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.

Bugs

 * Sometimes after defeating all the ghouls, the Brotherhood of Steel members refuse to talk, as if there are more ghouls incoming. A suggested fix is to use the console command.
 * Sometimes after defeating all the ghouls, the Brotherhood of Steel members refuse to talk, as if there are more ghouls incoming. As yet there is no fix for this. You need to reload your last save and try again until it does not bug out. It can take many attempts (10+).
 * This worked on xbox game, after triggering the 'check fire' bug if you continue the main story quests to 'The Molecular Level, Virgil gives you an optional quest objective to talk to the Brotherhood and you get a map marker to speak to dance at which point you have speach options, you need to tell him you are there for help, then say no to his offer to join up, which kicks you out of dialogue, but gives you 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 you to carry on with the Brotherhood storyline.

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