How to unlock: You must help Paladin Danse in defending the Cambridge Police Station (M5,1).
Helping the soldiers
In order to start this quest, you must go to the Cambridge Police Station and help the soldier in defending the place from waves of Ghouls. You should hear the battlefield from quite far away. The battle shouldn't be hard, even without your help the soldier should manage to win. Once the battle ends, don't forget to gather valuable items from the bodies of fallen enemies. Once you do it, talk to Paladin Danse. He will tell you that he is a member of the Brotherhood of Steel. During the conversation you can agree or not agree to help him and his organization. If you accept his offer, you will have to listen to another, very short dialogue. The quest will end and another one will start automatically - Call to Arms.
Quest reward: Experience points.