Gamma Energy visits the Aperture Science East facility, built after GLaDOS gained sentience. The goal was to create a facility where GLaDOS' research could be examined and tested without harm to employees or other people who actually... mattered, without her knowledge. Pick up a prototype Portal Device and try not to die.
This review may contain spoilers
Excellent concept and puzzles. Unlike other Portal mods (such as Portal: Prelude), I love how this game was actually puzzle-based, instead of skill/time based. I like a game where you are forced to sit down and think. I completed this game checking the walkthrough only once, so the difficulty was spot-on, in my opinion.
My major complaint comes from the story and glass shooting objectives. While the story concept was brilliant, I feel there is so much potential gone to waste. I didn't completely understand why Chell died (although, I assume she 'self-destructed') and the backstory was underdeveloped. Still good, though. Second, I didn't like the glass shooting objectives because I felt like I was cheating the system. It is okay outside the chambers, but not inside.
All in all, really fun mod. Difficult puzzles keep the game popping, but an underdeveloped story drags it down. I'd highly recommend this mod. One of the best I've played!