Portal's plot is revealed to the player via audio messages from GLaDOS and side rooms found in the later levels. The game begins with Chell waking up from a stasis bed and hearing instructions and warnings from GLaDOS about the upcoming test experience. This part of the game involves distinct test chambers that, in sequence, introduce players to the game's mechanics. GLaDOS's announcements serve not only to instruct Chell and help her progress through the game, but also to create atmosphere and develop the AI as a character. Chell is promised cake and grief counseling as her reward if she manages to complete all the test chambers.
A typical Portal test chamber, with both of the player's colored portals opened. The Weighted Companion Cube can also be seen.
Chell proceeds through the empty Enrichment Center, interacting only with GLaDOS. Over the course of the game, GLaDOS's motives are hinted to be more sinister than her helpful demeanor suggests. Although she is designed to appear helpful and encouraging, GLaDOS's actions and speech suggest insincerity and callous disregard for the safety and well-being of the test subjects. The test chambers become increasingly dangerous as Chell proceeds, and GLaDOS even directs Chell through a live-fire course designed for military androids as a result of "mandatory scheduled maintenance" in the regular test chamber. In another chamber, GLaDOS boasts about the fidelity and importance of the Weighted Companion Cube, a waist-high crate with a single large pink heart on each face, for helping Chell to complete the chamber. However, GLaDOS then declares that it "unfortunately must be euthanized" in an "emergency intelligence incinerator" before Chell can continue. Some of the later chambers include automated turrets with child-like voices (also voiced by McLain) that fire at Chell, only to sympathize with her after being disabled ("I don't blame you" and "No hard feelings").
After Chell completes the final test chamber, GLaDOS congratulates her and prepares her "victory candescence", maneuvering Chell into a pit of fire. As GLaDOS assures her that "all Aperture technologies remain safely operational up to 4,000 degrees [sic] kelvin", Chell escapes with the use of the portal gun and makes her way through the maintenance areas within the Enrichment Center. Throughout this section, GLaDOS still sends messages to Chell and it becomes clear that she has become corrupt and may have killed everyone else in the center. Chell makes her way through the maintenance areas and empty office spaces behind the chambers, sometimes following graffiti messages which point in the right direction. These backstage areas, which are in an extremely dilapidated state, stand in stark contrast to the pristine test chambers. The graffiti includes statements such as "the cake is a lie" and pastiches of Emily Dickinson's poem "The Chariot," Henry Wadsworth Longfellow's "The Reaper and the Flowers," and Emily Brontë's "No Coward Soul Is Mine," mourning the death of the Companion Cube.
GLaDOS attempts to dissuade Chell with threats of physical harm and misleading statements claiming that she is going the wrong way as Chell makes her way deeper into the maintenance areas. Eventually, Chell reaches a large chamber where GLaDOS's hardware hangs overhead. GLaDOS continues to plead with Chell, but during the exchange one of GLaDOS' personality core spheres falls off; Chell drops it in an incinerator. GLaDOS reveals that Chell has just destroyed the morality core, which the Aperture Science employees allegedly installed after GLaDOS flooded the enrichment center with a deadly neurotoxin, and goes on to state that now there is nothing to prevent her from doing so once again. A six-minute countdown starts as Chell dislodges and incinerates more pieces of GLaDOS, while GLaDOS attempts to discourage her both verbally with a series of taunts and increasingly juvenile insults and physically by firing rockets at Chell. After she has destroyed the final piece, a portal malfunction tears the room apart and transports everything to the surface. Chell is then seen lying outside the facility's gates amid the remains of GLaDOS. The final scene is changed through a patch of the PC version that was made available a few days before Portal 2's announcement; in this retroactive continuity, Chell is dragged away from the scene by an unseen entity speaking in a robotic voice, thanking her for assuming the party escort submission position (a reference to GLaDOS's request that she assume this position after escaping).