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 who's 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, but 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...
To Install: All you need to do is to install Portal: Prelude to your "SourceMods" directory, inside of the "SteamApps" directory of Steam. Be sure to restart Steam after the installation. This will allow Steam to update your game list and include Portal: Prelude in it. Also it is mandatory that you have the source engine before installation.