One of the biggest surprises of last year's holiday releases was Left 4 Dead; a first person shooter developed by Valve taking place after a zombie apocalypse. Though critically acclaimed for its focus on co-operative play, limitless replay value, and intense zombie-slaughtering action, many felt that when the game launched, it felt incomplete. Only two of the four campaign maps were available to play in Versus mode, there was virtually no story, and playing alone practically ruined the experience. Thanks to downloadable content, Valve added new maps and modes, which has kept the Left 4 Dead faithful happy. Now, just a short year later, Valve has released Left 4 Dead 2. Though the game certainly feels more complete than its predecessor with tons of new content, it disappointingly does not address some of the problems from the original.

Like the first game, Left 4 Dead 2 presents its campaigns in the form of horror movies. Whereas last year had each campaign separate from each other, this year has all five connected via one story, which focuses on four immune survivors making their way from Georgia to New Orleans in their attempt to be rescued. Other than that, the story is pretty non-existent; each campaign consists of five parts, with parts 1 to 4 requiring you to get to a safehouse, and the finale having you make an escape, be it by car, boat, or helicopter. Throughout the game, you'll visit such locales as a mall (which is quickly moving from "homage" to "cliche"), an abandoned carnival, and an eerie swamp. Although the story disappoints, the fact that there is one makes it an automatic improvement over last year. What really will capture your attention are the survivors, who will chatter amongst themselves as you sit in the safehouse before venturing out into the world. Ellis, the redneck mechanic of the group, will divulge stories involving his friend Keith that will leave all but the most humorless zombie killers laughing out loud. Each survivor is just brimming with character, and it seems like Valve missed a great opportunity to make an entertaining story with them.

Of course, story is for single player games, and L4D2 certainly isn't that. To get the most out of the game, you need to play it with three other people, especially on higher difficulties where the AI companions are all but useless. They'll do a good enough job at shooting zombies, but often get sidetracked at killing a few straggling common infected, while your busy fighting a Tank. Or, they'll continually shout "We need to stick together!" when you try and keep moving, but they just won't follow you. The most frustrating is during the game's gauntlet events, where you have to go and turn off something like an alarm, and the AI just won't keep up. If you don't have access to Xbox Live, then L4D2 will get old really quick.

With friends, the game is still a blast. The AI Director returns, and again changes the maps according to how well you are doing. If you're full on ammo and health packs, prepare to meet up with a host of special infected. Be on the verge of death with only a frying pan, however, and you won't face quite as many. And nothing is as exhilarating as going into a room to find a pipe bomb in one game, and trying to grab it in another game only to find a Witch sobbing in the corner. These alterations are reminiscent of the changing dungeons found in the Diablo series, and help keep each campaign highly replayable. There is also a new Realism mode, which can be turned on in any difficulty, which adds a little extra challenge. Your allies don't have the color outline showing where they are, and if they die and aren't defibrillated, they're gone for good. The mode adds another layer of difficulty if you and your friends are looking for a challenge, but is otherwise pretty forgettable.

Versus mode returns, and is still the most fun of the bunch. In the eight player mode, four people are survivors, and four are the infected. All of the special infected return from the first game, including the repulsive Boomer, the ferocious Tank, and the creepy Smoker. Three new infected join the roster this time around, and fit in just great. The Charger is able to pick up a survivor, carry them, and pummel them into the ground. The Spitter spits deadly poison that lingers on the ground. And the Jockey can jump on a survivors back, steering them into danger zones such as fire or a Spitter's poison patch. Each type of special infected are unique and fun, and the addition of three more is just icing on the cake. Like the first game, playing as the special infected is still a blast - there's nothing quite like setting up an ambush with your fellow zombies to kill the survivors in Versus mode.

Survival mode makes another appearance as well. Released as DLC in the first game, it comes to L4D2 mostly unchanged, having the four survivors face waves of endless infected coming to save them, with rescue never coming. It's fun, and seems to be the hot trend in video games right now (Horde in Gears…Firefight in ODST…Nazi Zombies in Call of Duty…) but gets repetitive quickly, and is not near as engaging as Versus. Scavenge is a totally new four on four mode which has the survivors attempt to gather gas canisters, and the infected try to stop them. After the round is over, the teams switch sides and go at it again. The games are quick, not lasting more than a few minutes, and result in some frantic action. Your team will need strong co-operation to win, and be able to play both offense and defense, in a sense. The mode is a welcome addition, as it adds some variation to the online play; however, at times it feels just like a shorter version of Versus that sacrifices the tense, on-edge atmosphere for a more traditional FPS experience. For those who enjoy Left 4 Dead for the survival horror-esque elements, Scavenge may be a turn off.

The graphics are mostly unchanged from last year, but the environments are much more detailed and atmospheric. The rain effects are especially mind-blowing, and can make the threat of an oncoming zombie assault quite stressful when you can't see five feet in front of your face. The ways in which zombies die are also gruesomely spectacular. With the introduction of melee weapons, you can hack at certain body parts of the infected, taking off a leg, chunk of head, or outright decapitating them. Bullets also tear off flesh to reveal ribcages, and chainsaws literally rip the infected apart. There are some nagging clipping issues, such as having zombies' heads protrude through walls, or being able to walk through your allies, having their body disappear into a shell of polygons. The sound direction is stellar, from the powerful gun shots to the creepy, ambient groans and moans warning you of nearby special infected. The soundtrack is fitting for the American South, but could've been longer, as it repeats itself too often.

Sure, L4D2 features new weapons, new game modes and new infected, but in the end, these are all pretty minor additions. What makes L4D2 a great game is what made the original so popular: exhilarating co-operative action with three buddies… and zombies. In a way, it feels like this is the game that should've been released a year ago. Valve certainly went with the "if it ain't broke, don't fix it" mantra, and it seems to have worked well enough. However, with a fully fleshed out story and some better AI companions for those playing alone, the sequel could've been so much more. Nonetheless, if you're in the mood to slay some zombies with a few friends, then Left 4 Dead 2 is the shooter for you.