The story takes place in an Alternate Continuity, and begins during the "Homestead" portion of the original game. The Marstons are enjoying an evening by the fire together but are worried about Uncle, who hasn't returned home yet. The weather has taken a turn for worse so John decides he can't go looking for him until morning and heads to bed.
Later that night, John Marston and his wife Abigail are rudely awakened by Uncle who has been zombified. In the struggle Uncle bites Abigail, infecting her, and John shoots Uncle dead. Jack runs to the aid of his mother upon hearing the gunshots and is bitten by her. John hogties his family and leaves them raw meat while he leaves to find out what the hell is going on and to hopefully find a cure.
- Abnormal Ammo: One of the new weapons for Undead Nightmare, the Civil War-era Blunderbuss, can be "loaded with just about anything". Once this weapon is unlocked, zombies without ammo belts, who previously gave you nothing (except an angry Marston), now provide you with "undead parts" to stuff into your blunderbuss and instantly gib your targets with.
- Affectionate Parody: The overall plot of the game, music cues, hammier voice acting, and incompetent character actions are all very reminiscent of a B-Movie. Given Rockstar's pedigree, it's blatantly obvious that it was intentional.
- All Myths Are True: Alongside zombies, there are sasquatches, chubacapras, unicorns, and the Four Horses of the Apocalypse.
- And Then John Was a Zombie: This happens literally to John Marston at the end of the game. He rises from his grave not too long after being killed by Edgar Ross' men, and you play as his zombie in free-roam. Kinda makes the cover a Spoiler Title.
- All Your Base Are Belong to Us: One of the Achievement is untitled "All your town are belong to us".
- An Axe to Grind: The tomahawk is a remarkably effective melee weapon.
- And Your Reward Is Clothes: The game features 3 new outfits two of which have to be earned by completing challenges.
- Angel Unaware: You know that woman you haven't been noticing that appears in a lot of missions briefly? Turns out she's an Aztec goddess that needs your help putting the mask back in it's proper place. However, it seems she can't do this herself despite, well, being a goddess.
- Anticlimax Boss You may spend a good long while waiting for the Chubacapra to show up and fight you, but it'll take you only one good shot to bring it down.
- Artifact of Doom: Everything started when your old "friend" Abraham Reyes stole a jade mask from an Aztec tomb beneath his estate, thinking it would make him immortal. The final plot mission has you descend into the tomb to return the mask, ending the curse. At least, until it get stolen again by your other "friend", Seth...
- Ascended Extra: Some deceased characters from the original story reappear as "boss" zombies when you clear out graveyards..
- Asshole Victim: After you find him, Herbert Moon goes on a lengthy rant about how much he hates women, Catholics, Jews, homosexuals, etc. etc. He's eaten alive five seconds later.
Herbert: You can't eat me! I'M HERBERT MOOOOOOOON!
- And of course, MacDougal early on in the game.
- Also Reyes, who caused all of this to happen.
- Despite how close he was to them, Seth manages to avoid this fate. Of course Seth isn't an asshole, just crazy.
- Badass Boast: Given by a zombie, no less.
"You wanna dance with the devil? Well HERE I AM!"
- Badass Bandolier: Just like in the main game. However, here you have it from the start - and you'll need all the ammo you can carry.
- Badass Grandpa: Landon Ricketts continues in this role from the main game. The town he lives in is designated a permanent safe zone, meaning it's one of only three settlements in the entire game that can never be overrun by the undead. The other two are secure, heavily fortified military installations that can only be accessed by ladder. His settlement, on the other hand, has no walls, no defenses, and no other guards - the doors aren't even locked. He literally keeps the town secure and zombie-free single-handedly - he's just that good.
- Gender Flipped with Mother Superior.
- Bag of Spilling: Even though the game takes place late in the story line of the original one, John Marston doesn't start with all the guns.
- Be Careful What You Wish For: Meta-example: After completing the vanilla game many fans were upset by the ending, with some even forming petitions for a patch or update that returns John to life. In Undead Nightmare that wish is granted and John rises from the grave... as an unintelligible zombie. Enjoy!
- Big Damn Heroes: You can randomly find people being attacked by zombies. Whether they set up a makeshift barricade in a futile attempt to protect themselves against the horde, or are simply being rundown by one zombie they didn't notice until the last minute, you can save them in the nick of time.
- Bigfoot, Sasquatch, and Yeti
- Big Ham: Herbert MOOOOOOON
- Black Comedy
- Call Back: The achievement/trophy for killing Sasquatch is "Six Years in the Making." Now, take a wild guess as to which game made by Rockstar was released in October 2004... Hilarious in Hindsight, indeed. The French version even calls the achievement/trophy "No need to seek it anymore, CJ."
- Captain Ersatz: There are three special zombie types; charging Bruisers, fast and crouching Bolters, and acid-spitting Retchers. Don't they sound similar to Chargers, Hunters, and Spitters?
- Or mabye Zombies, Dogs, and Crawlers?
- ~Chekhov's Lecture~: Jack's story about the Aztecs comes back in the end and turns out to be true.
- Chupacabra: Present as a hunting challenge in Mexico. Interestingly, it appears to be some kind of monstrous, mutant hyena rather than the typical alien lizard that it's usually portrayed as.
- Conspiracy Kitchen Sink: Herbert Moon hates virtually everything and is convinced they're all in an alliance against him. Marston confusedly points out he's never heard of a "Jewish British Catholic Homosexual Conspiracy" before.
- Cool Horse: The zombie horses, the Unicorn and the Four Horses of the Apocalypse.
- Death by Racism: Herbert Moon. Many other characters deserve the same, but we don't get to see if they were killed or not.
- Did Not Do the Research: A lot of what the mother superior says. After killing a zombie she says that she's just committed a mortal sin; even if zombies did count as alive, the Catholic Church has always taught that it's justifiable to kill in self-defense. Later, she tries to use holy water on a zombie, and tells John that she blessed it. Only a priest or bishop can bless objects, which might explain why the holy water didn't actually succeed in curing the zombie.
- Downloadable Content: Originally released as a downloadable content pack that requires the Red Dead Redemption disc to play. However, as of November 2010 it was available for retail purchase, along with the other DLCs, on disc in a single box.
- Dying Like Animals: The survivors John comes across are generally more or less unhelpful, and are more interested in arguing about whether science, God or the government is to blame for the ongoing Zombie Apocalypse and who should be their new leader. Of course, this gives the zombies plenty of ample opportunities to sneak up on them.
- Enemy Mine: All of the former gang hideouts that haven't been overrun are now friendly settlements, and the surviving gang members (who previously would have shot you on sight) will happily accept your assistance in dealing with the undead hordes, and repay you with weapons, ammunition, supplies, and a warm bed, just like any other town.
- Epic Fail: Nigel West Dickens, in an attempt to make Zombie Repellant manages to make Zombie Bait. Even for a Snake Oil Salesman that's a pretty big fail right there.
- Everything's Deader with Zombies: Um, duh?
- Everything Is Worse With Bears: You thought they were terrifying in the main game? Well, It Got Worse: Keep on the lookout for Zombie Bears! Although despite their more frightful appearance, they're a bit easier to kill than a normal bear.
- Fantasy Kitchen Sink: Aside from multiple varieties of undead humans and animals, we also have sasquatches, horses of the Apocalypse, a unicorn, a chupacabra...
- Genre Blind: "I'm going to wander down that lonely, deserted street and get my bag." Seriously, exactly how many seconds did you expect MacDougal to last after he said that?
- Gentle Giant: The Sasquatches don't eat babies, like everyone says... rather, they eat mushrooms and berries, try to avoid human contact, and live a peaceful existence in the forest. Too bad you don't find this out until after you've killed five of them, and are facing the only one left. And even then, he doesn't attack you for killing his family and friends, but instead cries and begs you to "Make it stop!"
- Red Dead was depressing, brutal and sometimes funny; Undead Nightmare pretty much just goes all-out for the black comedy...so why that incredibly depressing, obviously-not-right mission? Player Punch much?
- Even though the mission itself is depressing, the way John keeps repeating "...you eat babies!" has definite memetic humor potential.
- Good Bad Bug: Killing the Unicorn causes it to spasm all over the place randomly. 
- Halloween Episode: Complete with a spooky narrator in the intro.
- Here We Go Again: What happens at the end of Undead Nightmare. Turns out not so bad for John though...
- Hellish Horse: Once again, the undead horse and the Four Horses of the Apocalypse.
- Hero of Another Story: You run into several of these as recurring minor characters, such as a doctor who is performing field studies on the undead with a mobile lab and a machine gun, a professional undead hunter who works alone and regularly challenges you to zombie-shooting contests for fun, and a few of the surviving characters from the first game, such as Marshal Johnson in Armadillo, who is keeping what's left of the town alive and functioning (for a given value of 'functioning', anyway) through what basically boils down to sheer force of will.
- Horsemen of the Apocalypse: Well, horses at least. You can find, break, and ride the Four Horses of the Apocalypse - each of which has its own special abilities:
- War: Lights enemies on fire.
- Famine: Doesn't slow down when going off-road. Also the fastest of the four horses.
- Pestilence: Is nigh-invincible.
- Death: Instant headshots on any enemies it tramples.
- Idiot Ball: Why did John need to run out to the barn to grab his shotgun? Wouldn't a experienced gunman like him be smart enough to keep a gun in his bedroom? Furthermore, what was his shotgun doing out in the barn instead of in a room in his house?
- And it appears that along with his clothes, he was keeping his revolver in the wardrobe. Right next to where he was standing.
- Improvised Weapon: Several. It turns out that West Dickens' miracle tonic makes remarkably good zombie bait, Landon Ricketts devises a rather handy anti-zombie bomb by simply taping a stick of dynamite to a bottle of said bait, and although the blunderbuss is actually a purpose-built weapon, its ammunition is improvised out of whatever crap you can find lying around - mostly bits and pieces of your enemies.
- Incendiary Exponent: The War Horse's mane, tail and hoofs are on fire and it burns every zombie it tremples.
- You can also coat your bullets in white phosphorous, which causes everything you shoot to spectacularly burst into flames.
- Indian Burial Ground: It was probably a bad idea to build Abraham Reyes's mansion on top of one.
- It Will Never Catch On: Doubles as a Take That to the Zombie Apocalypse Genre.
The Film Maker: "Kindhearted neighbors turned into savage flesh eating monsters. It will make a fantastic movie!"
John: "Who would enjoy that?!"
The Film Maker: "What?"
John: "What kind of a sick person would like that?!!"
The Film Maker: "My kind sir, THE LOWEST COMMON DENOMINATOR!!! My people."
- Kill It with Fire: The fire bottle, incendiary ammunition, and the torch, the latter two exclusive to this mode and especially effective against the undead.
- Laser-Guided Karma: Most of the assholes are promptly zombie snack food seconds after they rant.
- Late Arrival Spoiler: Since the campaign starts at a point that was near the tail end of the original story, expect a lot of spoilery stuff from the original game to be mentioned liberally.
- Lighter and Softer: Despite it being a freaking Zombie Apocalypse, this DLC is more softer than the main game. Think about it: you're shooting zombies instead of your fellow human beings, there's a lot less Shoot the Shaggy Dog stories, a lot of the unpreventable deaths happen to arseholes who survived the main game, and while John still gets killed by Ross, He gets to come back as a zombie. When zombies actually make the world better, you know the wild west is a Crapsack World!
- Lost Forever: If all the survivors of a town under attack die, the town is permanently overrun.
- Ludicrous Gibs: Zombies + Blunderbuss = huge red explosion.
- Mercy Kill: When forced to kill Uncle, you are instructed to "put (him) out of his misery", so by extent you would be Mercy Killing every zombie you encounter. And when you face the final Sasquatch, he cries "MAKE IT STOP!" and sits down, ready to be killed by your hand. It's up to you whether he lives or dies.
- Mood Whiplash: The unicorn which emits butterflies when stationary and leaves a rainbow in it's wake when it runs.
- Nerves of Steel: John Marston sees a Zombie Apocalypse in action, including his own family zombified, and pretty much just rolls with it.
- Never Mess with Granny: Mother Superior again. She is one kickass old lady.
- Nice Hat: The Legend of the Apocalypse outfit comes with a stylish wide-brimmed hat complete with a feather. It's also the only outfit Zombie John can wear that has a hat.
- Nigh Invulnerability: Pestilence. It can be killed, but this is highly unlikely due to its massive endurance. Not even cougars can fell it easily.
- And then there's Death, which literally can't be killed unless it falls from a great height or falls into a lake or river. And even then, you automatically gain its horse deed once you acquire it, so if it does die, you can just respawn it.
- Ninja Pirate Zombie Robot: Zombie outlaws, zombie horses, zombie bears...
- Not Using the Z Word: The timeframe naturally justifies this. The first Zombie film was White Zombie in 1932.
- Mackenna the Film Maker actually uses it if you stand near him and listen to him. But he's ahead of his time, so it might pass.
- Our Zombies Are Different: These zombies pretty closely follow classic Romero rules: All dead bodies are reanimated at the time of the curse, the zombie plague can be spread through biting, and only headshots kill. An addition not found in Romero films is that fire and holy water kill them as well. The townsfolk often describe them as "drinking blood" and "howling at the moon," which is technically correct if you include their moaning.
- Out-of-Genre Experience: When's the last time you heard of a cowboy fighting zombies?
- Perpetual Poverty: Oh, yes. Especially ammo. Ammo is as rare as gold.
- Playable Epilogue: Like in the main game, John dies (though it isn't shown here)... but instead of taking up his guns as his son Jack, you play as a (sentient) zombie John!
- Precision F-Strike: Players know John Marston, especially if they've played through the vanilla game, to be cool-headed and have a high tolerance for pressure, and rarely raising his voice in character-building cutscenes, despite all he's been through. However, should he come across a zombie's corpse that has no ammo or spare body parts, Marston will have a surprisingly short fuse, muttering with frustration, or even just plain screaming "GODDAMN IT, THIS GOD FORSAKEN SHITHOLE!".
- Shaggy Dog Story: After all John goes through to save the world from a zombie apocalypse, Seth goes and steals the mask again, restarting the Zombie Apocalypse. This, however, allows John to come back from the dead.
- Shout-Out: Killing the chupacabra in Undead Nightmare (yes, there's a chupacabra) earns you the "Chupathingy" achievement. And there are several nods to horror movies, such as multiplayer characters named for Wes Craven, Sam Raimi, Michael Jackson (or possibly mean-spirited joke about NBA player Magic Johnson and his disease), and yes, even Uwe Boll. Beyond that, the Undead Hunter outfit effectively dresses John as Ashley "Ash" Williams and the Legend of the Apocalypse outfit has John dressed as Stone from Deadlands. And during the post-game free roam, when John is a zombie himself, the outfit's name changes to "Army of the Undead".
- So Proud of You: A non-spoken, internal version occurs in the first mission. Jack starts to explain the story he's reading to John, and as the view changes to John, the audio begins to fade and is replaced by light piano notes, implying John lost his concentration in favor of admiring his son.
- Too Dumb to Live: Did the Film Maker really think it was a good idea to set two zombies loose and try filming them only a few steps away from them?
- To be fair, John was with him, and he did cry for John to help him, so he may have thought that John would actually save him
- "I'm just gonna wander down that lonely, deserted street, and get my bag." Even MacDougal sounds like he can't believe what he is saying.
- Urban Legend of Zelda: The achievement for killing a sasquatch is titled "Six Years in the Making," a reference to 2004's Grand Theft Auto San Andreas where players insisted a sasquatch could be found in the forest.
- Violation of Common Sense: Many of the "safe" rooms where John can bed down for the night are wooden buildings that are currently on fire. Even many of the relatively intact ones are unsecure, unguarded, and on ground level - one in particular is a tent, and they aren't exactly known for their sturdy defenses.
- Vitriolic Best Buds: When we first see Jack and Abigale in the DLC, they are playfully trading insults. She does end the conversation saying she's proud of him.
- What the Hell, Player?: The Sasquatch. You spend a stranger quest hunting six of them in the hills, but when you finally find the last one, he's sitting under a tree, crying because his docile species is functionally extinct. And he proceeds to explain this to you in perfect English.
- Yank the Dog's Chain / Player Punch: Undead Nightmare presents a What If story of what if a zombie outbreak occured before John had his last fatal standoff with the FBI. Even though John is alive, his family is zombified. John simply can't catch a break.
- This lampshaded by D. S. MacKenna
John: "I'm trying to save my family."
D. S. MacKenna: "That is an interesting hobby you have."
- You All Look Familiar: Subverted, many of female npcs are revealed to be the same person, an Aztec Goddess trapped on Earth because of the Zombie outbreak.
- Your Head Asplode: One of the only ways to put down the undead for good is to shoot them in the head - and nine times out of then, their head will violently (and messily) explode.
- Unicorn: It has a rainbow trailing behind it as it runs and butterflies come out under it when it stands still. It has unlimited stamina and is very tough, but can be killed by a blow to the head, but its death is very glitched.
- Zombie Apocalypse: The premise of the game is John Marston against a zombie plague. Fuck yes.
- Which includes both zombie bears and zombie cougars. OH SHI- * ZOMBIE-COUGAR'D*
- In a rare variation of this trope, you can actually cure the mass zombification.
- Zombie John Marston Is About To Shoot You: And he's holding a blunderbuss.
- ↑ He lasts about 4