Something is rotten in the state of Denmark. Actually, that's not quite right - there are several rotting things in the state of New Austin, and they're climbing out of their graves, chomping on the living, and generally making a George A. Romero-style nuisance of themselves. It's quite bizarre, really: Rockstar San Diego spends years carefully recreating the Old West in loving detail, resulting in one of the finest open world environments we've ever had - and then a few months after release, it uses DLC to infest the place with zombies, mythical creatures and gloomy weather.

It's a move that's confused quite a few people - not least John Marston, the (anti)hero of Red Dead Redemption. One early cutscene in Undead Nightmare finds Marston seeking out his old buddy Seth, the desiccated grave-robber, in a bid to discover what the hell is going on. When he finds him, Seth is playing cards with his friend Moses - a situation that wouldn't be too strange, if it weren't for the fact that Moses is quite clearly a walking corpse. Seth flits back and forth in front of his dead chum, helping him to "play" the cards in his putrefied hand, and Marston begins to lose patience. Eventually he snaps and demands an answer: why are there zombies shuffling across the land? Why has this happened?

Seth seems bemused by the question. "Well," he replies, "why not?"

Why not indeed. There's a rather whimsical feel to Undead Nightmare. The main body of Red Dead Redemption pulled off the rare feat of telling a video game story with genuine emotional depth, but this expansion clearly has its tongue wedged into the wettest corner of its cheek. That's not to dismiss the whole thing as a joke, however; on the contrary, this is a far bigger offering than all of the game's previous DLC packs - comprising of a fresh set of story missions, weapons and enemies, as well as new music, side missions and challenges. It's a significant departure from the core game, with a whole set of new mechanics underpinning the action.

For starters, there's the question of ammo. Red Dead veterans are used to galloping around with enough bullets to kill half of Mexico (which, if memory serves, was actually one of the missions in the main game). Now that the zombie plague has wafted into town, you're going to have to make every round count. All the gun shops are now closed, presumably because the owners have never been to business school. It's the zombie apocalypse, after all. Everyone is going to want a boomstick! Maybe everywhere has just sold out of stock? I don't know, but what I do know is that you'll have to make do with what you can find. Slain zombies will occasionally drop a few spare bullets, but you really can't afford to be reckless. Carefully aimed shots are the order of the day: a single enemy can absorb a good five or six hits to the body before they hit the dirt, but a headshot will blow their noggin clean off.

Luckily John now has a range of new toys to help him thin out the decomposing denizens of New Austin. Holy Water acts like a sort of Church-endorsed Molotov Cocktail, engulfing your foes in a pretty blue flame; if you'd prefer non-magical fire, you can always set them alight with a flaming torch. You can distract your enemies by throwing out Zombait (think catnip, but for the living dead), or if you're really lucky you might find some Explosive Zombait, which blows up after a short delay (thankfully there's no catnip equivalent to this). All of these tools have their uses, but the handiest new weapon is the Blunderbuss - a devastating trumpet of death that blows the opposition into a shower of giblets. The downside of this potency is that it utterly disintegrates its targets, meaning that there's no opportunity to search the corpse for bullets, or indeed for the loose teeth and bones that the Blunderbuss takes for ammunition.

This setup creates a pleasing sense of balance: you can't rely too heavily on your normal guns, as there's not enough ammo - but if you use the Blunderbuss too often, you'll never be able to salvage anything. You need to keep a clear head during combat, but that's often easier said than done. Wonderfully, a single zombie bite will result in instant death; when Marston gets attacked he'll usually have a chance to save himself via a QTE, but this won't stop you from getting quite panicky when you're utterly surrounded. The standard zombies shamble about in a manner than shows off Red Dead's lovely animation and physics effects, and they'll easily overwhelm you if you don't keep track of your surroundings. In Left 4 Dead style, you'll also come across "special" variants of enemy who pose even more of a threat: Bolters scurry about on all fours at disconcerting speed, while hefty Bruiser zombies charge you and knock Marston to the floor. Unlike Left 4 Dead, however, Undead Nightmare gives you a zombified horse to ride. Thankfully this rotting nag is still capable of galloping about - in fact it's faster than flesh-and-blood horses - although it doesn't have much in the way of stamina... which is hardly surprising, given that it doesn't have any guts.

As you might expect, most of the gameplay is built around the challenge of crowd management. One story mission from Seth involves clearing three graveyards dotted around the map; in each instance the player takes on several waves of zombies while using the torch to burn a set number of coffins. Elsewhere there are settlements that you can help to clear - either by wiping out all the local zombies, or by bringing requested items to other survivors in the camp. Where Red Dead Redemption had Wanted posters that dished out bounty hunter mini-quests, Undead Nightmare has Missing Person notices; if you activate one of these, you'll have to go rescue an NPC and safely transport him back to Fort Mercer.

While the principle objective changes in each of these assignments, the basic principles remain the same: keep the hungry buggers away from you and don't slip up. Even with the benefit of Dead Eye's slow-mo targeting, staying in control is easier said than done. All it takes is a moment's lapsed concentration for the zombies to take the upper hand - pulling you down from your steed, or swarming over you as you reload at an inopportune moment. Marston's close-up execution moves take on a new relevance as they save your sorry hide, time and time again, while the explosive effects of the Blunderbuss can lead to moments of unexpected hilarity: in the heat of one battle, I desperately tugged on the trigger and took out two zombies at point blank range; as bloody gizzards rained down around me, I realised I that the blast had also vaporized my undead horse.

These over-the-top antics might surprise some players, particularly people who enjoyed Redemption's serious tone and who would happily settle for more of the same. Still, it's interesting to see a developer using DLC to drastically change the core elements of its game. It looks like there'll be loads of content here: there's no word on the length of the new story, but in addition to everything I've mentioned thus far there's also a new multiplayer mode - news on this should be released shortly - as well as random events and "mythical beasts" to deal with. Rockstar won't go say too much about the latter just yet, so your guess is as good as mine - my money's on Bigfoot making a cameo appearance, if only to satisfy all those people who claimed he was hiding in San Andreas.

Red Dead Redemption: Undead Nightmare will be released on PS3 and Xbox 360 in the near future.