A Meeting of the Minds

A Meeting of the Minds is a Fallout 3 main 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. Along the way most players may be able to hack computers to disable the turrets. As for the Robo Brains, if the player has the perk which allows them to deactivate robots the robobrains are more likely to be distracted by a Shotgun wielding ghoul than the Lone Wanderer sneaking up to them. The player should be able to deactivate them while they are so distracted. The fighting will occasionally pause after you've cleared the area, use this time to find a clearance badge in order to open the 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 ammunition (including Mesmetron power cells, microfusion cells, laser rifles/pistols, and even 2 Gatling lasers), and also get some background information on the lab.

Whom 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. 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 ammunition-boxes, a safe, 2 first aid boxes, some energy weapon ammunition, 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".

Killing Desmond
If you decide to kill Desmond, 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.
 * 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 when you reach the first door that slams shut and requires you to hack or find a card, if you decide to hack it, when the door opens the player will not be able to proceed, an invisible barrier is blocking the door. This occasionally happens with others doors. To fix this, simply save where you are then reload the save. You should be able to go through the door.
 * Some ammunition boxes, items and containers cannot be opened; they can be reached by using the console command through the floor.
 * It is possible that Desmond will run into the cupboard room when you open the first door after you meet up with him and then he doesn't move from there unless you proceed into the next area, where he still may not proceed with you.