Setting Seeds: Life After People
The PCs awaken in a bright white room, sparsely furnished. They have no idea who they are or how they got there. They are wearing simple white tunics and pants, and there is no identification of any kind.
After a few moments, as they get to know each other, a seam appears along one of the walls, and a door opens with a hiss.
Eventually, the PCs discover that they’re in some sort of obstacle course. It starts off somewhat innocuous, mostly requiring physical abilities like climbing and jumping. It soon begins to turn more dangerous, however, as automated turrets, lasers and vats of acid turn up as new obstacles.
The PCs might eventually discover areas where the facade of the obstacle course is broken; panels they can crawl behind, observation windows they can reach, etc. There is a great deal of computer equipment actively logging their progress, but nobody monitoring them. Eventually they find a factory where robots are being assembled– robots identical to themselves.
This facility was an automated testing grounds created to develop the ultimate robot soldiers. Humanity destroyed itself some indeterminate time ago, but the underground bunker housing this place was left untouched, and was specifically created to be self-sufficient. The PCs may be the first of their kind to escape the facility, forced now to make their way as the only intelligent beings on the planet, or they may find sporadic groups of earlier robots which escaped.

the cake is a lie.
Hmm… very interesting. I mirror Anonymous on this one: good take on Portal. Another lead off might be that they discover other people in the facility and lead the fight to escape.
I don’t know what you guys are talking about. The cake is alive.
Ah! Cake Mimic! Final boss for Glados.