The Lost Patrol

The Lost Patrol is a Brotherhood of Steel side quest in Fallout 4.

Detailed walkthrough
The player character will detect a distress signal once they are near any one of the three locations where Brotherhood of Steel followers perished. To complete the objective, the player must loot holotapes from the fallen soldiers, then play them from their Pip-Boy.

Knight Varham's Battlefield holotape is located in a ruined house north of Relay Tower 0MC-810, south of Greentop Nursery and east of Taffington boathouse.

Knight Astlin can be found behind a table in the recruitment office inside the National Guard training yard.

Scribe Faris is found at the Revere satellite array. He is in a metal and wood structure above the ground connected to the southeast array tower. This area is inhabited by super mutants.

Once all three holotapes have been played, a quest marker will appear at Recon Bunker Theta which is otherwise inaccessible. Once there, one will be accosted by Paladin Brandis who will demand to know why the Sole Survivor is there. If the player character tells him the fate of the soldiers, he will allow the Sole Survivor to loot the items in his home, which includes a fusion core. Otherwise he will initialise combat.

The Sole Survivor may have the option of persuading him to rejoin the Brotherhood of Steel. They must pass three conversation checks and if any of them fail, he will remain where he is. If successful, they will find him aboard the The Prydwen where he will reward them with the Survivor's Special.

To complete the quest, the Sole Survivor must report to Paladin Danse, or Captain Kells if the quest Shadow of Steel has been started and the Prydwen entered the Commonwealth.

Regarding Paladin Danse

 * There is no way to start the quest with Danse after he becomes available as a companion.
 * For anyone who wishes to take Paladin Danse with them as a companion, it may be worth waiting until Captain Kells gives you this quest aboard the Prydwen. If Danse is available as a companion when you embark on this quest, and you take him with you, he has a lot of unique dialogue and scripted reactions throughout, and will even help you out in conversation when you confront Paladin Brandis and talks to you about it afterward.
 * Be warned if you have Paladin Danse with you as a companion, and have completed Blind Betrayal Brandis will behave towards you and Danse as any Brotherhood member would.

Regarding Revere satellite array

 * To get to the distress signal in the Revere Satellite Array, the player character will need to kill a large number of super mutants, including several higher level variants. It is a difficult challenge for one not equipped or skilled properly.
 * Just north of Revere satellite array is a vehicle that contains a partial suit of level-adjusted Power Armor, which may prove useful for clearing the array (assuming you don't already have one of the two suits located at the Training Yard). It is, however, guarded by a higher level Mister Gutsy - who may prove difficult early on for lower leveled characters. There is also an empty power armor frame on one of the dishes at the array, accessible via some rickety stairs and walkways.
 * The Revere satellite array is one of the locations you can direct the prototype sentry bot from the Robotics disposal ground to patrol. Although it can be tricky to direct, it can make short work of even legendary super mutants if it spots them.

Others

 * If you didn't complete Semper Invicta before starting the quests on the Prydwen, you can start the quest by discovering one of the lost patrols on your own or from Kells as mentioned above.
 * If the player stumbles across the battle site at random the quest will start even if Danse has not assigned it. If the player speaks to Brandis before being briefed on it by Danse (i.e. if Call to Arms has not been completed) the dialog options for convincing him to rejoin the Brotherhood will not be available, and the mission will end after speaking to Brandis. The same will occur if the player is hostile to the Brotherhood of Steel. However, if the player returns to Recon Bunker Theta after joining the Brotherhood of Steel, they will be able to talk with Brandis again and attempt to convince him to rejoin the Brotherhood.
 * If the player starts the quest by finding the distress signal and completes it before being briefed by Danse, the patrol's holotapes will still be marked as quest items and cannot be dropped. This can be resolved by speaking to Danse about the patrol after completing Semper Invicta. He will request the tapes from the player and they will be removed from their inventory.
 * If Preston Garvey is brought to Knight Astlin's body, he will state that if the same thing were to occur to him, it would be his "worst nightmare".
 * Picking up the quest by first finding and listening to Knight Astlin's holotape then going onto the satellite array, the player will need to then discover the initial Battlefield holotape (see details above) in order to progress.

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