You were looting a building near the Cambridge Police Station, being careful not to be to loud to not alert any ghouls or possible enemies.

But then you heard shooting and got curious, so you climbed onto a roof and saw some brotherhood soldiers fighting ghouls.

You grabbed your Gauss Rifle and shot at the ghouls, helping the soldiers who looked up at the roof you were on once the ghouls were dead, the one who caught your attention was the one with power armor, Paladin Danse.