You've got to raise at least one eyebrow when Valve's buddies at Turtle Rock Studios claim that with Left 4 Dead, they're out to beat Counter-Strike - but that's exactly what they're planning on doing. They're adamant that with their four-way zombie co-op survival horror extravaganza they can supersede the most widely played online shooter on the planet, and having flown to Valve, taken part in the undead slaughter and relished the sort of interdependent gameplay so rarely seen in PC games, we can say that Turtle Rock Studios has a good chance of doing just that.

With one foot on the back of Valve's tried-and-tested Source engine, and another on the head of Mike Booth, master bot AI designer who earned his stripes developing Counter-Strike's dead-eyed CPU foes, Turtle Rock is peeking over the wall of potential, and into the garden of wondrous, original co-op fun. Left 4 Dead is what they see.

We took our place in front of the play-testing PC with Valve writer Chet Faliszek by our side, as one of Left 4 Dead 's four campaigns is loaded and players from throughout Valve's Bellevue offices begin to fill the server. We are one of the four human survivors, a John Everyman business type who's found himself in the middle of a zombie virus outbreak without even a suit for comfort. To our left and right, stocking up on guns and ammo from a handy weapons cache on the rooftop we've started on, are the other survivors.

There's a big biker-looking dude, a war veteran probably secretly loving the opportunity to shoot things, and a "pick me to make people want to give you their med-packs" pretty rich girl who, at odds with her appearance, knows how to blast holes in zombie torsos. In practice, of course, they're all just different player models and voice-samples. The only real choice is in what you grab from the table of weapons.

To make a total player count of eight, four other players have spawned as four special zombies who live happily among the ravenous hordes. The Boomer, a fat flesh-balloon filled with explosive gases and zombie-attracting vomit. The Hunter, a quick and agile zombie who jumps out of dark corners. The Smoker, with his 50-foot prehensile tongue. And The Hulk, who is not unlike his greener namesake. As the survivors make their way through the level, it's these infected players who must stop them with their particular zombie powers. Helpfully, they've got infinite lives, so they won't be worried about being a tad gung-ho.