The mission is going badly. My partner and I are both riddled with bullets, and 10 seconds ago I took down a guy by shotgunning his legs as he dashed for cover. It was an efficient kill - but unfortunately the man in question was an innocent civilian. Still, there's no time for me to feel guilty about this: as I round the corner of a run-down shack, I'm greeted by a fresh wave of gunfire.

My vision blurs as the rounds strike home, but through the bloody haze I can just about tell that it's my buddy who's trying to kill me. "FRIENDLY FIRE, FRIENDLY FIRE!" I scream, slumping to the floor in a gory mess. My apologetic co-worker lollops over to help me back on my feet, but as he does he accidentally throws a frag grenade at the floor. A few tense seconds follow as the pair of us yelp and panic, then the mission comes to an abrupt, explosive end.

That pretty much describes my first attempt at getting to grips with Modern Warfare 2's co-operative Spec Ops mode. It was hardly my most professional performance as a games journalist, but it was certainly a hugely enjoyable ride. Previous Call of Duty games have always been chaotic, but it seems that the mad factor ramps up to 10 when there are two of you fighting side-by-side against a mass of 30 bad guys. There's all the unpredictability of standard multiplayer CoD, but there's also a lot of the cinematic atmosphere that the single-player game does so well.

True, there may not be a plot to the 23 missions on offer, but as you and a friend storm through a Brazilian slum - gunning down gangsters, covering each other's backs and coming to the rescue when one of you gets shot - you'll find yourself mentally filling in the blanks. It's like a flashback to childhood, when you ran around with your hands shaped like pistols, yelling 'pyow pyow!'. You're still making up stories as you go, but this time it feels frighteningly real. When one of you takes too much damage, you go into a sort of Last Stand mode: you can crawl about very slowly, and you can defend yourself with a pistol, but if your mate doesn't get to you within a minute or two, you'll bleed to death. It's dramatic stuff, and it's the kind of feature that will provide you with great tales to tell your mates.

Shooting people with a friend is brilliant

The missions in Spec Ops will be split into four groups - Alpha, Beta, Charlie and Delta. Initially only the first set will be available, but as you play through these you'll unlock further challenges via a Rock Band-like system that awards you a number of stars based upon the difficulty you've selected. In addition to changing the usual factors (stamina, enemy damage), increasing the difficulty level will also change your goals. In O Cristo Redentor, the Rio-based level I played, the objective was to wipe out a set number of bad guys without killing too many innocents. On the default setting we had to kill 30 enemies, and no more than six civilians; when we were playing on Hardened, the first number went up to 40 while the second dropped to four - and for added thrills, we also had to deal with attack dogs. It's actually possible for each player to use different difficulty levels, although I'm not clear on how this affects the objectives.

While shooting clearly plays a primary role in MW2's action, the 23 Spec Ops missions use a wide range of formats. There are stages where the only objective is to reach the end of the map in one piece, while others take the form of vehicle races and time trials. During my latest hands on with the game I was also shown a level type that roughly follows the trend set by Gears of War 2's Horde mode. Once again the goal here was to kill off a set number of hostiles, but this time they came at the player in waves, with a notable difference in the ways different enemies behaved.

The gameplay is incredibly tense

The setting for this battle was a huge, isolated house surrounded by hills. As with all the Spec Ops levels, the player starts the mission with a prescribed weapon load-out, but thankfully there's a good selection of alternative weapons nearby. In fact, that's a massive understatement: there's a blanket covered with boomsticks on the floor in the front room, and nearby there's a sort of terrorist's larder with an entire wall full of things to murder people with. The guy demonstrating this level was playing alone (you can do this with any spec op mission), so he immediately started laying claymores in the mansion's many doorways. The stage was set for an epic battle, and what followed did not disappoint.

Some 30 seconds after the stage began, gunmen started bearing down on the house. My demonstrator was clearly well-versed in the level's best hiding spots, but since the enemy generation is different each time you play, even he wasn't sure exactly what to expect. He'd run to one end of the house, trading bullets with baddies across open-plan rooms, then dash back to the entrances to deploy more mines. Amid all the carnage I took a moment to appreciate the detail in the level design, the abundance of objects that brought the environment to life. The house itself was filled with domestic clutter - at one point the player took refuge behind a stack of printer paper - and these decorations really helped to make the house feel like a lived-in place (and subsequently, a died-in place).

A short while into the mission, something unexpected occurred. To the sound of a sinister drum beat, a new enemy type showed up - the Juggernaut. This guy is a ridiculously tough nasty who wears thick, padded armour; as a result, he can take a metric tonne of punishment before he'll finally lie down to die. Dealing with the Juggernaut looks to be a major challenge. At one point the demonstrator hammered a 'naut with rounds from the lethal Barrett .50cal sniper rifle, and even here it took four or five shots to drop him. During another confrontation the player resorted to a combination of flashbangs and close-range shotgun blasts to get the job done. You can get four or five Juggernauts in one of these maps, and sometimes you may need to face more than one at once.

As is probably clear by now, the whole level was an incredibly tense affair. Despite the obvious skill of the person walking me through the action, there were several intervals where it looked like he wouldn't make it. There were plenty of surprises, too: in addition to the appearance of the Juggernaut, there was a great moment when the player left the house and moved out into the surrounding hills. Almost immediately I caught a glimpse of a flash in the treeline. It took me a moment to realise that it was a sniper, and that the flash was sunlight hitting the lens of his scope.

Single-player impressions are coming soon

One of the great things about Modern Warfare 2 is that there's an awful lot to look forward to. We already know that the multiplayer is looking superb, and none of us can wait to get our teeth into the main campaign, but after playing Spec Ops I have no doubt that this part of the game is going to be just as sublime. Infinity Ward has mastered the art of satisfying gunplay, and when you marry that to all the good stuff that accompanies co-op play - the competition, the strategy, the sheer buzz of rescuing your fallen comrade - well, then you have something truly special. And hey, it even works brilliantly when played in offline split-screen - good news for gamers without an internet connection.

As it happens, Special Ops wasn't all that was on display at this week's showcase event. I was also lucky enough to get a look at one of the campaign levels - but rather than rushing through that now, we're going to give this material its own article. If you're hungry for more details Modern Warfare 2 (and I suspect that quite a few of you are), then keep an eye out for another preview early next week.

Call of Duty: Modern Warfare 2 is scheduled for release on November 10.