A Meeting of the Minds

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

Walkthrough
After placing the cog wave blocker you are called back to the Calvert Mansion where you arrive just in time to see it completely destroyed. You can find Desmond hiding in his underground panic room. He will tell you that Professor Calvert is hiding in the lighthouse. Follow Desmond to the lighthouse and through the tunnel (there is various random loot in the rooms to each side of you). After a few robots and turrets you will reach the main room in which professor Calvert's brain is suspended in. The good Professor will tell you to kill Desmond.

Reward
If you do, the Professor will reward you with the best thing any human could ever hope for...death.

Or at least he'll try, he will send the robots (difficulty will dictate which type robots they are) kill them and loot Desmond's body and find his unique glasses (5 points to small guns and explosives).

If you don't want to kill Desmond, shoot the brain until the glass shatters and the water completely drains and then shoot the brain itself. After the brain is dead talk to Desmond, he will give you a key which unlocks the door which requires a key nearby. Inside this door is a lot of ammo for energy weapons and weapon components as well as the Micro-Wave Emitter.