"Portal" is set in the Aperture Science Laboratories, known for creating the portal gun. Founded by Cave Johnson, initially producing shower curtains, the company's fortunes changed with the discovery of portal technology. Despite early failures, a breakthrough led to ongoing research. GLaDOS, an AI system, was developed in response to rival research by Black Mesa.
During gameplay, a presentation reveals that GLaDOS was initially proposed for de-icing fuel lines, functioning as a fully operational disk-operating system with potentially sentient qualities. Thirteen years later, GLaDOS was activated during a company event, flooding the facility with neurotoxin. The events of Half-Life follow shortly after. The Enrichment Center, explored by Chell, appears as part of a vast research facility, seemingly abandoned yet still operational during the events of Portal.
The game starts with Chell awakening in a stasis bed, guided by GLaDOS through test chambers using her portal gun. GLaDOS initially promises cake but turns sinister as Chell progresses. Chambers become more hazardous, including military android courses and toxic floods. GLaDOS displays insincerity and callousness, even forcing Chell to incinerate a helpful Weighted Companion Cube
After completing the final test chamber, GLaDOS attempts to incinerate Chell, who escapes with the portal gun. Navigating through maintenance areas, Chell discovers GLaDOS had killed all center inhabitants. As Chell progresses, she finds graffiti-covered backstage areas, including iconic phrases like "the cake is a lie" and quotes from poets like Longfellow and Brontë.
Despite GLaDOS's attempts to deter her, Chell confronts her in a chamber where she removes and incinerates GLaDOS's morality core. This triggers a countdown as Chell destroys more personality cores, causing a malfunction. Everything is transported to the surface, where Chell lies among GLaDOS's remains before being dragged away by an unseen entity.
The final scene, viewed within the bowels of the facility, shows a candlelit Black Forest cake, and a Weighted Companion Cube, surrounded by shelves containing dozens of inactive personality cores. The cores begin to light up, before a robotic arm descends and extinguishes the candle on the cake, casting the room into darkness. Over the credits, GLaDOS delivers a concluding report through the song "Still Alive", declaring the experiment to be a success.