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Once you have all three pieces of technology, you can make the big push into Dr. Mobius’ lab. To get this mission, just go back and talk to Dr. Klein, and with a little help, he’ll figure out that the technology must be needed to get into the Forbidden Zone Dome.
Note that going to see Dr. Mobius sets off a point of no return with the Think Tank, so you will probably want to take care of each of their quests beforehand.
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A Brain's Best Friend
This is the quest for Dr. Borous. When you talk to him, you can talk about his research and potentially express anger for his creation of nightstalkers and cazadors. The key conversation is about Gabe though. If you’ve been to X-8 and went to the observation deck, he should have a memory of turning Gabe into a massive cyberdog.
You should be able to talk about Gabe to Borous, and pick up the official quest. To complete it, you’ll need to travel to the marked location and enter Higgs Village. Then just look for the house with the doghouse behind it.
If you have Wild Wasteland on, then Stripe will be in the doghouse. Stripe is a miniature deathclaw, that can still hit as hard as any normal one. The good news is that he won’t stray too far from his doghouse, so you can just attack him from the street and be fairly safe. You still have to be careful though, as one hit will be enough to nearly kill lower level players, and it can still take off half of a high-level character’s health.
When you're done, grab Gabe’s Bowl from in front of it. If you turn it back in to Dr. Borous and pass a Speech check of 45, he should have some feelings of guilt for the first time in years and switch over to your side.
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On the Same Wavelength
If you talk to Dr. 8, you will just get static for several questions. After you ask a few things though, you’ll get the opportunity for a skill check. With 75 points in Science, or 7 points in Perception, you can figure out that he’s speaking in repeating lines of code.
You can then use the same 75 points in Science to threaten to hack him. Evil characters can then force his compliance and make him hand over some ammo, while good characters can comfort him and say that they’d never do it.
After comforting him a bit, you should finish the quest.
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Coming Out of Her Shell
This is a fairly easy one. You just need to talk to Dr. Dala and question her about her unusual interest in the human body. After this point, you have a few options. Those with Lady Killer or Cherchez La Femme can just use that to get Dala to open up. There’s also an option for those with a decent amount in Speech, or apparently, you should be able to show her an actual teddy bear to stir up some feelings in her. You can easily get one of these from her room in the Think Tank (just go up a level and look in the sealed rooms) or from her old home in Higgs Village.
Regardless of how you get her to be honest with you about her strange attraction to your bio-rhythms, you can then agree to let her “examine" you. After an awkward moment, you’ll get some energy cells and her warm thanks.
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What's in A Name
T his is the easiest one, since it’s compact at least. While you can talk to Dr. O about a few things, the key issue is his name. You can get him to open up with 65 points in Speech. Once he explains that it’s “Dr. Zero" and not “O", you will have a possible skill check. Those with Math Wrath or 9 points in Intelligence can point out that a slash through the zero will denote that it’s a number and solve his problem.
He’ll be a very grateful ally afterwards.
Note that if you can’t pass the skill check, there are plenty of mentats lying around the base. That’s an easy +2.
There are also Mad Scientist Scrubs in your bedroom in The Sink. That’s a +1.
If you can pick an Average lock, you can go to the locked room near Dr. Klein in the Think Tank to find Dr. Mobius’ old room. On the floor, you can find Dr. Mobius’s Glasses, which grant a +2 boost to intelligence.
You can also easily pick up Dr. Klein’s scrubs from his bedroom in Higgs Village, at house # 101.
If you have less than 3 points in Intelligence, then you’re just out of luck. Try to do Intense Training or just focus on the other scientists’ quests.
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A Fight to the Forbidden Zone
It’s a long walk through a dangerous valley. You’ll probably find a number of lobotomites and even some Y-17 harness suits. I assume that you’re comfortable fighting these enemies by now though. The real trick will be what happens when you actually reach the door to the Forbidden Zone.
There is an army of roboscorpions waiting at the door. When you approach the door for the X-42 war room, a lot of scorpions will spawn in. I strongly suggest that you get to the edge of the battlefield quickly. Fire as you move. You should be able to use a proton axe or the antenna to keep them at bay. One swipe will disable them temporarily, which makes the fight much more manageable.
If you keep falling back, you should be able to just shrug off the laser damage and thin them out. With a few stimpaks, you’ll be fine. Also note that this is basically the final battle for the DLC, so don’t bother holding back. If you’ve got good weapons, fire away.
Once the path is clear, go into the bunker to find the X-42 giant roboscorpion.
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X-42 - The Giant Robot Scorpion
If you want, feel free to just fight it. You can use the sonic emitter or the LAER and you’ll be fine, for the most part. You better have a lot of stimpaks to heal up the damage though, and a lot of ammo. There are some other options though.
The giant scorpion has about 2,500 HP, good defenses and a strong laser attack. The blast takes a few seconds to charge, but it has a huge blast radius and emits radiation too. You really want to be inside one of the side buildings when it goes off, and stay away from the windows.
There are some creative options for this course though. You can start by running into the nearest bunker to the right. If you jump onto this computer, you can set up some target drones and have them enter the course. This will draw the X-42’s attention for a long time, which will let you continue to the other bunker on the right. Note that if you have 100 points in Science, don’t bother with the following steps. In the back left corner of the room, you can access the emergency shutdown terminal. If you can hack this Very Hard terminal, you can end the fight now. Use the drones for cover and run for the other set of stairs and then look for the computer.
If you don’t have the points in Science, you can then go to the other computer inside the bunker to the right. You should be able to just use the computer and activate the backup generators, which will cause a massive explosion and take about 1,000 HP off of its health.
If you want, you can also go to a computer in the front-left corner. The one closest to the door doesn't have anything, but the other has a terminal that will let you activate the turrets and turn them on the giant scorpion.
The last step, if you can’t hack a Very Hard computer, is to grab a mint condition LAER off of the table in the bunker in the back right. Note that you should be able to jump over the rails to save yourself some pain. This weapon will do a lot of damage to the X-42 giant scorpion, so if you don’t have something equally good, run inside and grab it and the ammo. Then just start firing and drain the rest of its health while using the buildings for cover.
Once it’s dead, make sure that you search the remains. There’s a final setting for the sonic emitter, “roboscorpion," inside it. When that’s done, get onto the raised catwalk and go through the door to reach Dr. Mobius’ lair.
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Once the surprise has worn off from the meeting of the minds, you’ll need to make a decision. If you wish, you can go ahead and kill Dr. Mobius. He’s surprisingly tough, since he’s fast and has a good laser built into him, but he’s far from invulnerable. Just make sure that you’re not too hurt from the last fight.
The only other wild card is that he’ll spawn some roboscorpions behind you, which will make things more frustrating. Just keep on the move and focus on killing Mobius first, then use your usual tactics to take out the scorpions.
Note that I don’t believe there’s a concrete benefit for killing him, it seems to mainly be there for roleplaying. It's up to you.
If you don’t wish to kill him, just say goodbye when you’re done talking to him.
When you’re done with Dr. Mobius, walk over to the center machine on the top floor to find your own brain in a tank. You can then talk to it. Due to a potential bug, make sure that you save now.
There are a few things to note. If you want, you can decide to just leave it behind after it helps you defeat the Think Tank. Otherwise, you have to talk it into coming with you. There are several options, although Science and Medicine checks seem to work well. Note that for the time being, the speech path that goes roughly to the “you’ll just stay here forever?" passive aggressive option will have him get stuck on an endless loop, merely mentioning your adventures with Jason Bright and Helios 1 forever. You’ll need to reload a save if you trigger this, and do the other path.
If you push and promise to protect the wasteland against the Think Tank, you should be able to get him to come with you.
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Confronting the Think Tank
Travel back to the Think Tank, and consider swapping your organs back into place at the Auto-Doc.
As the screen warns, talking to the Think Tank will force an ending. Make sure that you've done everything that you want to do, although you can come back and do most of the quests after the ending. . You now have a few options to deal with them.
If you did all of their quests, you can just tell Dr. Klein to “confer with his colleagues", who will refuse to fight you. You can then make him surrender and agree to stay in the Big Empty without any skill checks.
If you didn’t do the quests, you can still make them surrender with a good bluff. 75 points in Speech will let you convince them that you’re Dr. Mobius, which means that you can present his deal to them. 85 points in Medicine or 100 in Science will convince them that there really was a brain implantation, and they’ll stand down.
If you can’t do either, then there will be a fight. Make sure that you grab your LAER. The one that I had from the roboscorpion fight was able to kill each tank in a few shots, which turns the battle into a simple shooting gallery. Their lasers are fairly powerful, so you’ll need to be careful, but if your armor is holding up, you should be fine. Their power decreases greatly once you’ve picked off the other bots. Klein by himself shouldn’t be much of a threat to your superior firepower.
Once you’re done, regardless of your choice, just walk out of the Think Tank to get your ending, and receive the Big Mountain Transportalponder. You can just pull this weapon out and fire it to teleport back to the Wasteland.
Congratulations! You have beaten the DLC, and you now have a whole new home to visit and fill up with new gadgets.
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All screenshots and information from Fallout: New Vegas - Old World Blues