There's many reasons why post-apocalyptic scenarios appeal to us. Mostly it's due to our innate desire to be a part of something interesting, something unexpected that thrusts us into a position of authority amongst our fellow humans. It's the belief that you're somehow better than the mindless rabble that have been so deservedly erased from the planet.

But, a general concept involving a world suddenly plunged into the depths of suffering, sickness, mutation and death is only ever going to pass muster as a B-grade horror film unless you've got some talented individuals to pad out the rest of the story. Create some protagonists - believable heroes who have found themselves unwittingly thrown into the maelstrom, and who are prepared to fight to the death to protect their unblemished DNA. Give them personalities, customise their appearance, provide them with weapons, and now you have a story.

The original Left 4 Dead, released almost exactly a year ago, was exactly that kind of rollercoaster experience. Admittedly it wasn't without fault, as several areas required a level of refinement that seemed to be in short supply. Weapon choices were limited, as were environmental objects, and due to what many derided as a short campaign it all became a bit "samey" after several plays. Despite this, it was recognised as an exemplary Valve title, and easily capable of being referred to as more than just a stop-gap between Half-life releases.

Those who have played the demo released earlier this month are probably wondering if the sequel that forms the basis of this review is merely, as my friend put it to me, "Left 4 Dead 1.5", of if indeed Valve has provided gamers with a new experience worthy of an additional whole digit. From a purely aesthetic point of view, Valve probably shouldn't have made the subject of that demo "The Parish", as it's set during the daytime. Light freaks people out. It makes them think their beloved Left 4 Dead has suddenly become a cutesy open-world adventure where the objective is to collect rainbows and dodge unicorns, when the reality couldn't be further from that.

Left 4 Dead 2 is every part as brutal, as violent, as gore-ridden and as freaky as the first. It's expanded the scope of the original in virtually every direction. There are more zombies. More weapons. More environments, more special infected, and much more gameplay overall. Yet throughout all this, these long-awaited additions fail to derail the title from the core premise, which is to basically shoot anything that doesn't look like you.

The five new campaigns consist of progressive locations that enable your characters to move throughout the world in an attempt to team up with other survivors of a deadly plague which has all but destroyed the human race. The action is set in the US deep South, which has equipped Valve with a whole new set of cultural stereotypes to exploit. Whoever was in charge of recording the sound bites must have had a field day; the one-liners vary from clichéd to downright hilarious, and are all delivered with impeccable timing. It's hard to say if the new team of survivors are any more dynamic or likeable than the original lot, Rochelle in particular seems a bit dull, but they still eschew the type of camaraderie you've come to expect, and it's all served up by the more-than-capable AI Director.

Herein lies the beauty of this game. Having recently finished another extremely popular game that involves a lot of warfare, there's simply no comparison when it comes to the pathing and general control of the AI. Valve's system ensures that you'll rarely (if ever) encounter the same scenario more than once, unless it's part of an end-chapter boss event. Just like the original Left 4 Dead, you know that you'll probably have to face a Tank, but you don't always know when, or where this will occur. With Left 4 Dead 2, the AI Director is smarter - more involved, and much more refined. You may think that because you've nailed a Boomer and a Tank that the way is clear, but that's precisely when you'll limp through the safehouse door and discover a Hunter waiting for you. Where the original Left 4 Dead involved unpredictable routines that steadily became more predictable over time, Left 4 Dead 2 will leave you standing in the middle of a field with no ammunition listening to the approaching horde and wondering how the hell it came to this.

Key to this is monster variation. Not only do you have the new Spitter, Charger and Jockey mutants, you also have a range of special infected types. These vary from Mud Men who charge at you on all fours and look suspiciously like boss creatures, to infected that wear HazChem suits, or bulletproof vests. It's chaotic and designed to keep the tension high, and you'll discover that keeping your primary ammunition stocked for the final battle is much more crucial this time around.

Fortunately however, you have much more in the way of melee weapons. We covered this in the demo, so if you're not sure of the base additions to the new arsenal then scoot back there and take a look. What we didn't get to try in the demo however was the chainsaw, and this has definitely become a contender for the best off-hand weapon in any game to date. Sure, it runs out of petrol after a while and you'll have to swap it out, but once that horde closes in, there's nothing like charging full-speed through the middle of a stack of zombies with the tinny sound of a two-stroke ringing in your ears whilst your screen drips with blood. If you're in the mood to startle a witch, you could do a lot worse than firing up a Husqvarna in her ear.

The primary weapons are excellent too, in particular the new range of assault and sniper rifles. Levels such as the Swamp suit sniping particularly well, whereas open spaces such as the Carnival level are made that much more fluid by liberal use of the new automatic rifles and silenced SMG. Without ruining things too much, you'll probably want to spend a good bit of time replaying the Carnival map, it's easily one of the best campaigns out of both Left 4 Dead titles so far.

In addition to the conventional co-operative modes which can be played either online or local with friends, or offline using AI stand-ins, Valve have also added a Realism mode, Versus and Scavenger. The Realism mode modifies the overall game to remove things such as the glowing outlines of your party members, as well as introducing the requirement to use the defibrillator to bring back party members from the dead. It's a nice addition but ultimately with the game being so incredibly hard in many places it seems a tad unnecessary. Versus is very much the same as the original Left 4 Dead mode in which infected challenge the survivors, and it's here that you'll discover the Charger has rendered the Hunter somewhat obsolete. The original Left 4 Dead placed Hunters near the top of the food chain, and any occurrence was cause to race to the assistance of whoever was under attack - this time around they take more of a back seat.

Scavenge extends the life of the game considerably. Essentially the idea is to survive as long as possible by facing rounds of special infected whilst gathering fuel for a generator. You swap sides regularly, and the team with the most number of gas cans collected takes the match. It's a great concept and I can see it becoming an instant favourite amongst late-night LAN participants.

Left 4 Dead 2 is a perfect example of what happens when a competent developer takes an existing game and adds just enough to make it better, without subtracting from its initial appeal. I was somewhat annoyed that Half-life Episode 3 wasn't announced for 2009 ahead of Left 4 Dead 2, but now I'm not so sure. This begs the question - what on earth can Valve do now to improve this game? Perhaps allow both groups of survivors to team up in an eight-person co-operative environment?

It's Valve, so whatever they do, chances are it'll be excellent.