Halo. Microsoft's great Xbox seller. Bungie's tour de force. It knows no bounds. It conquers all. It is our Star Wars.

Halo's enduring popularity is remarkable. Some two years after Halo 3 broke global entertainment launch records it's still the most played game on Xbox LIVE. Master Chief is Mario for the console FPS generation.

Why is Halo great? Answers are varied, and objective. But ask many why they keep flooding back for more, and they'll answer with one, simple word: multiplayer. Arguably Bungie's most influential innovation, Halo's multiplayer paved the way for console shooters that followed it. Halo: Combat Evolved made system linked Xboxs fashionable. Halo 2's ground-breaking online matchmaking carried Xbox LIVE's fledgling years on the crest of a wave, breaking only when Halo 3 refined the process further on the Xbox 360. Without Halo, would we have enjoyed the brutal online thrills of Modern Warfare or Killzone 2? Now, in the post CoD/Gears of War world, it's time for Halo to return.

And returned it has, this time with Halo 3: ODST, a two-disc release described by Bungie as "much more than an expansion". We've finished the brand new campaign, checked out all the new weapons, explored the abandoned city streets of New Mombasa, pumped hours into the brand new co-operative multiplayer mode Firefight and killed more Covenant than we could possibly count. So, is Bungie right? Is Halo 3: ODST much more than an expansion? Is it worth your hard-earned cash?

With ODST, it often feels as if Bungie desperately wanted to create something different, but, for whatever reason, wasn't able to go as far as it wanted. There are obvious influences from other games, most clearly 2K Games' BioShock, and distinct tweaks in the tried and trusted Halo formula, but ODST is still very much a Halo game that fans will instantly find familiar.

Meet the Rookie, the

The most obvious change is the introduction of the Rookie, a UNSC Orbital Drop Shock Trooper, as central playable character. Master Chief is nowhere to be found. He's off gallivanting in space. He's saving the world. He's doing his thing. The Rookie, being a human bereft of the cybernetic enhancements that make John-117 a super soldier, has less grandiose ambitions.

Set during the events of Halo 2, ODST begins with a cutscene as testosterone-fuelled as it is typically Halo. Buck, voiced by Firefly star Nathan Fillion, is preparing his five-man squad for a drop on the African city of New Mombasa. The Covenant, Halo's deranged cult-like alien brotherhood, has taken complete control over the city. As the ODSTs lock and load, you get a sense that you're embarking on something of a suicide mission. But with Master Chief otherwise occupied, you're New Mombasa's last hope.

New Mombasa's streets are dark and abandoned - save for Covenant patrols.

The drop, somewhat predictably, goes horribly wrong. You crash land. Six hours later you wake. You're alone. Night has drenched New Mombasa in a pitch black. The only illumination comes from the flames of burning vehicles and flickering street lights. The Rookie steps out onto the cold urban streets. You see a Covenant patrol - a few Grunts and a Brute - skulking about in search of strays. Immediately ODST reveals itself: this is not the Halo we're used to.

New Mombasa has been designed to be a sandbox, and is in fact the single largest level Bungie has ever created for a Halo game. It's no Liberty City or Capital Wasteland, of course, but it is fully explorable (after about half an hour's play), with no loading required. A curious mechanic, however, breaks up this slower-paced, somewhat stealthy exploration - flashback missions. The Rookie's sole goal is to hook up with his squad. To do this, and to advance the story, he needs to find various objects, remnants of the ODST's actions during the last six hours. When you do, you flashback to before night overcame the city, taking control of one of the Rookie's squad mates and playing out a self contained mission reminiscent of a traditional Halo level. Once completed, you jump straight back into the Rookie's armour plated shoes, and head out in search of the next piece in the puzzle.

The first of these flashback missions triggers when you find the helmet of Dare, Ms Naval Intelligence, as she's described by Buck. Set immediately after drop (each flashback begins with an indication of how many hours after drop it occurs and who you're controlling), you jump into Buck's shoes as he battles the Covenant in Tayari Plaza. Buck discovers dead Elites, apparently killed by Brutes - it's classified, Dare says. We meet a new member of the Covenant family: an Engineer - floating plant-like organic supercomputers forced to wear bombs to protect the military secrets they contain. Dare, guiding Buck via inner ear communication, is in trouble. You go in desperate search of your damsel in distress. As the Covenant close in on her position Buck fears her dead. You eventually find her pod. It's empty. Only her helmet remains...

By the end of the game you'll have stepped into the shoes of all five members of the ODST squad, piecing together the mystery of the last six hours before discovering the truth behind your mission on New Mombasa. The order in which you play these flashback missions is entirely up to you. There's a clear path the game wants you to take in order for it to make the most sense plot wise, one that's encouraged by the Superintendent's (the city-wide AI) less than subtle nudges, but after completing the first two you're free to give the Superintendent the finger and stretch your wings. The only problem is Covenant patrols and Phantom drop-offs have the annoying habit of interrupting your sightseeing just when you don't want them to.

The feared Hunters return, with the same weakspots.

The Rookie, being all human, isn't as powerful as Master Chief. He's not as strong, durable or as fast. He can't jump as high either, although his leaps still have a somewhat unrealistic floaty motion. He can't throw grenades as far, although can still lob them a fair old distance. He can't dual wield, but he can pull turrets out of the ground - an ability Bungie left in for the fun factor despite it making little sense. The most notable difference, however, is that the Rookie doesn't have a shield. Instead he's got stamina. Take damage and the screen will flash red. That's your stamina telling you to get in cover. Take sustained damage and your actual health will deplete. The only way to restore it is to grab a medipack or use an Optican MediGel First Aid station, many of which are dotted around the city. There's no hiding behind a rock and waiting for your shield to recharge in ODST.

The system is a clear nod to Halo: Combat Evolved, which also allowed players to regain health with medipacks, but the gameplay is affected differently. ODST's New Mombasa is more dangerous than any Halo environment ever created, simply because the Rookie doesn't have a shield. In past Halo games, Bungie's defining "ten minutes of fun" mechanic challenged players by virtue of the sheer number of enemies you had to face. In ODST it's different. The number of enemies you encounter at any given time while out and about in New Mombasa is smaller, but because you're much flimsier, leaping in like a madman is likely to get you killed, quickly. Instead, you're forced to pick your fights. You might ignore a Covenant patrol completely, for example. When you're forced into a scrap, when a flashback clue is surrounded by Brutes, Jackals and Grunts, perhaps, it pays to plan your attack carefully, identifying cover, escape routes, sniping positions, flanks and higher ground. It's initially a refreshing change for Halo, and it's hard to think of another FPS set in a futuristic open world urban city.

The Rookie, however, is not completely defenceless. His advanced VISR allows him to outline what he sees, identifying friendlies in green, foes in red and objects of interest in yellow. Essentially sci-fi night vision goggles, the VISR is most useful when exploring the abandoned streets of New Mombasa as the Rookie at night. During the day it makes it nigh on impossible to see what's going on.

The problem with the VISR, however, is that it's linked to the Superintendent to such an extent that it often feels as if your hand is being held too tightly. Way points clearly show you where you need to go to find the next flashback clue. They're not pinpointed to the square inch, but they may as well be. Like in EA's flawed gem Dead Space, in ODST you're never in any doubt which way to run to get to where you need to go. You can even press up on the d-pad for a reminder of where to go. You can ignore the Superintendent's help, of course, but on a subconscious level it makes you less inclined to go off the beaten track and simply stumble across stuff, perhaps the greatest joy open world games can bring.

The VISR is one of the UNSC's new toys, and exclusive to the elite ODSTs.

Accompanying the VISR are two new weapons. The new M7 submachine gun is a medium distance weapon great for reducing enemy shields to zero in the blink of an eye. Halo fans, however, will be more excited by the new pistol, the M6C/SOCOM. It's a powerful, incredibly accurate weapon that comes complete with a sound suppressor, a zoom and a rate of fire limited only by the speed of your trigger finger - basically it's the pistol from Halo: CE. It's incredibly satisfying to use, just a complete and utter joy. Dropping Grunts and Jackals with headshots from ODST's pistol is thrilling. Using the SMG to wear down a Brute's armour then, the millisecond before the clip runs out, switching to the pistol to drop him with a headshot, is the stuff of FPS dreams. One of Halo's greatest strengths is how tight and satisfying simply shooting stuff is. ODST's pistol is the pinnacle of that, and is the best video game pistol ever created.

The story is classic sci-fi blockbuster silliness

You might think that the night/flashback dynamic, and the differences between the Rookie and Master Chief, would combine to create a very different Halo experience. It doesn't. After half an hour or so, ODST feels just like Halo. No, you can't jump as high or run as fast, but the Rookie's limitations soon become unnoticeable as you get to grips with killing impromptu Covenant patrols that drop unannounced from low-flying Phantoms. The flashback missions feel particularly familiar - they're signature Bungie and classic Halo. There are Scorpion tank road trips down highways, Scarabs to destroy, Banshees to fly, Hunter face-offs, Warthog madness and, of course, Wraiths. There are even one or two comedy moments where you'll find UNSC soldiers talking to themselves - something of a Halo trademark. Put simply, ODST's memorable missions and set pieces mostly feel like more of the same.

Graphically, too, ODST is classic Halo. While the game's art has significantly improved, with some stunning blood red night skies complete with crumbling skyscrapers and billowing smoke clouds, the quality of the graphics overall are as they were two years ago. This isn't surprising given ODST has been built using the Halo 3 engine, and like Halo 3 it displays at just under a 720p resolution. Halo's look is certainly distinct, a colourful, bright style that contrasts Covenant purples with UNSC greys and browns. It doesn't quite match the heights of Gears of War 2 or Killzone 2 of course, but it occupies a unique place - ODST is quintessentially, unmistakeably Halo, and many fans will be delighted by this.

Some, though, may have been hoping that Bungie's next Halo game - and remember it's been some two years since the release of Halo 3 - would be significantly advanced not just graphically, but in gameplay terms. The argument will go that ODST is an extension of Halo 3, hence the title: Halo 3: ODST. This argument, however, will again be countered by those who feel the game's campaign is too short - I beat ODST on Heroic difficulty in just over five hours.

As always, multiplayer is absolutely brilliant.

Length aside, the campaign is not without other problems. The story is a largely un-engaging affair. The mysteriously silent Rookie is hard to love, and certainly lacks the heroic appeal of Master Chief. His squad mates are classic cliché-ridden space marines, with personalities that aren't explored to any great detail. The plot makes more sense than previous Halo titles, but is still hugely silly. The ending is barmy, and seems as if it should have had a massive bearing on the Halo universe as a whole, but clearly didn't because it's ramifications never came up in Halo 3.

Bungie will point to the hidden BioShock-esque audio recordings (you can listen to them as you continue your exploration) that tell the tale of a young African woman, called Sadie, as she fights to survive the initial Covenant attack on the city as well as rescue her father. The collection of the 30 audio recordings is entirely optional, acting as a sort of meta-story that runs underneath the main plot. You don't have to find them, and to be honest there's little motivation - the voice acting is awful and laughably hammy.

Prepare to drop - ODST won't wait.

The thing is, Halo has always been like this. It's always made little sense, always been more sci-fi fan fiction than Philip K. Dick. And it's never really mattered. Halo's managed to side step such single-player criticism because the multiplayer is just so damn good. This is not to say that ODST's campaign isn't worth soldiering through. It is. In fact at times it's downright heart-thumping. At the end of most of the flashback missions you'll be chucked into an enclosed area and asked to fend off waves and waves of Covenant. These last stand moments are far and away the best ODST's campaign has to offer. One sees you and your squad taking out multiple Phantoms with heavy weapons while on a rooftop - wicked. No wonder then, that in isolation they make such a great co-op multiplayer mode.

I'm talking, of course, about Firefight, ODST's saving grace. Firefight is, after all is said and done, what ODST is worth shelling out for. Put simply, it's brilliant fun. Like Gears of War 2's Horde mode (Bungie and Microsoft insist Firefight was conceived long ago), Firefight is a co-operative game mode that tasks players with working together to fend off increasingly difficult waves of enemies. Unlike Horde, however, Firefight is four-player, and it never ends. You could, if you were good enough, play it forever, although you might run up an electricity bill so scary it would send even Master Chief running for cover.

Firefight's genius is in how tactical it is. You have a set number of lives shared between the squad. Once they're gone, it's game over. And, because you're an ODST, you've no shield to fall back on. This makes you much more cautious than you would otherwise be. Hanging back in strategic positions, coordinating choke points, for example, and letting the Covenant come for you, is a smart strategy. Running and gunning won't get you anywhere, especially against Wraiths, which eat your puny stamina for breakfast. What each wave brings is completely random. Will you be up against four Hunters? Two Wraiths? Or simply scores of Brute Chieftains hell bent on smashing your skull into smithereens with gravity hammers? In Firefight, when you hear the dull vibrations of an approaching Phantom, your heart fills with as much dread as it does excitement.

Firefight is a guaranteed hit online - it's just too much fun not to be. Will it usurp the traditional competitive Halo 3 multiplayer (ODST's second disc comes with all the premium and free Halo 3 maps ever released, as well as three brand new maps)? Probably not. But it will be one hell of a diversion.

Little over a week ago, a website ran a story carrying quotes from an unnamed Halo 3: ODST reviewer who called the game "underwhelming". He said, and I quote: "You just look at it and think 'so what?'. Sure, the multiplayer is fun (it's Halo after all) but it's all built on an old engine (which looks pretty much untouched) and looks nowhere near as good as games such as Gears of War 2 or Killzone 2. It smacks of no effort and is a teensy bit cynical. [Microsoft] needed a big franchise title for this Christmas and presumably there wasn't enough time to build something new, so they gave us new missions on old, ageing technology that didn't really look that great first time round."

The thing is he's got a point. A number of them, in fact. ODST does feel like an expansion, despite Bungie's claims to the contrary. But there is a convincing explanation for each and every criticism. The campaign is short, but that didn't stop CoD 4 from being brilliant. The campaign doesn't offer much replayability - finding audio recordings unlocks weapons caches, which grant the Rookie access to secret weapons and even a Mongoose - but playing it co-operatively with three friends on Legendary difficulty is a blast. Most hardcore Halo 3 fans will already have all of the competitive multiplayer maps that ODST's second disc comes with, but the upshot is Halo 3's matchmaking will be more inclusive. For many ODST will be worth a purchase for two reasons only: Firefight and an invite into the Halo: Reach beta. Well, for many, that will be a purchase well made.

This brings me back to the point I made way back at the beginning of the review. ODST shows Bungie has new ideas. It shows it wants to stretch its creative wings. Every now and then, as you're exploring the unnerving New Mombasa streets, you'll feel it clear as day - the open world, the flashbacks, the adventure experienced from different perspectives, the environmental storytelling, the scrawled slogans, "They lied to us!", the corpses, the stealth. But as quickly as you feel it the wind of change is blown away by a tornado of familiarity. It's almost as if someone stood hovering over Bungie's shoulder as it slouched in front of the glare of a computer monitor, slapping its hand every time a stray mouse click dared to deviate from its very own Halo formula.

It seems a silly conclusion, but Halo 3: ODST is still Halo. It's still cracking fun. ODST is an absolutely essential purchase if you're a Halo fan, but if you're not, if you've never quite understood what all the fuss has been about, then ODST will do nothing to convince you to join the fight. Some will be disappointed with it. Some will see it just the way the unnamed reviewer sees it - as a cynical release. Some will bemoan the lack of change - I guess that'll have to wait till next year and Halo: Reach (hopefully). Until then, there's a gap in the office lunchtime Firefight match with my name on it.