There's a gift shop. Of course there's a gift shop: Halo is one of the most lucrative and loved franchises of our times, and the chance to buy limited edition Grunt plushies (maximum three per customer) and replica Master Chief helmets is a source of genuine excitement for the thousands of fans pouring through the doors of Halofest at PAX 2011.

Halofest, running from the 26-28 August in Seattle, was organised to commemorate the series' tenth anniversary. It was just about the most perfect place on Earth - and I learnt this the hard way - to get inadvertently roped into a heated argument about whether the Needle Rifle can stack up to the DMR in Halo: Reach. (I have it on some very good authority that it can't).

Such passion and devotion from its fans is well deserved, and Halo is one of the few series that command enough attention to warrant such an event. One person in the queue remarked that he didn't even have an Xbox but wanted to see what was going on. The game's innovations, such as recharging health and a two weapon arsenal, have become commonplace, but its soul - lush greenery, the incredible musical score, the simple pleasure of shooting grunts - is still very much its own.

But just how did Halo become the franchise that it is today? Dan Ayoub, executive producer at 343 Industries, should know; he's currently working on Halo: Combat Evolved Anniversary, a remastered version of the Xbox's defining game. "There's something very special about Halo that I think a lot of franchises haven't been able to replicate," he says.

Like what? "First of all, it's a story - there's a fantastic story around it, there's a universe that I think people relate to. There's a very human struggle in the franchise as well, and I think people are very able to identify with. I think what's really cool about Halo is the whole UNSC push for survival. It's really about human potential. What can humanity aspire to? What can we get to? That we actually can achieve greatness. I think that's very special and appeals to a lot of people."

This year's Halo is a remastered version of the original Halo: Combat Evolved, to be released on November 15. That's the same day the original game came out in 2001, and a date precious enough to Microsoft that it's prepared to have the remade Halo go toe-to-toe with Modern Warfare 3 in 2011's crowded Christmas market. But why, exactly, does Microsoft have so much faith in this being a date worth celebrating?

The story of Halo goes back way before November 15 2001. Originally conceived as a real-time strategy game set in a sci-fi universe, Halo made its debut in 1999 with a trailer that looked nothing short of revolutionary. Bungie's game was to be the jewel in the crown of Mac gaming, announced by none other than Steve Jobs at the Macworld conference.

It wasn't to be, of course: Microsoft acquired Halo creator Bungie from Take Two in 2000, transporting the studio from their offices in Chicago - described as "a big smelly frat house located in a former Catholic girls school" by Bungie luminary (and composer of that theme) Marty O'Donnell in a 2004 blog post - to the other side of the USA. Almost all of Bungie's employees chose to make the 2000 mile trek to the new Redmond office, which O'Donnell described as "an even, bigger smellier frat house located in a former accounting firm's office space".

By November 15 2001, then, the game was already legendary, stoked by a heavy mix of dissidents and optimists perennially clashing over issues such as graphical potency and analogue stick controls, a then-daily argument catalysed with a multi-million pound marketing campaign from Microsoft and its first-party status.

Like many twenty-something gamers, I distinctly remember the release of Halo: Combat Evolved. Though I'm ashamed to admit it now, I was sceptical of the series at the start. After a few minutes of the game's opening level, I was ready to dismiss it as little more than empty hype and go back to playing Counter-Strike on the PC. Though it's taken me a few years to realise it, I was wrong about Halo.

I was definitely in the minority at that point, however, and by 2004's Halo 2 the quality and importance of the original game was firmly cast into gaming canon, but topping the original's mammoth success with the sequel would prove problematic. Bungie had gone from a cult developer with the privilege of time to one with a strict development schedule, the pedigree of a critical hit, and weight of an entire platform resting on its continued success.

It was never going to end happily, and Halo 2 will always be remembered for two things. The first, the one quickly cited by its naysayers, is its unforgivably dire ending, which closes on an unsatisfying cliff-hanger in the middle of a pivotal sequence. "The ending of Halo 2 is Halo 3", jokes Bungie cinematic director Joseph Staten during a Halo Fest panel before being corrected by O'Donnell, who says he's stored away Halo 2's actual ending so it's never seen or heard by anybody.

Where the campaign faltered, however, the multiplayer shone. Bungie's second Halo would become Xbox LIVE's poster child for over three years - until Halo 3 launched in 2007. Halo 2 popularised online gaming, introduced it to a whole new generation, and offered an enormously respectful multiplayer experience to boot. In some circles of the mainstream media, "Halo" and "multiplayer" were virtually interchangeable terms for years.

The second game's weak resolution obviously stung Bungie, and the studio has made an obvious effort to up their storytelling with each subsequent title - Halo 3, Halo: ODST and eventually in last year's Halo: Reach.

"I think there was a time in Bungie's history where we weren't all storytellers," says Staten. "I think there was a big disagreement in Halo 1 about how important story really was. There were lots of people who thought we didn't need a game with a story in it, we just needed to ship a fun game and the story was kind of a pain in the ass, actually. I think over the years we've all realised the importance of not just telling a story but creating a real living world".

Few games do a better job of crafting a sense of place than Bungie's Halo games, notably Halo: Reach, which not only conjures up a beautiful living world, but a dying one a couple of hours later. Crafting worlds and fleshing out environments is perhaps Bungie's greatest narrative talent, and it's something the team (and underlying technology) accomplish far more successfully than, say, subtle characterisation.

Whether it's due to its single or multiplayer triumphs - or both - Bungie has clearly created worlds worth exploring, and enjoyed a sizable community of adoring fans as a reward. But few developers have ever seemed to have reciprocated in the same way as Bungie, with the team revelling and nurturing their neighbourhoods of fans with such passion and zeal; the team has been entertaining its community long before it became an industry buzzword a couple of years ago. Services like Call of Duty: Elite and Battlelog currently take up their fair share of column inches, but all current developers will have used Bungie's pioneering work over the last decade as a source of inspiration for their own respective projects.

In our modern climate, words like "passion" have become nebulous marketing terms, but among the Bungie community and its creators the dedication seems tangible: when you listen to the people involved in creating Halo talk about the series, you get a sense of empowerment and pride. Pete Stacker has lent his vocal talents to the franchise since the beginning, playing Sergeant Stacker and uttering the first game's opening line as Captain Keyes. Talking on the Halo Universe panel at Halo Fest - which, coincidentally, was the first time Master Chief and Cortana voice actors Steve Downes and Jen Taylor ever met in real life - Stacker reflects on the last decade with sincere nostalgia and a lump in his throat:

"I just think being part of something that is actually part of the crust of America... I know that sounds really cliché or whatever, but over 10 years to see the amount of people who are playing it, that's what's really cool. People from all over the world, man, in different languages. And everybody is playing the game. To be part of that is really unique. I've been part of a lot of cool things, I've been a very lucky guy, but this is especially cool because, beyond anything else, [the team at Bungie] are my friends. So for us to get to be able to play with you is especially cool. And the pictures and the art, and the amount of work that goes into this, by everyone involved... everybody is going to the extra level because we're into it and, when everything is all brought together, that makes it about the coolest damn thing I've ever been part of."

Halo is currently in a transitional period, with development and maintenance of the mammoth franchise being passed from Bungie - now working with Activision to create a multiplatform IP - to new studio 343 Industries.

Even if you think the big floaty jumps are silly and the guns a bit too weak, there's no denying that Halo has become a cultural phenomenon over the last ten years - I know more than a few people who refer to it as the Star Wars of our generation. Surely Bungie is finding it hard to say goodbye to Halo?

"We leaned many years ago, not months ago, that it wasn't really ours anymore - it belonged to all of you guys," says Staten during another Halo Fest panel. "It hasn't been too hard to say goodbye because it has a home. It has people who care about it."

O'Donnell also takes a moment to reflect on what Bungie accomplished, but also to wonder about if they'll ever be able to make something that popular ever again. "I talk to Jason [Jones, Bungie co-founder] about how, wow, we really caught lightning a bottle here - what are the odds of being able to do this again? And he goes, 'Yeah, slim to none, but we're sure as hell going to try'. We have a bunch of people back at Bungie and our goal is to do this again."

One thing is clear: 343 Industries certainly has a tough task over the next few years, with the fledging studio now taking Master Chief forward into the Reclaimer trilogy. "We've got a big challenge ahead of us," says Ayoub. "We've got a franchise that has a huge legacy, a ton of very serious diehard fans, and I think that is really the key."

"As I say all the time," he adds, "the fans are the reason this franchise is here after 10 years, and the fans are the reason we're going to be here in another 10 years."