDOS' chest directly at a Strider (a "tripod" from Half-Life 2).
The scene changes to black with the word "Portal" coming into focus quickly followed by an image of a portal with the number 3 in it.
GlaDOS' voice is heard one more time saying, "Dare I say it? This is actually quite fun."
Okay, not really, but wouldn't that have been cool?
