Fire Support

Fire Support is a Brotherhood of Steel main quest in Fallout 4.

Detailed walkthrough
While traveling near Lexington and College Square (or just south of the Corvega assembly plant if heading southeast from Concord) you receive a military radio broadcast (AF95). The broadcast details a cry for help from the Brotherhood of Steek stationed at the Cambridge Police Station.

Head towards the location and you will be faced with a horde of feral ghouls attacking the Brotherhood members stationed there. Assist the soldiers and when the fight ends speak to the commander Paladin Danse and agree to assist him.

You can also persuade Paladin Danse to hire you. If successful, you will receive a higher sum of caps at the end of Call to Arms.