Fire Support

Fire Support is a Brotherhood of Steel main quest

Detailed walkthrough
While traveling near Lexington and College Square (or just south of the Corvega assembly plant if heading southeast from Concord), the Sole Survivor's Pip-Boy will pick up a military radio broadcast. It details a cry for help from the Brotherhood of Steel stationed at the Cambridge Police Station.

Head towards the location and the player 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 the Sole Survivor agrees to assist him, the Call to Arms quest will begin. Alternatively, they can persuade Danse to hire them as a mercenary, resulting in a higher sum of caps for the same quest.

If one has 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 the Sole Survivor his laser rifle and send them on their way.

Bugs

 * Sometimes after defeating all the ghouls, the Brotherhood of Steel members refuse to talk, as if there are more ghouls incoming.
 * On PC, this may be fixed using console command . If this does not work, will advance the quest, but may cause other issues later on.
 * After triggering the 'check fire' bug if one continues the main story quests to The Molecular Level, Virgil give an optional quest objective to talk to the Brotherhood and one will get a map marker to speak to Danse at which point one will have speech options. One needs to tell Danse they are there for help, then say no to his offer to join up, which kicks you out of dialogue, but gives the chance to talk to him again which then has the post 'Fire Support' fight dialogue options, accept there and 'Fire Support' should complete and leave one free to carry on with the Brotherhood storyline.
 * It is possible that when one arrives that Paladin Danse will become hostile, attacking the Sole Survivor and making it impossible to engage in conversation. One fix is to simply go to Cambridge Police Station after getting the Reveille quest (if one did indeed receive it). At this time it is unclear what causes the bug but skipping the trip to Boston International Airport altogether and going directly to Cambridge Police Station seems to fix it. Upon arriving at the station, Reveille will automatically complete itself and Danse will then speak to the player as normal after the ghouls have all been dealt with, allowing the quest to be completed.