The co-op mode in "Portal 2" is as pleasant a surprise as the first "Portal" was back in 2007.

Back in 2007, “Portal” arrived with virtually no hype. One of five games in Valve’s “The Orange Box,” the quirky title about a test subject using experimental technology to solve increasingly difficult, physics-based tests took a back seat to the latest installments in the “Half-Life” and “Team Fortress’ series. Initially, at least, most gaming outlets didn’t even give the game its own review.

But once “The Orange Box” hit retail and people started to play “Portal,” it became a sensation. Catchphrases like, “The cake is a lie” permeated gamer culture while “Still Alive,” the song sung by the sadistic, test-giving artificial intelligence GLaDOS at the end of the game, garnered millions of views on YouTube. Sensing an appetite for more, Valve released “Portal: Still Alive,” a deluxe downloadable version of the game with even more puzzles that helped whet appetites before the inevitable sequel.

By contrast, “Portal 2” hits the marketplace in anything but stealth mode. This time, it has a full, $60 retail game all to itself, with a 12- to 15-hour story, and a new cooperative mode that’s as long as the original game. With three-and-a-half years having passed since the first game, “Portal” fans are ravenous for a sequel, even though it required an unusual, after-the-fact rewrite of the first game’s ending. (Thank the miracle of downloadable title updates for that one.)

GLaDOS has had a long time to think about what went down at the end of "Portal."

For the most part, “Portal 2” (rated E10+, $60 for Xbox 360 and PlayStation 3, $50 for PC) should satisfy all but the grumpiest fan. Sarcastic, sing-song GLaDOS is as entertaining and murderous as ever, while new character Wheatley, voiced by nerd darling Stephen Merchant, provides bumbling comic relief. The best part of the single-player game, though, is the middle act, a tour through the wreckage of Aperture Science that tells the story of the company, founder Cave Johnson (voiced by J.K. Simmons) and GLaDOS.

The gameplay in “Portal 2” works largely the same as the first one. Though it’s technically a first-person shooter, the gun main character Chell fires can’t kill anyone directly. You’ll press one button to place a blue portal onto a surface. Another button press fires an orange portal. You can then walk through one oval-shaped entry point and emerge from the other one. The game frequently uses its unusual arsenal to subvert common video game conventions. You might, for example, find yourself staring down a long corridor riddled with traps. Where a game like “Super Mario Galaxy” might require the most careful timing and quick reflexes, “Portal 2” simply wants you to make two portals, one at your end of the hallway and one on the other side, bypassing the whole challenge.

The laws of physics apply. If you’re traveling at a high velocity when you enter a portal, you’ll come flying out of the other one as if shot from a cannon. Though “Portal 2” introduces a number of new wrinkles to its puzzles, such as three different kinds of gel that cause surfaces to behave differently, each element is introduced gradually, and most players should have time to master it before moving onto the next variant.

“Portal 2’s” cooperative campaign, which can be played either online or split-screen, is as much of a revelation as the original game. The campaign takes place after the conclusion of the single-player game, and helps set the stage for an eventual “Portal 3,” or at least more downloadable content. Instead of controlling Chell, however, the two co-op players run through the levels as Atlas and P-body, two robots created by GLaDOS. Obviously, having two players allows for extra portals, adding complexity to the proceedings.

As much fun as it is to play “Portal” and the single-player “Portal 2” game, it’s even more of a blast to work through puzzles with a friend, preferably a smart one. As the cliché goes, two heads are better than one, and it’s satisfying to work together to explore each test chamber and try out ideas. While you’re sussing out solutions, the whimsical gestures and constant chastising from GLaDOS about how “human” Atlas and P-body are acting, provide comic relief.

With the core gameplay concept proved by critical acclaim and fan worship in the three-and-a-half years after the first “Portal’s” stealth release, Valve has, in a way, created the first full “Portal” game. Rather than merely bring back the menacing computer and mute hero for another go, “Portal 2” boasts well-developed characters, a real story, a compelling game world and even a little heart. It equals and surpasses everything that made the first title great and will no doubt end up on the inevitable game-of-the-year shortlists.