Portal is Valve just showing off. When Portal was released in 2007 for Windows, Valve already had the Team Fortress, CounterStrike, and Half-Life franchises to their credit, in addition to developing the quintessential gaming digital download service, Steam. So when Valve bundled the little-known sci-fi physics puzzler in The Orange Box compilation, it seemed almost an afterthought. Of course, the game met with universal acclaim, garnered dozens of awards, and has earned a franchise tag itself, due in large part to its imminent sequel set to launch this holiday season.

On Wednesday, Valve’s Steam service launches on the Mac and not surprisingly, Portal is Valve’s launch title for the platform. Portal, in many ways, is the most approachable game Valve has ever made. The simple concept, clean interface, and quirky humor of the game are reminiscent of themes normally associated with Apple—fanatic following included.

When you first enter the world of Portal, you find yourself waking up in a small antiseptic holding cell in what looks to be a futuristic lab or maybe the back room of an Apple Store. You have no idea who you are or what you’re doing in this strange place. Concrete walls, a digital clock, and weirdly perky dance music immediately give you a sense of what is to come: You’re in a science lab, a strangely efficient and clean place that still eerily tries to be happy about it. The epitome of this strange new place is GLaDOS, an AI with a feminine voice that oversees the “testing” process at the Enrichment Center.

GLaDOS has a unique way of talking, and her personality and its progression throughout the course of the story is one of the most entertaining aspects of Portal. She tells you that the primary reasons for the tests are “fun” and “learning,” but warns you to “refrain from…” before suffering some kind of glitch. Many times throughout the testing process, GLaDOS will tell the player that they are doing great, but cast hints of the darker subtext. “The taste of blood is not part of the testing protocol,” she explains, and later “the effects of prolonged exposure” to a particular device are not the purpose of this test. While ominous, she does promise the player they'll be rewarded with cake at the end.

When not contemplating GLaDOS’ strange way with words, you get to navigate 19 levels of some of the cleverest platform-based puzzles you’ll ever encounter. Each level tasks you with reaching an elevator to advance. To do so, you’ll need to utilize the Portal Gun. With the portal gun, Valve once again proves it has a knack for re-inventing genre. Half-Life 2’s gravity gun completely changed how the player interacted with objects in the environment, but the portal gun’s effect on physics is even more revolutionary.

The portal gun initially lets you place a blue portal on certain surfaces. Go through the blue portal and you’ll end up coming out an orange portal. Later, you’ll be able to place an orange portal as well as a blue. Put a blue portal on a wall and then put one on the other side of a ravine, and by walking through the blue portal and out the orange, you’ve avoided the obstacle. The trick is that the portals maintain momentum. So if you’re going at high speeds through a portal, you’ll exit at high speeds. This may seem like a simple concept, but its application in gameplay is revolutionary. You can place a portal behind you high atop a wall and then a portal on a level below you. You’ll fall through the first portal and come shooting out the second—often enabling you to access areas you would not be able to.

You’ll need to utilize the portal gun cleverly, because the 19 tests become increasingly difficult. You’ll need to utilize weighted storage cubes (as well as a heart-emblazoned companion cube), hit buttons, redirect power conduits, hop platforms, and avoid falling to your death. There are also several life-fire turrets sprinkled throughout the game, and not only will they kill you if they see you, but their deadly designs are offset by some adorable dialogue: “I don’t blame you,” they’ll squeal as you throw them to their doom.

The game’s story (told mainly through GLaDOS’ increasingly ominous comments to the player) is one of the most entertaining and original aspects of Portal. Spoiler alert: after finishing your tests, GLaDOS tries to murder you, and you must escape Aperture’s labs. Instead of maneuvering through the sterile whitewashed testing rooms, you’ll be navigating the industrial inner workings of the huge underground complex. GLaDOS begins to taunt you and you’ll see clues that you are not the first test subject to try to escape. Oh, and that promise of cake? One of the more disturbing (and funny) lines you’ll see sprawled on the wall by some former test subject: “The cake is a lie.”

By far, the biggest complaint people have about Portal is that it’s too short. Even with the bonus maps, the game only takes a few hours to completely beat.

On my 27-inch 2.8GHz Core i7 iMac, the game looked absolutely gorgeous. I experienced no frame-rate drops, GLaDOs’s unique voice came through crystal clear, and the gameplay was as crisp and satisfying as when it first came out in 2007.

Macworld’s buying advice

In 2007, Portal took many people by surprise. Included in The Orange Box, most gamers were more familiar with the Team Fortress and Half-Life franchises than the quirky puzzle game. But after numerous awards and nearly universal acclaim, Portal has become a franchise in its own right. The sequel, due out this holiday season, is one of the most anticipated games of 2010. Both casual and serious gamers should make Portal the first game you download on Steam.

[Chris Holt is a Macworld associate editor.]