Old World Blues (quest)

Old World Blues is the final main quest in the Fallout: New Vegas add-on, Old World Blues.

After completing this quest, the player will earn the Make up Your Mind achievement/trophy and the Outsmarted achievement/trophy.

Quick walkthrough

 * 1) Talk to Dr. Klein.
 * 2) Enter the Forbidden Zone Dome entrance.
 * 3) Defeat or disable the X-42 giant robo-scorpion.
 * 4) Enter Mobius' lair and reason with or kill him.
 * 5) Reclaim or reason with your brain.
 * 6) Reason with or defeat the Think Tank.

Detailed walkthrough
The player can start the quest by speaking to Dr. Klein, where the good doctor will tell them that it's time to go to the Forbidden Zone and get their brain back. He also asks the player to deal with Dr. Mobius. Optional speech checks are available:
 * Lockpick 75
 * Repair 75
 * Science 65
 * Sneak 45
 * Followed by Science 85 or Medicine 85.

The player should begin by traveling to the Forbidden Zone. Once there, the player will encounter several robo-scorpions surrounding the entrance. After dealing with them and entering the zone itself, Dr. Mobius will release his most powerful guardian, the X-42 giant robo-scorpion.
 * The player can fight it.
 * Enter the room farther to the right with stairs that lead to a terminal, which gives the player the option of activating the X-42 backup generator, overloading it and causing some damage.
 * Enter the room to the left and use the terminal to access turret controls (Average lock) to turn them upon the X-42 giant robo-scorpion. Although they will not be sufficient to destroy the X-42, they cause it a lot of damage.
 * Activating the protectrons in the room to the right from the dome's entrance will distract the robot long enough to get to the shutdown terminal or other terminals.
 * Enter the balcony room on the far left side of the zone from the entrance, that contains a computer terminal (Very Hard lock). If hacked, it gives the option to permanently disable the X-42 giant robo-scorpion. Access to this room is via the stairs on the far left side of the room. If the player lacks the necessary Science skill the password to the computer is on a holotape to the right of the rightmost protectron bay.

Alternatively, if the player has the Robotics Expert perk, they can sneak up and disable it but the X-42 giant robo-scorpion will restart itself after a set amount of time has passed. However, if they do not have the required Science level to hack the terminals, nor the Robotics Expert perk, an LAER is available behind a force field, just past the exit balcony. This LAER is in good condition, and it can be sufficient to take out the X-42 giant robo-scorpion.

After defeating it, the player must confront Dr. Mobius. To do so, take the stairs to the left of the entrance and make their way to the door on the right. There the player will meet Dr. Mobius, who, strangely enough, is not hostile.

Here the player has two choices: reason with him or kill him. If they choose the former, he will direct the player to their brain and ask to convince it to rejoin the player. Optional speech checks are available with Mobius:
 * Repair 75
 * Science 50
 * Speech 65
 * Intelligence 7 (x2)
 * Perception 7 or Medicine 65 or Guns 75
 * Intelligence 7.

The player must then speak to their brain, and convince it to join them again. Having high Medicine, Speech or Science will give them a slight advantage and make the dialogue much easier. The use of the unique dialogue provided by Confirmed Bachelor/Cherchez La Femme/Black Widow will result in a somewhat humorous, if mortified response from the brain that doesn't help the player's case. Talking to the brain and convincing it to help in the final quest stage will grant the "Make up Your Mind" achievement/trophy. Optional speech checks:
 * Speech 75 (x2)

After acquiring the brain, the player needs to speak to Dr. Mobius again. He'll tell them more about what the Think Tank is all about and suggests dealing with them, instead of him. The choice is up to the player.

After making the choice, the player needs to return to The Think Tank. There they have the option of using the Sink's Auto-Doc to retrieve their original heart and spine. If the player decides to leave their heart and spine, they can be replaced at any time the player so chooses. Then it's either violence or reason. For those wishing to reason, the speech checks are:
 * Speech 75
 * Medicine 85 or Science 100.
 * If the player has completed at least three of the four side quests What's In A Name?, On The Same Wavelength, A Brain's Best Friend and Coming Out of Her Shell, they can tell Klein to confer with his colleagues and the others support the view of the Courier, which allows the player to reason with the Think Tank without using the high skill checks.

After completing the quest, and watching the ending slide show, the player is given the Big Mountain Transportalponder!. This will allow one to return to the Big MT via The Sink balcony at any time, or from Big MT to the Mojave Drive-in, although using it is limited to locations within the Mojave Wasteland, and it cannot be used in areas such as The Strip or while in combat. Furthermore, upon completion, all active personalities in The Sink will cheer for the Courier's victory, with a particularly drawn-out and rambling congratulations from Blind Diode Jefferson.

After the slide show ends, (and the player convinced the brain to come back), the brain will be located in the far left room, (where the heart and spine have been stored). The player will then be able to re-attach and detach his/her brain later at any time or leave it in the storage facility. Doing so will change brain, heart, and spine-related perks.

Journal entries
¹ Optional depending on speech paths taken during initial confrontation.

Behind the scenes

 * If you finished more than half of the Think Tank's personal quests and ask Dr. Klein to consult his colleagues, having completed 8|Doctor 8's quest, when it is his turn to speak, Klein will say, "Et 2, 8?" in response, referring to Gaius Julius Caesar's last words to Marcus Junius Brutus. It is also a reference to another quest in Fallout: New Vegas, Et Tumor, Brute?.

Bugs

 * When the brain is asked if it wants to be reunited with you, choosing the compromise option, the second choice, will result in a never ending loop of the brain talking about the quests. This only seems to happen if the player has completed Come Fly With Me and That Lucky Old Sun. If this happens, reload the autosave where you entered Mobius' room and pick alternate dialogue options.
 * Before entering the Forbidden Zone Dome, it is advisable to make a new separate save as when entering through the main entrance into the arena with the X-42 giant robo-scorpion, it is possible to load under the catwalk and become stuck there.
 * Be mindful before completing the quest and make a save somewhere outside the Think Tank. When the quest is completed and the player is asked to leave Big MT, the moment the slide show starts the game crashes. This forces the player to reload an earlier save. If the last save is inside the dome, they will be trapped.
 * After finishing this quest by either killing, or talking to the Doctors in the Think Tank, the game may crash as the player tries to leave. This bug occurs when you have a hunger greater than zero, and, inherently, only in Hardcore Mode.
 * The game may crash as the slide show ends, on the last slide with the Old World flag in the foreground and a sunset in the background, when the screen begins to fade. An Xbox message will say "This Disc is un-readable please turn off the console"
 * When speaking with Dr. Klein at the end of the DLC, some dialogue may repeat when the other brains are trying to keep the player alive.
 * If you enter the Forbidden Zone without first killing the robo-scorpions outside, upon exiting, all of the robo-scorpions will be gone.
 * If certain choices are made in dialogue, it is impossible to reason with Mobius and killing him becomes the only option, even if the correct skill checks are completed. This is a similar situation to what may happen with Dean Domino in the Dead Money add-on.
 * A possible fix for this bug involves a repair check to fix Mobius' monitor, which can re-initiate the dialogue path needed to avoid killing Mobius.
 * Even with all the holotapes located & installed, the only options available while interacting with the Sink Auto-Doc are heal yourself & respec your traits. This bug seems related only to the Spanish version of the game (Confirmed with the latest Spanish version).
 * While entering the Forbidden Zone, the X-42 giant robo-scorpion may not attack you unless you approach it. This will remain the case even when attacking the X-42.
 * When the Protectrons inside the Forbidden Zone Dome are activated, a seemingly unlimited/invincible number of them may keep attacking the X-42, creating an effective distraction when attempting to destroy the X-42.