Portal 2 isn't merely a sequel, it's a viable application of what was essentially a trial run. Valve Software has stated that it never expected Portal to gain the extended following it has, which is why The Orange Box acted as a safety net. A most unnecessary safety net, since Portal managed to eclipse both Half-Life 2: Episode 2 and Team Fortress 2 in popularity. And now Portal 2 has evolved into a standalone title that does just that: standing alone. It should come as no surprise that Portal 2 is expertly written. The overall story is an intriguing one, even if it is a series of excuses to keep putting the player through more and more numbered test chambers. And even though the characters often utter lines that are only loosely related to that story, they are so darkly humorous that this doesn't grate in the least. It's often worth it not to follow instructions right away, just to see the many follow-up comments that are provided. One of the best lines in the whole game comes from Cave Johnson, whose already hilarious lines are lent even more bite by J.K. Simmons' wonderfully cantankerous performance. Gameplay-wise, Portal 2 intelligently keeps the player's main mode of interaction with the game to its bare essentials: a portal gun that shoots two portals. Every new addition is something that exists in the world. Still, I sometimes found myself oddly focused on such additions. For instance, I spent quite a while trying to figure out how to direct an excursion funnel over a chasm so I could cross it. Then I realised I could simply place regular portals at both sides of the chasm and pass through those. As such, the increase in scale and content that naturally accompanies a sequel does preclude some of the tight-knit and intimate puzzles (and storytelling) that made Portal so appealing. After finishing the single-player game, I played the co-op campaign with my brother. It's one of the most entertaining and rewarding multiplayer experiences I've ever had. In one of the earlier levels, there's a maze that needs to be navigated by one player while the other operates buttons to shift its layout. There's of course the immediate temptation to trap or crush each other. Valve Software knows its audience. GLaDOS' lines are naturally very funny (especially the enmity she attempts to sow between the robots), but figuring out and solving a puzzle alongside someone else puts just as wide a smile on my face. Then there's the music. Whenever a game boasts a "dynamic soundtrack", it usually means that each level has two distinct musical cues that flow along a single theme tied to that level: one for calm exploration and one for frantic action. Portal 2 features a soundtrack that labels itself as dynamic, but takes it much further than that. Whenever part of a puzzle is solved, the music reacts by adding a layer of riffs, increasing its complexity. The player can also go back by undoing his actions and removing those layers, which allows for a very organic indication of progression. This is most apparent in a test chamber that requires the simultaneous redirection of three lasers -- sorry -- thermal discouragement beams. The gels also have their own musical cues, and I'm curious to see whether players will upload videos of the emergent music they've "composed" by shooting across different gels.