It is usually not until after completing one of BioWare's games that one realizes what an expert command of the role-playing game (RPG) form the developers have. It's only after experiencing the breadth of one of their worlds that the magnitude of what they've built comes into focus. Not so with Mass Effect, and this is only partly because it's been such a long time coming. From the moment you start it, you are left with the feeling that you are digging into a colossal work -- one that, by means of its stellar quality, fundamentally redefines what's acceptable in terms of a game's narrative presentation. Indeed, it's not the way the game plays that draws you in; in fact, it may actually turn you off at first, if you're not keen to dig into the expansive world that's on offer. But from those relatively weak opening sequences till the moment the game truly opens up and begs you to explore it, there's little room to doubt that BioWare has taken its craft to the next level.

Eschewing the convention that dictates most RPG protagonists should start out as nameless, humble nobodies, Mass Effect immediately thrusts you into a position of mass consequence. You are Commander Shepard, executive officer of the SSV Normandy (the product of a rare human/alien co-action and the most advanced ship in the Alliance fleet), and a certifiably notable personage -- the particulars of your backstory depend on which of the three background archetypes you choose during character creation. People know your name, and they hold you in high esteem. Eventually, you even become something of a goodwill ambassador, representing our fledgling species as it begins to make moves on the galactic stage.

Very much in keeping with RPG conventions, though, Commander Shepard starts the game as a bona fide 100-pound weakling, lofty profile be damned. Thanks to the combination of a character-advancement system that takes a few levels to really get into gear and as a real-time combat system that sometimes tricks players into behaving like they're playing a shooter, you're probably going to have some rough battles in the early portions of Mass Effect.

The class you choose at the outset largely dictates how well you'll handle things, especially early on. Soldiers, able to specialize in heavy weapons and use higher-end armor, typically fare the best in the early game. Adepts, who focus on using "biotics" (read: Mass Effect's quantum-theory-inflected response to The Force), can muster some admirable offensive output, though they're at their best when supported by beefier allies. Engineers, meanwhile, get their really impressive abilities a bit later in the game, though once they've come into their own, they can wreak serious havoc, especially on robotic enemies (which are in no short supply). There are also hybrid classes, incorporating abilities from each of the abovementioned: vanguards, who use guns and biotics; sentinels, who use biotics and technology; and infiltrators, who focus on tech and guns.

Regardless of whether or not you're new to RPGs, you're going to undergo a serious baptism by fire the moment you get into your first real conflict in Mass Effect. For all intents and purposes, combat occurs in real-time. Unlike most of BioWare's RPGs, though, it isn't immediately apparent that it's a good idea to pause combat and issue orders to your NPC companions when the going gets rough. Before you learn better, you're probably going to be popping up from behind cover, firing off a few rounds, and hoping for the best. While you can indeed pause the game at any time and execute special abilities or swap weapons from a radial menu for both yourself and your squad mates, this isn't ever satisfactorily explained through any sort of tutorial. Neither is the fact that you can give rough positioning orders via the directional pad. This alone may turn off players who aren't fully invested in experiencing some BioWare-quality role-playing, as the game isn't particularly fun when played as a squad-based shooter, and it isn't until you're about halfway through the story (give or take, depending on how many experience-rich side-quests you do) that your characters become accomplished enough to do some of the more impressive stuff that the combat system allows.

Once you're there, though, things take a turn for the exhilarating. There's a moment in most RPGs when your character reaches a certain threshold of power where big battles are still challenging, but strategic acuity affords you spectacular means to overcome them. In Mass Effect, this moment is expressed in myriad ways: using the biotic ability "lift" to levitate a horde of attackers and pick them off with small weapons fire while they float helplessly; or hacking a 10-foot robotic walker in order to turn it on its allies. You definitely get access to some sensational abilities once you reach the higher levels in Mass Effect, and the best encounters in the game force you to utilize them expertly and efficiently. At times like this, the real-time nature of the combat turns from a nagging liability into an immense asset, as it forces you to be mindful of what's going on in a battle, lest your enemies gain an inch when they catch you slipping. Given that the enemies' levels scale relative to your own, sometimes that inch is all they need.

When you're actively pursuing the game's main story thread, every battle you fight has an immediate, imperative context. Things never feel quite linear, however; once the story starts in earnest, you're given a few choices as to where to go. If you decide to experience Mass Effect this way, you're allowing yourself to be propelled to its shocking endgame without needlessly fragmenting the experience. But given how frequently the various players in the cast try to sidetrack you, you'll probably give in.

Unfortunately, the farther you veer from the critical path, the more trivial things start to feel. Most of the side-quests feel woefully underdeveloped compared to scenarios that comprise the main story. Some will start out strong and putter into prosaic bunker-clearing-runs once you touch down on the planet where they're set. Many of these will trick you into thinking there will be a big, satisfying payoff once all is said and done, only to leave you with a sterile pop-up message of congratulations that someone must have put in as an afterthought. The best side-quests are the ones that manage to transcend this dull formula by incorporating some of the game's best elements -- like its amazing dialogue sequences. Unfortunately, these are few and far between.

It's too bad that the game's exploration elements largely focus on these sorts of side-quests. You can access a beautiful galaxy map from the navigation deck of the Normandy, and spending a few minutes plotting points through it reveals a staggering array or clusters and systems that are just begging to be explored. Relatively few planets, however, are available to land on, and apart from those where the main quest takes place, they're all pretty barren and lifeless, haphazardly populated by remote bunkers, mineral nodes, and a whole lot of (admittedly valuable) space garbage. If nothing else, however, these side-treks will allow you to collect scads of powerful loot, as well as experience points to inch you toward those supremelypowerful abilities.

You get to pilot an impressive-looking six-wheeled rover when the situation calls for you to touch down planetside. Apart from its ability to defy gravity by vertically traversing walls, the so-called Mako is equipped with heavy weaponry that is typically more than suitable for taking out the various foot patrols and encampments you'll encounter on these remote worlds. Again, the best Mako sequences are the ones that are integral to the story; all the rest are pretty forgettable. Even in the best cases, though, the rover is a bit of hassle to control -- three-point turns are sometimes a bit too much to ask of it, which can mean a lonely death on a backwater planet during tactically sensitive situations.

Vague, looming threats factor heavily into Mass Effect's story. After a botched covert operation on behalf of the Normandy's crew at the story's outset (not to mention some particularly aggravating red tape courtesy of the interspecies Galactic Council), you discover that a Council operative named Saren is up to some pretty heavy stuff involving a fanatical Artificial Intelligence (AI) army; ancient, nigh-mythic alien technology; and an unfathomably powerful starship. From a human's perspective, the Saren situation brings with it some serious baggage. First, he's a Turian, which happens to be the alien species of which humanity ran afoul the moment it stepped out of its home solar system. Secondly, he's a member of the Spectres, the Council's covert ops arm, and an elusive organization that's thus far excluded humans from its ranks. They're also more or less autonomous, and seldom held accountable for their actions by the Council, which leads to a whole lot of plausible deniability before Shepard is actually empowered to chase him down and stop whatever unspecified apocalypse he has planned for galactic civilization.

But in spite the magnitude of his designs, Saren is largely absent throughout the proceedings; his name is vilified by every other significant character you meet, but your actual encounters with him are fleeting. Instead, Mass Effect focuses on narrative minutiae to engage you in its world, loosing the spigot on transitory information about the colorful characters you meet, and particularly those in your employ. A Codex is even built into the game's menu system that catalogues the often overwhelming barrage of information that's embedded into seemingly every computer console and instrument panel with which you interact in the world. Perhaps this focus on embellishing worlds in this manner instead of telling stories in the ways to which we're accustomed will give games a new narrative competency.

You've probably guessed as much by now, but Mass Effect is essentially space opera at its most grandiose. And while it's often difficult to get into the beat of the story, given its fragmented exposition and how hard the game tries to distract you from it, one can't deny just how impressive the whole presentation is. Mass Effect has essentially set the high benchmark for in-game voice acting, in terms of both quality and comprehensiveness. As you'd expect from a BioWare game, many of the characters you encounter during your galactic jaunt have a whole lot to say. What might surprise you, however, is how exhaustive the voice work is. Every line is spoken, and in most cases, it all sounds great. Facial expressions are nuanced and naturalistic, and even when the content of the lines isn't terribly compelling, the quality of the delivery will often fool you into caring.

The mechanics of conversation are an appreciable step ahead of BioWare's dialogue trees of yore. Rather than transcribing thoroughly every possible response you can give during a conversation sequence, the individual dialogue branches simply convey the tone of your options. As you play the game, you'll learn that the topmost branches tend to lean toward cooperative, generous replies; the ones on the bottom, meanwhile, usually entail a more ruthless approach. If you want expository information, the branches on the left will give you them, while the ones on the right will drive you closer to resolution. The way the system is set up truly succeeds at encouraging you to role-play via your gut instincts, rather than pore over every possible reply in hopes of effecting the most desirable outcome. The fact that the game's alignment choices -- in Mass Effect's case, whether your character is seen as a paragon of the human race or a renegade intent on finishing the mission by any means necessary -- don't constitute a zero-sum game similarly fosters this kind of dynamic approach to role-playing. The result is an RPG that's truly satisfying beyond its hard game mechanics.

A game as big as Mass Effect is bound to show some seams. Most people will play it a manner that will fragment the experience, and the technical production is not without its flaws. But that's not what you'll remember about it. For one, after playing through it, you'll be rendered unable to palate the mediocre voice work that most games offer. Mass Effect has altered the playing field in a big way in this respect, and in doing so has highlighted one of the most glaring weaknesses of the interactive form today. Beyond this, though, it's hard to quantify just how engrossing it is to explore the game's exuberant world, even despite how underdeveloped some of it is. BioWare has built a great many permutations into Mass Effect that will keep RPG fiends playing and replaying till the inevitable sequel arrives. Games like this simply don't come along very often, and when they do, they're meant to be savored.

This review is based on a retail copy acquired through Microsoft's outside PR agency.