Headhunting

Make your preparations and fast-travel to the nearest landmark where the quest marker is pointing. In this case, the Fort Hagen Filling Station will do. Once there, take out the robot units including the targeted robo-brain. Once done, loot its wreckage and talk to Ada. Your mission objective will be updated and you have to head there to locate the last robobrain. Don't forget to loot the robot remains to unlock new modification options to your workbench.

Robot cleanup

The location is the Fort Hagen Satellite Array due west from where you retrieved the second beacon. Make your way to the Rust Devil den and take out the lookout robots and humans. The Rust Devils will be wearing custom robot armor that you can loot and give to your settlers/ followers later on. To access the den interior, you can use the unprotected terminal on the wooden ledge above the entrance.

Rustling the devils

Once inside, disarm the laser tripwire and the assaultron totem in the first corner. Once done, defeat the guards down the ramp then open the security door to engage several upgraded robots and rust devils inside. You may encounter at least two legendaries here so don't hesitate to pull back behind the security door if your armor can't hold off their firepower.

Cleaning up house

Once cleared, check out the rooms to loot components. There are power armor stations here that you can use to repair your power armor. Behind the expert security door near a tank is a power armor scaled to your level. If your level is high enough, you should find an X-01 power armor here. Once done looting and repairing your armor, you can either go through the two expert security doors or take the longer route by going upstairs to the north, then go west through the control room then downstairs west again to reach the hangar itself.

There are no hostiles in the room so follow the marker and talk to the robo-brain named Jezebel. After reaching an agreement, you have to take the brain from the contraption it's in. The modified Sentry Bot named Ahab will be activated as soon as you pick up the robo-brain. It's heavily armored and armed but like ordinary sentry bots, it's weakness is its fusion cores at the back. Defeat it and loot its unique helm.

Brain rescue

If you take the stairs and follow the walkway, you'll reach the upper ledge to the north with a missile launcher and a handful of missiles. The Weapons workbench beside it has several duct tapes (adhesives) as well. There are also several workbenches in the room that you can use.

Head to the NW and you should reach a generator room with an active fusion core and a bed and chemistry station in the corner. Grab the fusion core then locate the rust devil's gaming terminal in the corner of the room. Eject the Automatron holotape from the terminal and loot the advanced safe beside the terminal.

Head outside the room and access the terminal across the door. This will allow you to open the security door with power armor stations and toolboxes inside. You can make the necessary repairs here after you clear the room. Engage the hostiles in this immediate area, including a menacing-looking Assaultron Gorgon. Take care of the turrets on the upper ground as well.

Once cleared, loot the bodies and chunks of metal then continue upstairs to the NW. Here you'll encounter several moving laser tripwires and assaultron totems. You can disable these tripwires to salvage their components then disable the totems as well.

Once done, continue to the room and take the valuable loot you can find. Continue following the path until you reach a walkway with more laser tripwires and totems. Disarm them then continue forth to engage the leader of the dust devils, Ivey and her assaultron gorgon. Ivey has a Tesla Power Armor so don't forget to loot the parts upon her defeat. The room she's in is very small so there isn't really that much space for you to maneuver.

Continue past the security door in Ivey's room and you'll find a green chest in the middle and an Expert security door that has a Fat-Man and two mini-nukes inside. The green chest contains a Salved Assaultron Head. Finally, access the terminal in the last security gate and use it to unlock the gate. You'll be back in the starting area. Go upstairs and leave the area.

Once outside, talk to Ada to install the last beacon. After the conversation, head back to the settlement with a Robot Workbench then construct a new body for Jezebel. First, create a default automatron body then in the Head category, place Jezebel's head. You can give Jezebel whatever body you want for it. Once done, talk to Jezebel and ask it about how to enter the Mechanist's base. You'll learn that you have to outfit one of your robots with an M-SAT modification and it will upload the plans to your Pip-Boy. The quest will be finally complete afterward.