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 the "Underground Lab". Once inside the lab, begin making your way down with Desmond, fighting turrets and several robobrains along the way. You can let Desmond run ahead and engage first; if hit enough he will become unconscious but will not die. If you have the robot perk, then you may be able to deactivate the robobrains while they are distracted by Desmond. Fighting will pause after clearing each area.

The first room on the left from when you enter the lab includes three first aid boxes and a Gatling laser. In the next room directly downstairs, use the terminal to deactivate the turrets and pick up a Mesmetron power cell and microfusion cells. If you delay long enough, Desmond will start to dance impatiently in front of the door.

In the next section, the first room on the left includes several types of energy weapon cells. The first room on the right is Dr. Stewart's office and includes an average safe with a Stealth Boy and other random loot, an ammo box with microfusion cells, and three pre-War books. The terminals in this room and those thereafter contain background notes on the lab. The last room before the stairs includes several more energy weapon and mesmetron cells.

The stairs lead to a round room with two doors, one open. The room on the right has more power cells and a security badge. A robobrain sentry is stationed behind a locked door in this room and will come out once you enter the room. After handling the sentry, check the small room for more ammo and a stealth boy before proceeding. The next level down has operating rooms; the second room on the right houses a bunch of chems. Use the terminal to unlock the door at the end of the hall. The next room has an easy safe with a stealth boy and many caps, as well as another Gatling laser and more energy weapons/ammo.

More stairs lead down to a hallway with several doors, all leading to bunks. Continue forward and 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.

Who to kill?
If you wait a bit to talk to the Brain, Desmond and Calvert will have a short, angry conversation, each of them claiming to have your support. (If you wait long enough, the dialogue will begin to loop.) 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.