A Meeting of the Minds

 is a Fallout 3 quest and achievement that is added to the game with the Point Lookout add-on.

Making Your Way to the Brain
After telling Desmond that you are ready to go, you will both make your way to the Lighthouse. Once you make your way to the base of the Lighthouse, enter it. When you enter, a section of the Lighthouse floor slides away and reveals a door to what is called the "Underground Lab". Once inside the lab, you will begin making your way down with Desmond, fighting turrets and Robobrains along the way. The fighting will occasionally pause after you've cleared the area, where in you'll have to find a clearance badge in order to open locked doors (via terminals). This pause also gives you a chance to loot the surrounding rooms of weapons and ammo (including Mesmetron Power Cells, MFCs, Laser Rifles/Pistols, and even 2 Gatling Lasers), and also get some background information of the lab.

Who to kill?
You and Desmond will eventually reach Calvert's room, where you'll see the Brain himself in a massive glass tank surrounded by inactive protectrons. Desmond and Calvert will have a short conversation, each of them claiming to have your support. Eventually you'll be forced to decide which of them to kill. If you speak to Calvert, he'll offer the greatest thing any human could ever hope for in exchange for your killing Desmond. You now have a decision to make...

Killing Calvert
Desmond may be an ass but that crazy brain-guy has gotta go. If you decide to side with Desmond, simply shoot at the Brain's glass tank until the glass shatters (note that after beginning to fire on the brain, many of the once inactive Protectrons will attack). After the Brain is dead, any remaining robots will be disabled and you can talk to Desmond. He will be overjoyed to have finally won and remarks that everything that Calvert stole from him is finally all his. He will also give you a key which unlocks the nearby door. Inside this door are 2 ammo-boxes, a safe, 2 first aid boxes, some energy weapon ammo, some misc. items, and of course, the Microwave Emitter! The room also has a ladder leading outside.


 * If they're not already so, all unnamed tribals will become hostile to the player. However named tribals such as Croatoa and Woodrose will only become hostile if they see you attacking other tribals.
 * It is possible to still kill Desmond after completing this quest. This will allow you to take his glasses (though they can still be acquired through reverse pickpocketing better gear to replace their glasses and then stealing them).

Killing Desmond
Serves that asshole right for treating you like dirt! If you decide to kill Desmond (who will only become "killable" after you tell Calvert you're going to kill him), you'll be able to loot his corpse for his special glasses, his clothing, as well as any weapons he's carrying (which should include a combat shotgun and Sniper rifle along with the corresponding ammo).

After you're done, the Professor will reward you with the greatest thing any human could ever hope for... DEATH! At least he'll try, at which point he'll send some of the previously inactive protectrons after you. Destroy them and the professor (though killing the professor will disable all the robots anyway), then feel free to loot the area. You'll find a door has opened, giving you access to 2 ammo-boxes, a Safe, 2 first aid boxes, some energy weapon ammo, some misc. items, and of course, the Microwave Emitter! The room also has a ladder leading outside.

Bugs

 * In some cases if you choose to kill Calvert the Protectrons don't attack.
 * In the room where you need the alpha security badge, if you hack the terminal to unlock the door before you have the badge, the game still thinks the door is closed and there is an invisible barrier that you cannot pass. Obtaining the badge does not remove the barrier, and it is impossible to finish the quest.
 * Occasionally, Desmond will become stuck in one of the side rooms.
 * Sometimes Calvert's tank will be missing and the brain will be replaced by one of the bobbleheads that appeared after getting the seeds. In this case, Calvert is not able to be killed and after killing Desmond you will be unable to talk to him. In other words you will need to backtrack your saves in order to complete the quest.

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