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.
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 story revolves around the pre-GLaDOS epoch, even before she was plugged in. At this time, test subjects were monitored by real Aperture Science employees, whose work was tedious, lengthy and repetitive. This is why they decided to build a great 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 superiority. All employees of the Aperture Science complex are now eagerly awaiting GLaDOS. Maybe even a little too eagerly...
It all starts with a test subject (named Abby) going through the 19 test chambers, which 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 that a party will be held in her honour.But, GLaDOS is also being activated at the same time (even though her morality core isn't prepared yet), so, as a special treat, they invite Abby to go and watch from a secret platform.
However, soon after awakening, GLaDOS proceeds to kill all the people in the chamber, and attempts to kill Abby too, with a mixture of telekinesis and death rays. However, the remaining survivors manage to shut off the generators. Knowing they have only a little while before the emergency generators kick in, they tell Abby to get the morality core and install it before she kills everyone. After navigating through a now destroyed Aperture Science, she finds the core and manages to attach it to GLaDOS, just as she had begun flooding the place with a neurotoxin.
Abby then blacks out, and, when awakens, is seen being dragged out of the chamber by a female Aperture Science employee. The screen then fades, and the final cut-scene is that of Abby 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 can see is the G-Man, standing in the booth and adjusting his tie.