GLaDOS, the AI in charge of Aperture Science, wakes up her newest test subject Chell to put through a set of chambers using the Aperture Science Handheld Portal Device to solve various logic problems. Chell proves to be a decidedly single-minded and determined woman who refuses to so much as speak a word back as she makes her way through each chamber, even as GLaDOS’ instructions become increasingly passive-aggressive and mocking about the likelihood of her death.
As Chell becomes more skilled with the portal gun and learns tricks like using momentum from a long fall to cover distance horizontally, GLaDOS replaces one test chamber with a live fire course for military androids, filled with simple-minded and high-voiced turrets that provide an extra emotional torture with their childlike responses to being destroyed. Chell isn’t deterred and plows through them all, on the way discovering a space behind the walls where a previous subject had been living, covered in graffiti stating that GLaDOS has been lying and is just using the tests for her own sadistic amusement.
Another chamber requires Chell to carry a heart-covered Companion Cube throughout the entire puzzle, putting it to all sorts of uses until there is more of a bond formed with it than the regular single-use cubes. Then GLaDOS refuses to let her move on until she “euthanizes” it in a furnace, further twisting the knife by claiming she did it faster than any test subject had ever done it.
The final chamber makes use of almost every trick Chell has learned during the testing, only to end with her being sent into a massive fire pit. She manages to escape with a quick set of portals, and ventures into the facility's maintenance corridors despite GLaDOS pleading her to come back.
