For Auld Lang Syne

 is a quest in Fallout: New Vegas.

Pre-Quest Notes
Before this quest can be started an appropriate point in the New Vegas storyline must be reached so that you can tell the Remnants who to fight for, you must have progressed in any one of these quests to the point of being committed to this path: (You have to complete the part about the omertas and be told to go after the brotherhood, then arcade will initiate the mission) Also, you must reach a certain level of 'trust' with Arcade Gannon before he will open his past up to you, the following are ways to gain trust:
 * For the Republic, Part 2
 * Render Unto Caesar
 * The House Always Wins
 * Wild Card: Side Bets
 * Become accepted with the Followers of the Apocalypse.
 * Take Arcade to the Crashed Vertibird, he will talk about how he hasn't seen one in a long time before quickly adding it must have been in a book.
 * Take Arcade to the REPCONN Headquarters where he will talk about the history of REPCONN and it's research into plasma weapons after being bought by RobCo, with an intelligence of 6 you can inquire where he learned about this but he will dodge the question.
 * Taking Arcade Gannon into the Silver Rush in Freeside and he will start a dialog about plasma weapons. You can ask him how he knows so much about plasma weapons but he will dodge the question.
 * Going to Camp McCarran after completing the quest There Stands the Grass and talking to Doctor Hildern about OSI will gain trust.
 * Going to HELIOS One and speaking to the Fantastic, then assuring Arcade Gannon you'll reroute the power supply to Freeside.
 * The most beneficial option is to visit The Fort while Arcade is your companion, then remark that you "just want to see what Caesar's up to and get the hell out", this will gain enough trust to start the quest.

Walkthrough
Once you have obtained the quest you will need to visit various locations to recruit 5 old members of the Enclave to aid in the final battle at Hoover Dam. Note that these five members are killable in the world so be careful not to do anything to them should you run into them beforehand or this quest cannot be completed. You can go to the remnants in any order you choose. Your Pip-Boy will direct you to where you need to go. The 5 remnants are: Doctor Henry, Cannibal Johnson, Judah Kreger, Orion Moreno and Daisy Whitman. The last 4 are fairly easy to convince and any dialogue option should bring them to the meeting place, however Henry will take some work if you have not visited Jacobstown and done his questline yet. Guess Who I Saw Today will need to be completed before Henry will agree to go to the Remnant Bunker. Once you have convinced all 5 remnants to head to their secret bunker you will be told to head there and meet them to instruct them as to which side to support in the battle. The passphrases they give you upon recruiting them will be used to open the bunker's main door. You don't need to remember them or unscramble them, once you've sent all 5 there the full sentence is there as a conversation option when you interact with the entry terminal.

Upon entering the bunker head to the briefing room in the back and talk to Judah. You will then have to make a choice, have the Remnants support the NCR or Caesar's Legion in battle. Even if you are working with Mr. House or Yes Man, those questlines still will need them to side with the NCR. Should you ask them to side with the Legion, Cannibal Johnson will leave and there is no way to convince him to stay. You can then talk to Orion for training in Power Armor and will get Cannibal Johnson's power armor since he didn't take it when storming out. You can also ask them to side with the NCR. Orion Moreno will leave because he bears a strong hatred towards them. Leave the briefing room and talk to him in the hanger. You can convince him to stay and fight, with a Speech score of 80, and if you cannot do that, you will have to fight and kill him as he will immediately turn hostile. You can also tell him, with a high enough NCR infamy, that you do just as much bad as good for the NCR. Regardless of how you handled that situation, Judah will tell you to talk to Daisy for Power Armor training and can use the armor you can loot off of Orion if you killed him or armor from Daisy as she says it makes the cockpit too crowded if she's wearing it.

Judah will ask you to head on out and finish up your business while they plan their attack. When the battle at the dam takes place they will be dropped in by the Vertibird and battle whichever side you had them fight against. When you leave the bunker Arcade should approach you and ask what you think he should do. He wonders if he should aid the remnants in battle and follow in his father's footsteps, or stay out and make his own path. If you remind him of his duty to Freeside each time he talks to you about the Remnants, then have him not join the remnants he will give you his father's Enclave Tesla Armor with Helmet for your use (But to do this you must pass a speech check of 80). No matter which option you choose, this will complete the quest.

If you are working with Yes Man or Mr. House, then during the battle of the dam you will have to fight the Remnants if you recruit them as there is no option for them to support you. Although, if you pass a high speech check to the guards in the control room convincing them to leave, then remnants (and all NCR troopers as well) won't turn hostile.

Bugs

 * Player is unable to leave the briefing room (door is locked and requires a key) after using speech skill to convince Orion Moreno to stay. One work around is to pickpocket the lock down code from Orion Moreno. Another work around is to kill all the remnants. They will not show up at Hoover Dam but Arcade Gannon will not be hostile.


 * Player is unable to leave briefing room after killing Orion Moreno due to the key being removed from inventory upon quest completion. No other NPCs in the room have a key, and the only option is to reload a save. the Remnants will still help you during the final battle, but the quest will never be completed.


 * After completing the quest, Gannon will ask whether he should stay with the Enclave Remnants or return to the Followers of the Apocalypse, but the player will be unable to answer. The player then will be unable to open Gannon's follower wheel(Note:this can be fixed by dismissing him as a follower then recruiting him back and before you that pick pocket him him for the stuff you gave him since wen you try trading with it wont show the trading screen.)
 * Player is unable to start quest, despite having exchanged dialogue with Arcade that should allow the quest to begin. The quest does not appear in the pip boy quest list. (Console command: "setstage 00157e60 10" activates the quest.)


 * Gannon will leave your companionship, and there will be no dialogue to recruit him back. This is bug has been documented with Gannon outside of the quest, firing ED-E or Rex may restore the dialogue to hire him in some cases.
 * This may not actually be a bug as Gannon either goes in and joins the Remnant (presumably to fight with the final battle), which would make him unrecruitable as he is busy preparing, or returns to Freeside to be a doctor when TSHTF, so you cannot recruit him for the final battle (which you have to be near in order to activate this quest). In other words, it is unclear whether this is a bug or a feature.


 * Gannon will not give the dialogue for activating the quest no matter how many of the locations have been visited, making the quest impossible to start. If this happens and you are playing on the PC you may open the console click on Gannon to select him, then type AddScriptPackage 166d95. This will cause him to immediately approach you and give the quest.


 * Upon entering the Remnant Bunker, Daisy Whitman will randomly die, causing the mission to fail.


 * Doctor Henry will only agree to go to the Remnants reunion after completing his research on Nightkin, sometimes after handing in the chewed stealth boy and telling Lily to help, he will never use the stealth boy MK II on Lily. Thus making his research incomplete and making it impossible to get him to go to the Reunion.


 * When the Enclave Remnant appear during the fight at the Hoover Dam, if the player is sided with Mr. House or Yes-Man, the Remnant will be hostile.


 * If you are already wearing world looted remnant armour when you talk to Daisy to get her's then it will not spawn in your inventory.


 * When the Enclave remnants appear at Hoover Dam, Orion Moreno will be begin slowly walking towards the visitor's center rather than follow you, and a 2nd Doctor Henry will join you, wearing the Wasteland Doctor Outfit. The reason for this is unknown.

Behind the scenes

 * The name of this quest is in the Scots language. The translation of this quest is "For Old Times Sake"

Videos
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