Portal: Prelude 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, but also take responsibility for many other tasks within the complex and compete with Black Mesa's superiority. The v1.1.0 is shipped with more than 80 bug fixes and additions, along with slightly easier/less frustrating first levels. Savegames are not compatible - your progress with chapters unlocking is saved, but not savegames.