Portal Stories: Mel is a massive mod for Portal 2 that will be released on Steam on June 25. Representing four years of work by a dedicated team of modders, it contains five chapters, a custom story and original voice work, lots of new assets, and of course a buttload of new test chambers and levels to explore and solve. In other words, it's entirely new Portal game.
The mod begins with a little Half-Life flavor (a long tram ride) and then a scene reminiscent of Bioshock: a stroll through a town built by Aperture Science to house its employees. It's 1952, when Aperture is still a plucky startup with a bright future, and the facility beneath the town is filled with scientists, though unfortunately they're all out of reach so you can't actually interact with them.
Aperture being Aperture, it's not long before something goes horribly wrong. Having been preserved in a cryo-bed, you awaken to find the facility in a shambles. The environments are massive and incredibly well designed with tons of detail, and there are a few professional-looking cinematic sequences. The mod assumes you know what you're doing when it comes to portals: there's no slow build-up to the puzzle-solving as in the Portal games themselves. You get the portal device and are immediately faced with some challenging rooms to solve, and they remain tough for most of the game.
As a Portal player, I'm a bit more fond of the types of puzzles without searing laser fields and acid pits: I like to experiment freely without worrying about dying and having to start over. Many of the puzzles in Mel, however, are of the more fatal type, to be approached carefully and methodically. Gels, cubes, lasers, switches, turrets, and force fields are the main tools and obstacles you'll face in the sprawling and complex levels. Naturally, you'll engage in a showdown with a malignant A.I. near the end of the game.
As far as voice-over work goes, one actor provides a pretty good Cave Johnson impersonation, and another voices Virgil, the personality core who accompanies you through the majority of the game. I didn't find the humor particularly effective, and much of Virgil's dialogue feels like overkill as it's based around him encouraging you to find a way out of the facility. You'd be doing that anyway, right? At the very least, it's done with a lot of enthusiasm and an obvious fondness for the Portal series.
This massive mod is completely free if you own Portal 2, and provides around 6-10 hours of gameplay. Portal Stories: Mel will be available on June 25 via Steam.