This article is about a mod.
This article contains spoilers. Read at your own risk!
It might spoil your gaming experience when first playing the related games.
The story revolves around the pre-GLaDOS epoch, even before she was plugged in. At this time, test subjects were monitored by human Aperture Science employees, whose work was tedious, lengthy and repetitive. This is why they decided to build a massive artificial intelligence that could both replace them in these difficult tasks, and also take responsibility for many other tasks within the complex and compete with Black Mesa's project. All employees of the Aperture Science complex are now eagerly awaiting GLaDOS activation. Maybe even a little too eagerly, as the upcoming events will tell.
The game starts with the player taking control of a test subject named Abby, guided by several scientists, Mike, Erik and Peter as she goes through 19 heavily difficult test chambers that are considerably harder and more lethal then the ones of the original Portal (for example, turrets are encountered in the second chamber, whereas in the original Portal they don't appear until the 16th). After completing the test chambers, she is congratulated by the staff as being the first test subject to do such a feat, and is invited to a party that has been held while she was in the chambers, setting it in her honor and making a cake. Abby makes it to the party and meets Mike and Peter in person, but by the time she arrives, Aperature Science is in the near process of activating GLaDOS, so the party is cut short, as a special treat they invite Abby to go and watch GLaDOS' activation moment from a control room of her chamber, after which they will resume the party.
Abby arrives and witnesses GLaDOS' Anger Core being installed, with the Intelligence and Curiosity Cores having already been installed. She is told to flip a switch, which activates GLaDOS. However, soon after awakening, GLaDOS proceeds to kill all the people in the chamber and take control of the facility, she attempts to kill Abby too, with a mixture of anti-gravity force and lasers. Fortunately however, Mike managed to shut off the generators. Knowing they have only a little while before the emergency generators kick in, Mike informs Abby that both Erik and Peter along with a majority of the scientists in the facility have been killed by the Rocket Sentries and Manhacks infesting the facility, GLaDOS in the state she is currently in is practically unstoppable, but in a last attempt they can install the morality core into GLaDOS before she kills everyone, hopefully changing her moral decisions and ceasing the attack on all the people in the facility, Mike leaves the task up to Abby, before Mike leaves off, a Rocket Sentry can be heard in the background, presumably killing him. After navigating through a now-corrupted Aperture Science, she finds the core and manages to attach it to GLaDOS, however it does not make any change to GLaDOS' morality and she continues the attack, she grabs Abby with the anti-gravity force and slams her into a wall, knocking her unconscious and almost killing her.
Abby then briefly wakes up to see a scientist trying to reason with GLaDOS, but is immediatly killed with a laser just before she floods the room with neurotoxin. Succumbing to the neurotoxin, Abby blacks out, and awakens to being dragged out of the chamber by a female Aperture Science employee. In the post credits scene, an incredibly weak Abby is seen walking through the parking lot of Aperture Science (where it is now raining), collapsing, and (presumably) dying, as the screen fades to black. The last thing she see's is the G-Man, standing in the booth and adjusting his tie. Her fate is left unknown.
Portal: Prelude is a whole new story where the player takes the role of Abby, a Test Subject (whose model mostly consists of a blonde Chell model with other small changes), monitored by Aperture scientists with British accents who tend to make humorous comments. The game offers a few more gameplay hours than the original Portal, with 8 chapters, 19 Test Chambers, 48 challenges, 6 advanced maps, a brand new storyline and more than 400 lines of speech with various languages (as subtitles). The puzzles are also much more difficult than in the original Portal.
- G-Man is also seen many times in this mod, possibly to warn Aperture Science of unforeseen consequences.
- As the addon was created before Portal 2, no reference is made to Cave Johnson or Caroline.