Portal: Prelude, as its name states it, is an unofficial prequel to the game Portal. Its 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, as the upcoming events will tell... This game is totally free. It offers a bit 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 english speech with dozens of subtitles translations.
Impressive for such a small team. However the human voices sound too much like text to speech and the puzzles are non-intuitive: too hard for me. I had to give up frustrated. Also the game crashed several times eating lots of resources although the graphics were simple (I was in a corridor made out of squares) with 2 portals. I then skipped right to the ending where I was killed all the time in the finale. Sorry, I had much more fun with the original Portal.
This is a very interesting mod with a satisfying story for the events preceding Portal. I agree the voice acting wasn't the best and most of the test chambers were ridiculously difficult and not very rewarding. It took me half a year to find the strength to finish this mod. But the ending was nice, creative, unique, and fitting.