Popular mythology states that the only way to truly kill a zombie is to either put a bullet in the brain, or to completely sever the head from the body. Half measures simply won't do: even if you lop off an arm or blow a hole in their chest, there's every chance that they'll keep on coming at you. Valve's latest game has displayed a similar level of resilience this year, and neither a vocal online boycott campaign nor the gargantuan release of Modern Warfare 2 has done much to slow its momentum. And now it's finally here, the last big sequel of 2009.
The zombies themselves - or The Infected, to give them their official name - are probably as good a place as any to start this review. As in the first game, the walking dead are a constant threat to your adventures. There are zombified builders, zombified clowns, zombies dressed in hazmat suits and in a million other fashion styles - and they all want to use your colon as a skipping rope. The level of detail on these rampant rotters is superb, and they look even better when you start blowing them to pieces. The original game was gory enough, but here the Ick Factor is turned up to full volume. Smack a zombie in the head with one of the new melee weapons - perhaps Valve's trademark crowbar - and their skull will burst like an overripe melon. Fire a shotgun into someone's legs and you'll blow clean though their knees. Combat is a messy business: blood sprays, flesh tears, and limbs are left scattered on the reddened ground.
In a sense, this sums up Left 4 Dead 2 as a whole: it's more or less the same co-op FPS game that we played last year, but it's bolder, bloodier, and broader in its approach to the action. The concept for the five core campaigns remains the same. Four survivors attempt to battle their way through a zombie apocalypse, blasting through several interconnected stages in a bid to reach safety. Each player is limited to carrying one large firearm, a choice of pistol or melee weapon, and one single-use thrown weapon. Survivors are also able to carry a limited quantity of healing items - one medkit, and either some short-term-fix painkillers, or a new adrenaline shot that boosts your speed. As before, the distribution of health kits is a vital gameplay dynamic: it's important that you look out for yourself, but if you don't patch up your team-mates from time to time, there's a real risk that the group as a whole will fail.
Valve has added a number of new monsters and features and weapons for this sequel - elements that we'll cover in a moment - but arguably the most important changes lie with the designs of the campaigns themselves. As before, each scenario is presented as a B-movie that places the four survivors in a different environment. Dead Center forces the quartet through a ruined shopping mall, while the excellent Dark Carnival mostly takes place at a theme park. However, this time the levels themselves have greater variety and personality. The Hard Rain campaign abandons the game's traditional, linear structure in favour of a there-and-back-again quest to fetch petrol for your getaway boat, while the aforementioned theme park takes every opportunity to shepherd you into its rides - over a dodgem arena, up a rollercoaster track, and into a shady tunnel of love. At times the first game felt a bit samey, as if you were simply doing the same old zombie killing with a different set of wallpapers, but here each scenario carries its own distinct flavours and set pieces.
This welcome diversity is particularly evident during the climactic events that occur at the end of each campaign. In the past you'd invariably end each adventure with a siege-like stand-off against the undead hordes, a desperate struggle to survive until a rescue vehicle arrived. This time you'll find yourself suffering through all sorts of things: a marathon-length running battle across a suspended bridge; a zombie massacre in a rock stadium; a near-blind sprint over the roofs of flooded houses as you're battered by a tropical storm. The latter is a particularly well-crafted slice of level design, limiting the players' vision and battering them with impressive weather effects - and yet it's not quite the pick of the bunch. The best finale of all is to be found at the close of Dead Center: here the four survivors must race around a multi-tiered shopping complex in search of 16 petrol cans, which must then be brought to a stock car on display in the mall's main hall. The quickest way to gather the cans is to split up the group - but as is usually the case in L4D, this is an extremely dangerous tactic. The resulting scramble is always tense yet extremely enjoyable, and it feels genuinely different to the other parts of the game (with one exception, but we'll cover that later).