This quest becomes immediately available upon arrival at the Museum of Freedom location.
The last known Minuteman at this point in the timeline, by the name of Preston Garvey, will call down to the Sole Survivor as they approach the museum, and are forced to deal with the marauding raiders gang and their leader. Preston will then throw down a laser musket for the Sole Survivor to use, in order to safely get inside to him and the group of survivors that he is protecting.
After dealing with the first wave of raiders that have laid siege to the museum, both inside and out, Preston and his group will offer words of wisdom, revealing that a suit of power armor is on the roof of the museum, near the crash landing of a pre-WarVertibird.
If the Sole Survivor has not done so already, they must travel back to the ground floor of the museum, find the collapsed flooring revealing the building's basement, and retrieve the fusion core there to activate the derelict suit of power armor on the roof.
After activating the suit of power armor on the roof, the Sole Survivor will then want to grab the nearby minigun, still attached to the crashed Vertibird, and will then need to take down another two waves of raiders, their leader, and a deathclaw that erupts from a nearby sewer system after being disturbed by the fighting above.
Afterwards, Preston and his group will decide to continue traveling to Sanctuary Hills, in which the Sole Survivor may choose to help escort them there personally.
Should the Sole Survivor snipe the raiders on the streets below after obtaining the power armour, the second wave of raiders that appear, will trigger the scripted deathclaw event prematurely.
The area in which the deathclaw emerges, can later be visited as a hidden location.
Preston's line "Sturges? Tell him" has it's subtitle stay as Sturges describes the power armor.
The deathclaw will easily become stuck in the same hole that it emerged from, and will frequently get stuck in the red truck not far from its spawn position.
Without much cover on the nearby rooftops, the raiders that spawn on top of them will frequently walk off and fall to their deaths.