Portal: Prelude, as its name implies, is an unofficial prequel to the game Portal. Its story revolves around the pre-GlaDOS epoch way before she was plugged in. Back then, test subjects were still monitored by real Aperture Science employees, a tedious, dull, and repetitive job. One of the reasons why they decided to build a great artificial intelligence that could both replace them for these complex tasks and also take responsibility for many other functions within the complex while competing with Black Mesa's superiority. All employees of the Aperture Science complex are now eagerly awaiting GlaDOS. A little too eagerly maybe, as the unfolding 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 dialogue with dozens of subtitles languages.
If I were to give Portal: Prelude any credit, I'd say it was very ambitious, and I'd congratulate the developer for finishing it.
However, the mod is not entertaining. It has terrible voice acting for one, because the creator himself admitted he could not be bothered to find actual people. In addition to terrible voice acting, the characters that need the voices are all very badly written. They have an unecessary amount of malice for the player; while that was funny in Portal, it lacks any semblance of actual humor in Prelude.
Prelude's gameplay is also deplorable. The developer preps the player, telling them to prepare to "think with portals", but the mod is anything but that. The solution to the puzzle is often glaring you right in the face, whereas the actual challenge is just trick jumps that require perfect precision. The developer defends his decision to remove what made Portal so fun by saying "it would have taken too long to make actually challenging puzzles", and while I don't have a source in front of me, that is literally how I remember him saying it.
The scale of this mod is impressive, but overall it's not worth playing.