Old World Blues (quest)

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

After completing this quest, the Make up Your Mind and Outsmarted achievements/trophies will be earned.

Detailed walkthrough
The Courier can begin this quest by speaking to Dr. Klein, where they will tell them that it is time to go to the Forbidden Zone and get their brain back. He also asks the Courier 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 character must begin by traveling to the Forbidden Zone. Once there, the Courier will encounter several robo-scorpions surrounding the entrance. After dealing with them and entering the zone itself, Dr. Mobius will release his most powerful creation, the X-42 giant robo-scorpion.
 * Enter the room further to the right with stairs that leads to a terminal, which gives the player the option of activating the X-42 backup generator, or 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 will cause 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 Courier lacks the necessary Science skill, the password to the computer is on a holotape to the left of the leftmost Protectron bay.
 * Alternatively, if the Courier 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.

After defeating it, the Courier must confront Dr. Mobius. To do so, take the stairs to the left of the entrance and enter the door on the right. There they will meet Dr. Mobius, who is not hostile.

Here there are two choices: kill or reason with him.

Option One: Reason with Dr. Mobius

If reasoning with Dr. Mobius is chosen, he will direct the Courier to their brain where they must convince it to return. Optional speech checks are available with him:
 * Repair 75
 * Science 50
 * Speech 65
 * Intelligence 7 (x2)
 * Perception 7, Medicine 65, Guns 75 (do all 3, repeat conversation line)
 * Intelligence 7.

Option Two: Fight Mobius

If the Courier chooses to fight Mobius, Dr. Mobius will release five MK4B robo-scorpions. The player will need to defeat Mobius and the robo-scorpions in order to continue. 1066 XP will be awarded for defeating Mobius.

Once Dr. Mobius is dealt with, the Courier must then speak to the Brain Tank, and convince the brain to join them again. Having high Medicine, Speech or Science will give them a significant 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 not mortified response from the brain that does not help much. 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). Choosing to have the brain reinserted into the Courier's body will result in replacing the Brainless perk with the Big Brained perk, which yields +10% DT (Damage Threshold).

If Mobius is alive After acquiring the brain, the Courier needs to speak to Dr. Mobius again. He will tell them more about what the Think Tank is planning and suggests dealing with them, instead of him. The choice is up to the Courier.

If Mobius is dead Walk up to the Brain Tank and speak with it, thus re-acquiring the brain.

After making the choice, the Courier needs to return to the Think Tank. Then, the quest will end to a cut-scene. After that, The Courier will have their Brains installed.

There the Courier has the option of using the Sink's Auto-Doc to retrieve their original heart and spine.
 * Choosing to replace their original heart results in replacing the Heartless perk with the Cardiac Arrest perk (healing chems are 50% more effective)
 * Choosing to replace their original spine results in replacing the Spineless perk with the Reinforced Spine perk (STR and DT bonuses doubled (+2))
 * If the Courier decides to leave their heart and spine, they can later be replaced at any time. At this point the Courier must choose to deal with the five members of the Think Tank, through violence or reason.

Option One: Reason with the Think Tank

For those wishing to reason, the speech checks are:
 * Speech 75
 * Medicine 85 or Science 100.
 * If the Courier has completed all four of the 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 them to reason with the Think Tank without the high skill checks.

Option Two: Fight with the Think Tank

The Courier must destroy all five members of the Think Tank.

After completing the quest, and watching the ending slide show, the Courier 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 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 Courier convinced the brain to come back), the brain will either be located in the far left room, (where the heart and spine have been stored) or in the Courier's inventory. They will then be able to re-transplant or de-transplant their brain later at any time or leave it in the storage facility. Doing so will change brain, heart, and spine-related perks.

Bugs

 * 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 in Hardcore Mode when the player has a hunger greater than zero. If this is the case, turning hardcore off temporarily may allow to continue bug free to the end.
 * When speaking with Dr. Klein at the end of the add-on, some dialogue may repeat when the other brains are trying to keep the player alive.
 * If the Forbidden Zone is entered without first killing the robo-scorpions outside, upon exiting, all of the robo-scorpions will be gone.
 * While entering the Forbidden Zone, the X-42 giant robo-scorpion may not attack unless it is approached. This will remain the case even when attacking the X-42 giant robo-scorpion.
 * When the brain is asked if it wants to be reunited with the Courier, choosing the compromise option, the second choice, "So, what, you'd rather just stay here? Never leaving that tank of... whatever that stuff is?", 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 the player entered Mobius' room and pick alternate dialogue options.
 * An easy fix on the PC is to open the developer's console and enter . This closes out of the conversation for the player to try again.
 * 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.
 * 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.
 * When the Protectrons inside the Forbidden Zone dome are activated, a seemingly unlimited number of them may keep attacking the X-42, creating an effective distraction when attempting to destroy the X-42 giant robo-scorpion.