This quest will become available after completing the quest “Tradecraft” and officially joining the Railroad. Talk to Dr. Carrington and learn the next details of your quest. You’ll also get a Charisma speech check to get on his good side.
Once done, head to the marked location to retrieve the holotape from the mailbox. Next, head to Bunker Hill to talk to Old Man Stockton to learn about your mission.
Head to the unnamed chapel west of the BADTFL Regional Office and clear the area of raiders. Once done, sit in the pew and wait until evening. Stockton and the synth H2-22 will arrive. While waiting for another Railroad member to arrive, you can talk to H2-22 and pass a speech check to convince him to give you more information. Once done, the agent High Rise will arrive. Escort him and H2 to the safehouse.
You’ll encounter several raiders and supermutants along the way. Once you’ve reached the building’s entrance, the objective will be complete. You need to head back to the HQ and report to Carrington to complete the quest.