Coming from almost every direction, they pour out of darkened alleyways and side-streets, shadowy human figures spilling from the doorways of apartment blocks and shops. They clamber their way over cars, boxes and trash cans. Others scale nearby walls and fences with one single, solitary purpose.

Shrieking and snarling like feral dogs, bearing down upon you at speed, you can see their tattered clothing streaked with blood and dirt. Their sunken eyes are full of an almost tangible hatred and seething rage, their mouths flecked with blood and spit.

Shots ring out. The head of one explodes like an over-ripe fruit, bringing it to a crashing halt amid a shower of blood and brains. Its neighbour is peppered with bullets, bloody holes ripping through its torso and taking off an arm at the joint. But the rest keep on coming, instinctively shifting their weight and momentum to clear their fallen comrades without missing a step.

Within mere seconds of them first appearing, they are upon you, ploughing straight in without even slowing down, an unstoppable force of pushing, punching and kicking. Buckling under the assault you crumple to the floor as they land savage blow after savage blow. Everything fades to permanent darkness.

This is Left 4 Dead’s zombie horde. And they’re coming for you.

A mutation of the Rabies virus has devastated a nameless city, and, playing as one of four survivors, you must fight your way to safety against the infected inhabitants who are now hell-bent on killing you. More realistically than most titles, this doesn’t mean encounters with groups of three or four enemies slowly attacking one at a time. Whether inside the narrow corridors of apartment buildings or out in the city streets, Left 4 Dead’s running zombies literally mob the player, with numbers somewhere in the double digits descending from all sides at once. On higher difficulty settings there are often an insane amount of enemies to contend with and their number and speed means fire fights turn in to a delicious chaos that make them frantic, and exhilarating. Thanks to the massive amount of gore on offer, there’s nothing more entertaining than finishing a particularly hectic onslaught and surveying the blood-splashed scene left in its wake.

While there is a single-player element, with the AI taking competent control of the three other survivors, frankly that’s not even a third of the experience on offer. In order to get the most out of Left 4 Dead the game demands to be played co-operatively with other players and, accordingly, has been reviewed as such.

On paper it may all seem a bit limited. Aside from the opening cinematic, there’s no real plot to jig things along. The four playable survivors are identical except for their character models and voices. There’s only four (albeit quite lengthy) scenarios to play through, and there is only a small selection of guns and thrown incendiary-weapons with which to take down the zombies. But Valve has not one, but two aces up their sleeve, so that, in practice, none of these issues really matter.

The first is that the game is controlled by a clever piece of A.I. nicknamed 'The Director', which takes charge of the gameplay experience. This customises it depending on how the group are playing and rather than being some over-hyped smoke and mirrors, The Director really does affect the pace of the game. Accordingly the placement and number of zombies is never really the same, and this leads to the experience turning out differently each time you play. The simplest example is that, on one play-through, a room may be crowded with zombies, whilst on another it could be empty. It may only be some cleverly crafted lines of machine code, but at times The Director displays a genuine evil streak with its ambushing tactics. There are plenty of twists and turns to keep players on their toes and, because of the overwhelming odds, there’s no room for complacency, even when the level layout is familiar.

The second is its finely tuned emphasis on team-work. With the large mobs of enemies and unpredictable nature of their placement, it’s essential that players work together. Not only sharing health packs and rescuing each other when they get knocked down, but also keeping in close proximity. A single player separated from the rest makes an easy target for the zombies and they won’t last long. Of course the game is best enjoyed with a group of friends, but this emphasis on team tactics means that even playing online with random people keeps the usual stupid behaviour to a minimum.

As if your common or garden variety of zombie wasn’t enough, the survivors progress is further hampered by the five types of super-zombies which can be dropped in. Each of these types is mixed in by The Director to keep things varied, sometimes springing them in in the most unlikely of places.

These super-zombies include the Boomer, a distended gas-bag that explodes when shot and delights in spewing goo over players - not only blinding them but making them the centre of the horde's attention. Next is the Smoker, which strangles its prey from afar with its extra-long tongue, and the nimble Hunter which leaps on players and pins them down. Then there’s the Tank which, as its name suggests, is a speeding juggernaut able to withstand severe punishment and even tear up bits of scenery to throw. Last but not least, there’s The Witch. Detectable by her freakish sobbing and wailing, there’s a very good reason why you’re advised to turn off all torches and creep around her. If disturbed she’s deadly and relentless.

Along with the co-op campaigns, there’s also a Versus mode. Technically this is a straightforward versus for up to eight players - four survivors against four player-controlled super-zombies, the zombie horde still controlled by The Director. While this mode is standard fair for any online first-person shooter, Valve virtually admit in the Director's commentary that this part of the game is extremely unbalanced - the player-controlled zombies able to provide no end of problems for the survivors. This makes the mode just a nice addition rather than an absolute must play - unless you’re playing on the zombie side, in which case there’s lots of malicious fun to be had making the survivors' lives an absolute misery.

Left 4 Dead is a game which keeps things simple, but this to its benefit. It mixes all the right ingredients of tension, gore and rewards to make it thoroughly enjoyable. The fact that Valve has done their usual job of tweaking the game to perfection, should give some indication of the level of polish and thought that has gone into creating a must-play title. Grab a group of friends and settle into the best co-op game of 2008.