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 Robobrain sentries 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, or, you could just hack into them, provided your Science skill is high enough). This pause also gives you a chance to loot the surrounding rooms of weapons and ammo (including Mesmetron power cells, Microfusion cells, 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
If you decide to side with Desmond, simply shoot at the Brain's glass tank until the glass shatters, preferably with a missile launcher or a Fat-Man (although an automatic weapon will do fine). Note that after beginning to fire on the brain, many of the once inactive Protectrons will attack, so setting a few pulse charges in advance may help. 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 (although he's still a bit rude to you). 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.

After speaking to Desmond again, he will tell you that after he's done in the lab he will go north to confront another one of the "players". He refers to this contest as a "microcosm of the old world".


 * 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
If you decide to kill Desmond, 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.


 * Desmond doesn't need to be alive in the end, you can kill them in either order, and come out with the same loot. After killing the professor without Desmond, the door mentioned will automatically unlock, giving you access to the Microwave Emitter, and a way out.

Bugs

 * Sometimes it may be possible to look at the top of Calvert's tank and talk to him after killing him. In this case he says nothing (A blank subtitle appears if subtitles are on) and you gain the Vault Key.
 * Calvert's brain may actually be found relatively intact near the bottom after the glass tank is shattered. At this point it is also possible to destroy the brain itself with a single shot. Sometimes it may even just float where it was originally. If you grab Calvert's brain after destroying his tank, his brain will constantly be doing flips in the air while you are holding it.
 * If you use the Railway Rifle to shoot the glass which houses the Brain during the final showdown with Calvert, the glass will break and leave the Railway Spikes hovering in midair where they struck the glass.
 * In some cases even after killing Calvert, you will occasionally receive the popup "The Brain is unconscious". The Brain is located in an non-openable Polowski shelter accessable only by glitching into the building north of the ferris wheel. A tribal with a hunting rifle is in the shelter with the Brain shooting him after having clipped through the roof, however, as the Brain is immortal and the tribal is not, the Brain eventually kills the creature. The Brain can be seen on the players compass as a friendly NPC but is unlocatable unless glitching.
 * If Desmond is killed by the turrets or robots, Calvert will speak to you as if you had killed him.