Call of Duty is a series that has very much played it safe. It did World War II while the going was good, and much to the chagrin of hardcore fans, it even consented to change hands (from Infinity Ward to Treyarch) while its original creators did the heavy lifting involved in envisioning and crafting its true next-gen iteration. Call of Duty 4: Modern Warfare, the product of said labor, is unquestionably a return to form for the series (though, to be fair, the worst you could say about the third installment is that it did little more than tread water). So, does this fourth installment represent a substantial risk for a series that's historically kept itself reined in? Sort of, yes, but not for the reason you might assume. The expansive ideas aren't found in the game's "pulled-from-the-headlines" modern setting. It misses a few great opportunities there, in fact. You should be paying attention to the multiplayer -- and if you already like multiplayer shooters, chances are if you have a taste, you'll have no choice but to give it attention.

As you'd expect, Call of Duty 4's story revolves around gallant individuals selflessly taking up arms against the foes of freedom. The writers were aiming big here. In one corner is the most imminent bogeyman hiding in the folds of America's furrowed brow: fundamentalist Islam. In the other is a specter whose menace is just now starting to come into focus after a couple decades of seething slumber: the former Soviet Union. Heavy hitters indeed, and more than adequate for the purpose of rousing the vigilance of the Real American Hero? (and his avowed allies). But in a prime example of gaming's chicken-shit approach to telling real war stories, Call of Duty 4's script keeps the pertinent details purposefully vague -- and in some cases, takes them out of the picture completely, as with the unnamed Middle Eastern country where much of the action takes place. What's this Al-Asad dude all about, and why is he in league with Russian ultranationalist Zakhaev, apart from their mutual desire to spangle Old Glory with hot particles and viscera? No one knows, and none less than you, soldier. But it's enough that they want to do it and probably have the means, isn't it?

Anyway, the stalwarts you play as probably aren't tripping about the particulars of this maverick brand of foreign policy. Depending on the scenario, you're either U.S. Marine Sergeant Paul Jackson or Sergeant John "Soap" MacTavish of the British Special Air Service. Generally, the portions of the game that take place in Russia have you playing as MacTavish, while the levels in Whatchamacallitztan cast you as Jackson. True to the Call of Duty blueprint, though, you're never alone; you'll always operate as part of a squad, the size of which depends on the scenario at hand. This is a good thing, too, as both MacTavish and Jackson very much embody the archetypal voiceless first-person shooter protagonist, and without the constant chatter supplied courtesy of their more colorful comrades, the game would have felt pretty soulless. The whole production benefits from wonderful voice work, which makes the story engaging on a minute-to-minute level despite its big-picture vagueness. Series regular Captain Price is a particular treat, as this is a mentor whom you get to meet during an especially exciting blast from the past.

Call of Duty 4's story feels like it comes and goes in a heartbeat, however rife it may be with themes of sacrifice and courage. This is because it is both especially short and expertly paced. Almost every level feels like an intricate set piece through whose discrete points of interest you're propelled with severe urgency. You'll endure protracted firefights with entrenched assailants in desolate urban environments, scour the hostile countryside for errant informants, and wait in ambush for well-armed enemy convoys. Most sequences give you free rein to travel through the environments and choose your precise method of engagement (unless it chooses you first); others will guide you on a set path, be in it the back of a moving vehicle, or behind the sights of a gunship's armaments. The pace is generally relentless, and to dawdle in one position too long usually results in an unceremonious death, be it by a grenade that you neglected to lob back toward its point of origin, or an exploding piece of erstwhile cover (do mind those cars). Call of Duty 4 may indeed be a heavily scripted game, but unless you go out of your way to expose the man behind the curtain (by replaying the same scenario five times in a row on the highest difficulty level, in other words), you're not going to care when you're in the thick of things.

But as unrelenting as the single-player game is, Call of Duty 4's multiplayer is what will keep it relevant for a long time to come. Your usual spread of game modes are present -- including both team and solo deathmatch modes, plus a few whimsical variants on capture-and-hold and capture the flag. In truth, the game's tight, brutal mechanics and gorgeous visuals would have made for an intense enough multiplayer experience. But Infinity Ward has gone above and beyond, and implemented a feature that's sure to rob a great many individuals of sizeable portions of their lives: an RPG-style progression scheme.

The progression elements are every bit as consuming here as they are in the most insidiously-designed RPGs; you'll find yourself occupying the same mental space that games like Diablo encourage as they compel you to while away "just one more hour" in chase of the next dangling achievement. The structure is more or less the same as what you'd see in a modern massively-multiplayer online game. Killing enemies and completing match objectives nets you experience, and as you level up, you unlock weapon certifications and passive abilities called "perks." These elements allow you to create custom character classes, which are defined by their gear loadouts and perks. Perks run the gamut in terms of utility: One, for instance, will reveal the location of enemy explosives on the map, while another increases the accuracy of your aiming when firing from the hip. They're never quite spectacularly unbalancing, except on the extreme ends -- i.e., when a freshly-minted player goes up against a seasoned vet -- but devoted players will nonetheless aspire to have all of them at their disposal.

If this sounds at all enticing to you, please considering the following: Upon reaching the highest level of 55, you're allowed to trade in all your unlocked ranks and perks and start the grind over again as a humble level-one Private. In return, you get an insignia next to your name. You can do this up to 10 times, with the insignia growing more ornate with each go-around.

What Infinity Ward has created here is a stunning blockbuster that plays well and absolutely dazzles your senses. Call of Duty 4 is definitely the most completely-realized shooter to come out this year, and quite possibly the most memorable in spite of stiff competition. Its developers are unquestionably at the top of their game, and it's very enticing to consider just what they'll come up with next.

This review is based on a retail copy acquired through Activision PR.