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 Steel 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.

If you agree to assist him, the Call to Arms quest will begin. Alternatively, you can persuade Danse to hire you as a mercenary, resulting in a higher sum of caps for the same quest.

If you have completed The Nuclear Option in favor of the Commonwealth Minutemen prior to approaching the police station, and remain on amicable terms with the Brotherhood, Danse will instead gift you his laser rifle and send you on your way.