BROTHERHOOD OF STEEL QUEST 02 - Fire Support
Triggered by completing BoS Quest 01 - Reveille or by getting within range of Military Frequency AF95 (emanating from Cambridge police station)
Objective: Listen to military frequency AF95
Simply go into your pipboy and tune your radio to the right frequency. After a few seconds, the objective will update.
Objective: Proceed to Cambridge police station
Now it's time to tromp your way into Cambridge and find the marked police station. Stay alert as you enter Cambridge as there can be several mines dotted around the streets surrounding the police station. Use your V.A.T.S as you move to check for them. When you get close, you'll see ghouls running in from multiple directions to attack a brotherhood paladin at the front of the station.
Objective: Assist the soldiers
Despite the objective, you can simply stand to the side and let the paladin deal with all the ghouls if you like. But if you're feeling generous, shoot down a couple for the hell of it. On higher difficulties, use the barricade walkway (stand in the middle on top of it) to funnel the ghouls, making them easy targets as they can't dodge from side to side. There are some ammo containers on the barricade walkway to top up your supplies as well when the attack ends.
Objective: Speak to paladin Danse
After the battle, drop down and have a chat with the armoured paladin and ask him about joining up. Answer his questions and then agree to help him out. He'll give you a bit of a spiel and then head into the police station. The quest will end here, but you can loot the police station (there's a decent amount of stuff to grab inside) before talking to him again to start BoS Quest 03 - Call To Arms.