Fallout 3 random encounters

Random encounters in Fallout 3 can happen in certain pre-defined places in the Capital Wasteland. They usually consist of unusual situations, enemy encounters or small (and occasionally, not so small) tasks for the player.

The random element is the place where a given event will appear. Once a random encounter has been activated in a particular area no other random event will occur in this area. You can save outside the spawning area and keep reloading if you don't get the encounter you want.

Types
In principle, there are two different types of random encounters in the game: unique encounters and repeatable ones. Unique encounters will only occur once per playthrough whereas repeatable ones can occur multiple times. Another difference is that repeatable events can happen in any of the random encounter locations but unique encounters do not; there are 2 sets of unique events, each of which occurs only in a certain set of locations.

Repeatable encounters
These encounters may happen multiple times in a given game at any of the locations below.

Karma-related

 * A Talon Company ambush if you have a Karma rating of Good or Very Good, with the "Private Contract" note.
 * A Regulator ambush if you have a Karma rating of Evil or Very Evil. Sometimes there is a Speech challenge where you can bypass them.

Quest-related

 * A Scavenger heading to Girdershade. His entourage includes one or more of the following pets: a Brahmin, Dog or trained Yao Guai. He'll offer you a Nuka-Cola Quantum. You can barter with him into reducing the price.
 * Dead Vault 101 resident - if you drove the Vault 101 dwellers out of the Vault during Trouble on the Homefront.
 * A Wastelander killing and looting a Mole Rat or Mirelurk (only after completing the Wasteland Survival Guide quest). He has different dialogue options depending on whether or not the player completed all bonus objectives given by Moira and partially depending on chance (different Wastelanders may say slightly different things). If the player says he has helped with the book, a congratulatory or a mocking response is received. If the player says he has never heard of the guide, the Wastelander will provide him with a copy of the book.
 * The AntAgonizer and 3 Giant Worker Ants attacking 2-3 Wastelanders with another one cowering nearby. (only occurs after completing The Superhuman Gambit)

Hunters

 * 1-3 Hunters attacking a Giant Worker Ant, Giant Soldier Ant, Yao Guai, Vicious Dog, Super Mutant or Mole Rat. If they manage to kill the prey, they will take the meat from it. They will sell various sorts of meat to the player if asked to. The animal respawns every 72 hours. The Hunters themselves do not respawn if killed.
 * Three Hunters chasing and attacking a Wastelander. Killing the hunters results in positive Karma, and they carry Strange Meat, indicating they are roaming cannibals.

Scavengers

 * A normal Scavenger roaming the Wasteland with his/her pack Brahmin and possibly a pet (dog, robot, or Yao Guai). He/she has no unique dialogue, and instead has the standard Scavenger dialogue tree (offering to sell from his random inventory, or to repair the player's items).
 * A Scavenger in camp. A scavenger spawns standing stationary near a burning oil barrel. There is usually a nearby pack Brahmin and pet (either dog or Yao Guai). He usually has a limited number of items and small cash reserves. The inventory is randomized in each encounter.
 * Wasteland Doctor who can heal you and/or sell you medicine.
 * Wasteland Junkie who can sell you chems.
 * A Scavenger and his pet (either dog or Yao Guai) around a dead random robot (usually a Protectron or Robobrain). He will tell you to stay away from his loot and will attack if you persist on "stealing" from him. Even point your sights too long at the dead robot he may become hostile. The robot spawns alive but weakened and usually is quickly killed by the scavenger before you arrive. Even if you kill the robot before the scavenger can, he will claim it and become hostile if you loot it. Speak to him before he becomes hostile to get the vendor options. Killing him while he is hostile will result in negative karma, regardless of his reason for going hostile.
 * A Scavenger with his malfunctioning Mister Handy. If you repair it, the Scavenger will give you two Energy Cells. With a speech check, you can convince him to give you 100 caps and the energy cells. After fixing his robot, he will begin to travel the same route as the traders until he is killed. However, unlike the Caravan Merchants and Ben Canning, the Mr. Handy Scavenger will only travel if the player is in the same active map cell. Otherwise, the Mr. Handy Scavanger will remain frozen in his map cell. Waiting an hour while in the same active map cell as the Mr. Handy Scavenger will cause him to instantly travel to the next "trading stop" on the Caravan route (i.e. in front of Megaton, Rivet City, Agatha's House, etc.)
 * Raiders attacking a Scavenger with a robot buddy.

Slavers and slaves

 * Four Slavers seeking to eliminate you as competition because you have made them look bad - if you capture a large number of slaves (three or more).
 * A Hitman ambush, composed of Wastelanders instead of Talon Company Mercs, who call you "slaver scum" and attack you to avenge their friends. This encounter only occurs if the player has enslaved NPCs using the Mesmetron.
 * Three Slavers with three Slaves. Freeing the Slaves (using high Science or Explosives skill to disable their collars) earns you positive karma, but makes the Slavers hostile towards you.
 * A few Escaped Slaves who have a few dialogue options if you talk with them: if you know where the Temple of the Union is, you can give them the correct directions for some positive Karma, or lie to them for negative Karma, or just say goodbye.
 * Four escaped slaves who will ask you to remove their collars. Depending on your science skill, you could take it off for positive karma, or it could make their head explode.
 * Three Slavers looking for escaped slaves. You can lie for positive karma, or you can charge them for information and they'll give you 50 caps. However, they will become hostile no matter the dialogue if they have to give you caps. Killing these Slavers wont turn Paradise Falls hostile.
 * Two Slavers with 2-3 Slaves trying to escape from them.

Wastelanders

 * Two Wastelanders pondering an Enclave Eyebot that hovers around them consistently (as opposed to a long patrol like most). There is no dialogue possible with them, but they nevertheless hold a long dialogue with one another, discussing the eyebot.
 * Ghoul Wastelanders around a burning barrel, telling you they were headed for Underworld but couldn't make it because of Super Mutants (this contradicts Willow's statement that Super Mutants don't attack ghouls). They will tell you how to get to Underworld and mark it on your map.
 * Three Wastelanders wearing Merc Outfits and carrying mid-level weapons, who are planning to raid Oasis. If you approach and ask them what they're discussing, they will accidentally reveal the location of Oasis and turn hostile. If you act like it's none of your business, they'll shoo you off and you can no longer initiate conversation with them; they will then remain in that location indefinitely.
 * A Wastelander attacked by Mole Rats. If you save him/her he/she will give you a small amount of caps.
 * 1-3 Raiders attacking 1-3 Wasteland Settlers.

Brotherhood Outcasts

 * Three Brotherhood Outcasts (or two Outcasts and a robot) fighting Raiders.
 * Two or three Outcasts fighting three Enclave Soldiers (or 2 Enclave Soldiers and an Enclave-controlled Deathclaw)-
 * Three Brotherhood Outcasts fighting Talon Company Mercs.
 * A Brotherhood Outcasts scout being chased by two Deathclaws; an Outcast patrol joins battle.

Super mutants

 * Uncle Leo, a friendly Super Mutant. If you tell him you're going to rob him, he'll give you a Dirty Pre-War Businesswear as a present. If you attack him he'll run away instead of fighting.
 * Three Wastelanders fighting a Super Mutant or Centaur. If at least one survives and are spoken to afterwards, they'll thank the player and hand over an item.
 * Super mutants with captives.

Creatures

 * A weakened Deathclaw and a dead Wastelander. You can find Schematics - Deathclaw Gauntlet on the dead Wastelander.
 * A friendly Giant Radscorpion fighting 1-4 Giant Worker Ants. It will become sometimes hostile if you get too close to it.
 * One to four Giant Ants around a toxic barrel with one being bigger than the others.
 * 5-7 Radroaches.
 * A pack of Vicious Dogs.
 * A pack of dogs guarding a corpse.

Other

 * A Vertibird landing, dropping off a combination of Enclave soldiers and/or Sentry Bots, and then flying away. In certain areas, will repeat over and over again, dropping off endless armies of enclave.
 * Five Frag Mines which cannot be disarmed placed in a Square shape with one in the centre.

Unique encounters
These encounters only appear once in a given game and only in certain locations per type. See.

Type A
Water-Related
 * Ben Canning. He is dying of thirst and asks for water. He will accept Purified Water or Dirty Water. If you help him, he will join the Caravan Merchants' rounds around the wasteland (but not actually sell anything) until he dies.
 * Two Wasteland Settlers and two Escaped Slaves fighting over a refrigerator of 10 Purified Water. Opening the refrigerator will turn them hostile to the player, but sometimes just standing near it for too long can make them attack you. You can also convince them with a Speech check to share the water.
 * The player encounters a group of Wasteland wanderers who are trying to get water from an old fire hydrant (or refrigerator) guarded by an abnormally huge Radscorpion. The player may help the wanderers get to the water or take it for himself.

Misc
 * A group of Wastelanders fleeing from a Deathclaw in pursuit. When you speak to an NPC after killing the Deathclaw he/she gives you the location of Rock Creek Caverns. The "map" he gives you is listed in the "Notes" of your Pip-Boy with the description: "Mirelurk King's Treasure Chamber!"* Deathclaw bones
 * You receive the signal of a Chinese Radio Beacon and find a dead Chinese Commando at the spawning point.
 * Exploding UFO overhead, leaving Firelance and Alien Power Cells scattered on the ground
 * Mel, a modern Highwayman who will demand caps and items from you. You can either utilize speech options, or kill him.
 * Robot stuck in a conversation loop. On-looking technician requests help.
 * Two Wastelanders near Frag Mines; one Wastelander will unsuccessfully try to disarm one of the mines.
 * Five Raiders with four of them beating the fifth one (with baseball bats), who is in undergarments. It seems to be an initiation. If you sneak up and listen closely, you will hear what they are saying as they're beating the one without clothing.
 * A dead Scavenger with a Mini Nuke and a Fat Man in poor condition.
 * Laszlo Radford opens fire on you with his Railway Rifle.

Quest-related
 * Big Trouble in Big Town: A traveling merchant is headed to Big Town. His dialogue depends on the option you chose to finish the quest:
 * "Guns": he tells you that Big Town has made a large order for guns, and he is headed there.
 * "Robots": he will tell you that they ordered a bunch of robot parts.
 * "Sneak": he tells you that the residents have placed an order of a few Stealth Boy units.
 * "Explosives": he tells you that the residents have placed an order for a lot of mines.
 * Fought super mutants alone: he tells you that is safe there to trade.
 * Abandoned Big Town: he tells you he is on his way to loot Big Town because it was destroyed and left deserted by Super Mutant attack.
 * The Superhuman Gambit: A child will walk up to you and ask for your signature. Occurs when wearing either one of the superheros' armor
 * John's Treasure Box: Dead wastelander with John's Treasure Box Note.
 * Oasis: Player encounters Wasteland Merchant. After having a talk with him about the Great One, he'll collapse and die. You'll find a Wasteland Merchant Note on his body about him going to Oasis the next day.
 * Stealing Independence: Sydney and her friend Emaline attack the player. Occurs if Sydney is still alive and the player did not team with her or double-crossed her.
 * The Power of the Atom: Mr. Burke sends hitmen after you. Occurs after disarming the bomb in Megaton.
 * Megaton Refugees attack player. Occurs if you detonate the bomb in Megaton).
 * The Wasteland Survival Guide: A wastelander battling any creature or a protectron, help them win and tell them you helped write the Wasteland Survival Guide they will give you caps or stimpacks, radaway etc.
 * Trouble on the Homefront: Amata being questioned by two hostile Enclave soldiers and a non hostile Enclave officer; they will ask her where she obtained her Pip-Boy, and then ask for the location of her vault. After telling them however, the Officer will simply tell the troops to open fire, killing her; you can save her for no reward or dialog options, but she will only say "this is all your fault".

Type B
Quest-related
 * Reilly's Rangers: Donovan from Reilly's Rangers engaged in battle with three Super Mutants. Save him and he will give you a Mini Nuke. Only occurs if Donovan and Reilly are alive.
 * Trouble on the Homefront: Encounter with Susie Mack if you made Amata the Overseer of Vault 101. She will tell you that the Vault is doing fine and give you Purified Water.
 * Those!: Wastelander fleeing from Grayditch (Giving you the location of Grayditch)

Misc
 * Cannibals hunting their "prey".
 * Cannibals selling Strange Meat
 * "Human bomb" trap and raiders lying in wait.
 * Mad Brahmin attacks anyone in the vacinity. (Animal friend perk stops these attacks)
 * Sam Warrick shooting at a Wastelander. If he kills the Wastelander, he'll turn on you.

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