Mysterious Stranger

Mysterious Stranger is a perk in Fallout, Fallout 2, Fallout 3 and Fallout: New Vegas.

Fallout and Fallout 2
Only in hostile random encounters, there is a chance that your character will gain a temporary ally in leather jacket (male with a male character and female with a female character), details depend on the player's level.
 * $$\text{Chance of appearance }%=30+(2\times\text{Luck})$$

Fallout
The Mysterious Stranger remains on the map of the random encounter even after the end of the fight, until you leave. You can start a conversation with them but they will not say much.

Fallout 2
Contrary to Fallout, the Mysterious Stranger will not say anything and they will leave the map of the random encounter directly after the end of the fight.

Bugs

 * The Mysterious Stranger with the hunting rifle actually spawns shotgun shells as ammunition instead of .223 FMJ. As such, the Mysterious Stranger only has the 10 shots already provided in the clip.
 * In Fallout 2, if you save during a random encounter when the Mysterious Stranger is still on the map and then load that save they will remain frozen in place.

Fallout 3 and Fallout: New Vegas
Only in V.A.T.S., there is a flat 10% chance that your character will gain a temporary ally in fedora and overcoat armed with a powerful and unique .44 magnum, dealing exactly 18002 (9001 + 9001 critical damage with 100% critical multiplier) damage per shot and will kill any enemy in one hit. Regardless, he will still empty six shots into a chosen target.

Additional dialogue options

 * In the Fallout 3 add-on Mothership Zeta, when you first arrive aboard Zeta, having the Mysterious Stranger perk opens up a new line of dialogue with Somah, with you saying "maybe, if we're lucky, a friend of mine will show up to help us."

Bugs

 * The Stranger will sometimes be positioned unfavorably. This can include being stuck in a wall or such, being placed in water, being placed near a cliff, etc. This can result in his death (unconsciousness) or inability to effectively attack the enemy, or even in some cases to kill the player if the player is between the Stranger and his target. NOTE THAT THIS WILL RESULT IN YOUR GAME ENDING. Because the Stranger never attacks, even if all enemies have died, he will be forever trapped inside the level geometry and the trigger to return the game to normal never happens. Your only choice is to load a previous save.
 * During V.A.T.S., it isn't uncommon to hear the Mysterious Stranger music, even if he does not appear. This most likely happens at times when he would have appeared to finish off your enemy, but you killed the enemy instead. He may also have been affected by the bug above in some of these cases.
 * Sometimes when the Mysterious Stranger appears, he doesn't shoot. He'll just stand there, leaving you helpless. Another variant causes the Stranger to reload his magnum for the duration of his attack. If it jams, he may not attack at all. This also happens if his target is killed prematurely by an explosion or by you or a companion. V.A.T.S. will remain active for an extended time, leaving you open for attacks by any surrounding enemies.
 * While in the Operation: Anchorage simulation, the Mysterious Stranger will sometimes appear and then lie unconscious. When the V.A.T.S. sequence ends, gameplay will resume to normal.
 * At times, the Mysterious Stranger will aid you during V.A.T.S., but there will be a 5-10 second delay between killing the target and returning to normal gameplay. This can pose problems when fighting multiple enemies, as they will continue to attack both you and your companions during the delay. On occasion, this bug may cause normal play to not resume.
 * Sometimes, the game can freeze in the zoomed in style of V.A.T.S. when the Mysterious Stranger appears, making him stand there and watch.
 * Occasionally, the game may register that the Mysterious Stranger has appeared (you receive a progress update) but he does not appear, nor does the opponent die. You remain frozen in V.A.T.S. unable to move or die.

Behind the scenes

 * The Mysterious Stranger perk was originally designed by Scott Bennie.
 * The Mysterious Stranger in Fallout 3 and Fallout: New Vegas wears a fedora and overcoat, like the protagonists of 1950s film noir.

Gallery
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