After completing Mass Fusion, this quest will start immediately. Talk to Father to get more information about this quest. Travel to Graygarden then head west to find the Minutemen Contact. Talk to him and convince him to stand down. You can further order them to stay or start the attack.
Head inside the house and talk to Enrico. Next, talk to Wallace from the door. After successfully convincing Wallace, the quest will be complete and the next quest will become available