"What I wanted wasn't to just take the title and use it. I wanted what we all want: I don't want this stupid, fizzle-out ending - where they just lay everybody off and that's it. I want to play Duke Nukem Forever. I want to play the f***ing game I was supposed to play, the game I've been promised that I never got. I want to play that game. And we all do."

Randy Pitchford is on particularly good form today. The Gearbox CEO is explaining how he came to be the owner of Duke Nukem Forever - and indeed of the whole Nukem franchise. It's a tale that begins in 1996 with Pitchford's contribution to Duke Nukem 3D, the first game he ever worked on; from here the story builds and twists and swirls, encompassing all manner of asides and miniature anecdotes. There's the whole rise-and-fall of the original project - an epic chronicle of towering ambition, financial woes and bitter legal disputes. There are moments of heroism: in the weeks after they were fired by publisher Take-Two, some members of the development team refused to let Duke die, working on their code in their own time, with their own (non-existent) cash, whilst living off instant noodles. There are plenty of laughs, too - particularly when the game is described as "the biggest case of video gaming blue balls" in the history of the industry.

As the presentation continues, Duke Nukem Forever is compared to mythical creatures ("It's such a weird thing, right? He's like Bigfoot, or the Loch Ness Monster"). Pitchford, meanwhile, likens himself to a driver in the middle of the desert, stumbling across the aftermath of a brutal car crash. This metaphor is tweaked and revised a number of times, but the underlying message is clear: he desperately wanted - and still wants - to see this game released. It's a cracking yarn, and for 45 minutes (25 minutes longer than planned), Pitchford completely owns the room. Then, and only then, do we sit down to play Duke Nukem Forever.

And how does our 12-year wait end? Why, with a stream of piss hitting the back of a urinal, of course. Forever begins with Duke relieving himself in a run-down toilet - the first of many knowing nods to the game we played in '96. You can turn on the hand-dryers and admire your reflection in a spectacularly cracked mirror, but Duke refuses to take a second piss - explaining that he'll need something to drink first. Upon leaving the bathroom, we arrive at a circular chamber, littered with the mutilated bodies of fallen soldiers. A trio of surviving troopers are gathered around a white board; it turns out that you can interact with the latter, and before long the assembled (male) press are busy drawing cocks in slightly awkward, Etch-a-Sketch fashion. As we finish one of the soldiers enthusiastically appraises our work. "Wow, that's just... wow! Well, actually, I don't get it - but if I did, I bet that guy over there would still have his arm. And at least one of his nuts."

It soon becomes clear that Duke is in the bowels of some kind of sports stadium. Moments later we find ourselves in the main arena, battling the game's first enemy: a gigantic insectoid alien who charges around using enormous jets, raining down missiles from above. Luckily Duke is armed with the Devastator, a two-headed rocket launcher that sprays out its payload at impressive speed. The fight itself is piece of cake (this is the start of the game, after all), and before long the alien behemoth has doubled over in pain. A brief Quick Time Event finds Duke climbing the fallen beast and ripping out a handful of pipes from its back; in response the creature ends up vomiting up some kind of gooey-looking internal organ, which Duke promptly kicks over the field goal at the end of the pitch. "It's goooood!" he cries. And so it is.

I won't spoil the cutscene that immediately follows this fight, but suffice to say it seems that Gearbox is going to be pushing the ESRB and friends to the very limits of their tolerance. If you were worried that 12 years in development hell might have hurt Duke's crude sensibilities, think again. Shock humour isn't to everyone's taste, but it's arguably one of the most defining characteristics of the Duke Nukem franchise. The important thing is that the game seems to be genuinely funny, and yet it resists the temptation to spam the player with too many one-liners. All the same, it'll be interesting to see how everyone reacts to the tone of Forever. The mid-to-late '90s was home to a whole range of titles that celebrated balls-out humour and violence for violence sake, but these days such games are far less common.

The second half of the demo jumps forward to a point two thirds of the way through the game, when Duke is battling aliens in the canyons surrounding an old mine. There's a brief driving section - which feels fun, if a bit simple - then we're left to battle humanoid pigs with a variety of the game's weapons. All the old favourites from Duke 3D are here: the pipe bombs, the four-cylinder chaingun, and, best of all, the Shrinker. If you're not familiar with the previous game, know that the Shrinker is a beam weapon that turns enemies into terrified, six-inch-high wimps. You zap your foe, then run up and stamp on them. As you'd expect from the studio behind Borderlands, the gunplay feels solid and very satisfying. The graphics generally acquit themselves well, but it's the little details that make the combat gratifying. Shotgun blasts will occasionally tear off limbs, but from time to time you'll also drop a pig to his knees; at this point you can run up to him and perform a melee execution (another nod to the past), punching the poor sod's head into the wide blue yonder.

Unsurprisingly, there are a few sprinkles of old-school design nestling in among the modern conventions. There's an on-screen health meter, for example - or to be more accurate, there's an on-screen ego meter. When Duke gets hit by an enemy, it hurts his ego; when he kills a boss, his ego permanently swells. Duke's pride regenerates over time (just like a real ego), but it's still a bit odd to see a coloured indicator bar in this day and age. For the most part, however, the nostalgic vibe seems to stem from the feel and tone of the game, rather than its mechanics. Duke Nukem Forever doesn't want to be CoD or Halo; it wants to chuck loads of enemies at you and then let you blow them away in an entertaining manner, with plenty of jokes thrown in for good measure. It's a game that's focused on having a good time, in other words, and it's happy not to take itself too seriously.

The demo finishes all too soon, and after playing through twice I stop to compare notes with the other players. One guy claims that he shot a pig in the balls with the rail gun, at which point the enemy committed suicide by blowing out its brains. Not long after that, one of the event supervisors asks me if I picked up a poo. With any other game I might have thought he was kidding - but no, a third playthrough reveals that I missed the interactive turd in the toilets at the beginning. When you grab it, Duke will start to curse and moan about the things you're making him do. Throw it at a wall, and it'll leave behind a stomach-churning brown stain. Ladies and Gentlemen, Duke Nukem is back.

"It's just a game, guys," concludes Pitchford. "It's pretty cool, and I love it, but it's just a game. I know there's all this legend, but it's just a game. I don't know if anything can live up to the 12 years of time - but for me, if the game is fun, if it feels like the character and the personality and the attitude, and it's a good game, I'm thrilled that we finally get to play it."

I guess that makes two of us.

Duke Nukem Forever will be released next year (no, really!) on PC, PS3 and Xbox 360.