The Quincy Massacre was a successful attack on the town of Quincy by the Gunners, leading to the destruction of the settlement and the deaths of the Minutemen that tried to protect them. Only twenty Minutemen survived, including Preston Garvey.
In 2287, in response to a vision of Mama Murphy's, Sturges the mechanic requested aid from the Commonwealth Minutemen. A small detachment of Minutemen under the command of Colonel Ezra Hollis, including in their number Preston Garvey and Clint, arrived to defend the town. This turned out to be in the nick of time, as a large force of Gunners led by Baker were already laying siege to it. The Minutemen were initially able to drive them off by attacking from the rear.
Hollis moved his force into the town, where Sturges registered his unease about how few of them there were. Hollis requested more back-up from other units, none of whom arrived due to internal disputes. Clint, disgusted at the weakness shown by the Minutemen, defected to the Gunners and revealed how they could easily take the town by attacking it from the overhanging highway.
The Gunners renewed their assault from the highway, turning the town into a shooting gallery. The battle quickly devolved into a massacre for the Minutemen. All of the town's civilians and Minutemen were killed except for Preston Garvey and nineteen others, who were able to escape to Jamaica Plain. Hollis was taken prisoner, and according to Clint was not able to believe the latter's betrayal until Clint killed Hollis himself.
The battle dealt a killing blow to the Commonwealth Minutemen as they were, with the movement too few in number and rife with distrust to hold together. Those that remained who did not arrive to defend Quincy faded away.
The Gunners dispatched a further detail to finish off Preston's group. They killed several more of them at Jamaica Plain, but were unable to stop the rest of them escaping. Preston continued attempts to protect his group of survivors eventually took them to the ruins of Lexington. Feral ghouls whittled their number down to eight, before Jared's raider band killed a further three and forced the five survivors to take shelter in Concord's Museum of Freedom.
Clint was promoted to commander of Quincy by Captain Wes for his actions. Determined that the town would not be taken in the same way it had fallen to the Gunners, he fortified the highway, set up a radio tower and captured two sets of power armor in order to defend it.
If the Sole Survivor visits Quincy, they will see the armor being worn by Clint himself, and the other set by Tessa, who wields Tessa's Fist (a modified power armor piece). If the Survivor so chooses, the town can be attacked and Clint, Baker and Tessa can all be killed to avenge the Minutemen.
Despite what Preston believes it seems that not many people know of the Quincy Massacre; this can be seen with none of the guards making comments about it and Piper stating you're a trader from Quincy upon first meeting her.