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.
Portal: Prelude and Blue Portals apparently have something in common: They both revolve around fling puzzles instead of puzzles that actually make you think hard. The difficulty in Portal: Prelude is mainly caused by this excessive amount of momentum-focused test chambers. However, Portal: Prelude is one of the few mods for this game to actually have an interesting story. The story is the greatest part of this mod, and is worth playing for that reason.