Portal is part of The Orange Box. Don't forget to check out our reviews of the also-included Half-Life 2: Episode Two and Team Fortress 2.

As the only part of Valve's Orange Box package that isn't a proper sequel or expansion, Portal can be played without any prior knowledge of the Half-Life series. That's a plus in the sense that it makes this portion of the box more accessible to a general audience, as the title is without a doubt the most interesting take on the puzzle genre in years. It's essentially a combat-free game--having been spawned by a now Valve-employed team at DigiPen, which mandates its students game be nonviolent--though it features the first-person view and play mechanics most commonly associated with shooters. There are a few tweaks; weapons are replaced with the titular portal "gun," and levels require careful thought rather than twitch reactions. Perhaps most impressive for a puzzle game, Portal actually features a substantial narrative of skillfully subtle execution.

From the moment you start playing, the game's immersive, self-contained story unfolds without a break in the action, in signature Valve style. A feminine robotic voice greets you when you awake in a sterile, gray observation room and guides you from one test chamber to the next. This seemingly innocuous guide quickly gains a sort of benevolent taskmaster personality and becomes the game's central narrative device, lending encouragement or even humorous discouragement in some cases. "The enrichment center regrets to inform you that this next test is impossible," the voice says at one stage's outset. "Make no attempt to solve it." Though the comedic elements are a welcome addition, the tidbits of information offered by the monotonous voice actually reveal much about why you're running through this obstacle course of sorts inside the so-called Aperture Laboratories in the first place.

As the title suggests, the gameplay centers around a gun-like device allowing you to place two connected portals on nearly any flat surface. If your character or an object goes through one portal, it comes out the other, retaining its momentum and exiting at the same velocity with which the first portal was entered. Valve executed this system beautifully, and once you get the ability to place both portals, you'll undoubtedly spend a bit of time putting a portal directly above you and below you to simulate an infinite drop, your character plummeting faster with each cycle. Or try placing the two portals in convenient proximity to your character, using them as a looking glass of sorts to examine the protagonist--a woman, as it turns out.

Following standard puzzle dynamics, the stages start with easy challenges, acquainting you with the game's methods at a steady pace. Like any puzzle game, the central mechanics are simple: jumping, picking up objects, and eventually placing portals--starting with just one blue portal, and then adding the second orange portal later. You'll use portals to best complete each room by pressing buttons, manipulating blocks, sending energy balls on convenient trajectories, and becoming a human cannonball to reach each test chamber's exit. Through the 19 chambers, the challenge increases at a suitable pace, forcing you to build on what you've learned in increasingly innovative ways, but never demanding a large enough leap in puzzle logic to stump you.

Tips from your robotic guide are paired with a helpful diagram--masked as an elaborate warning sign--at each test chamber's entrance, revealing which skills you'll need to utilize in each level. A smaller caution sign depicting a figure being struck by a block indicates you'll need to manipulate objects in the current stage, for instance, while a black-and-white depiction of a desperately drowning caricature tells you the chamber floor won't be an available route through a level. On more complicated stages, squares with a sequence of dots placed conveniently near suggested portal placement locations indicate the order in which you'll need to perform a level's tasks.

Since travel through portals retains momentum, hurling yourself through these magic doors to cross chasms or other obstacles becomes an essential skill, and remains a thrillingly visceral exercise in physics throughout the extent of the game. After placing one portal on a wall or conveniently slanted platform, a second portal placed at a distance below your character serves as the point of entry. Using the momentum gained on the way down to the portal below, you'll shoot from your higher portal at a speed relative to the distance you dropped--and that's just the beginning. Didn't get enough air? Try replacing your portal on the ground with another below you before you hit the ground from your first launch, adding another cycle's momentum for a doubly forceful impetus.

Portal's 19 test chambers in its 10 chapters are a perfectly encapsulated gameplay experience, with each new test increased in length, and building on prior challenges an expertly tuned pace. The final chapter culminates in a thrilling test combining all prior exercises and wrapping up the surprisingly fleshed-out story as well. Still, despite Portal being a perfect slice of innovative gameplay, its two to three hours of gameplay end far too quickly.

Luckily, advanced versions of several stages as well as "least time," "least portals," and "least steps" challenges unlocked after the first playthrough significantly enhance its playability. Thought one of the levels was too easy? Try the advanced version, in which you can't touch the now-deadly floor, and that helpful block you used to hold down a button is nowhere to be found. While I'm not usually enthralled by time-attack modes, I couldn't help trying to test myself with Portal's extended playability options, which you'll be rewarded with achievements for completing in both the Xbox 360 and PC versions.

There were just a few sections of the game where using the Xbox 360's gamepad made things slightly difficult for me, but otherwise, it all felt extremely natural on the console. Of course, I'd be greatly envious of PC Portal players once the mod community goes nuts with the title and starts making Portal chambers and gameplay modes of its own--apparently, Valve has taken steps to fairly easily integrate custom content into the game.

Whichever version you play, you're going to be impressed again by Valve's commitment to innovation--not to mention the game's ending sequence, which I have no reservations in saying features the greatest original musical composition--courtesy of Jonathan Colton--ever included in a video game to date. The song is equal parts triumphant, humorous, and enjoyable--a fitting end to a game summed up by all three.