How to unlock: Meet with settlers in the location protected by the Minutemen, or with dwellers from a neutral homestead. This quest is one of the randomly unlocked ones.
It is one of random quests connected with Minutemen. There are several ways in which to unlock it, you can e.g. meet with Preston Garvey and receive the quest from him, you can talk to a settler from a settlement protected by Minutemen, or even by visiting a neutral homestead (e.g. the Warwick Homestead or the Abernathy Farm) and listen to the pleas of its dwellers. You may be asked to get rid of super mutants that have been harassing dwellers of that location. You will be directed to a random super mutant hideout, in the game world. After you have eliminated the marked monsters (not necessarily do you have to kill all of the mutants in the location), you need to return to the quest-giver and report. Additionally, you may be asked to report to Preston upon the quest's completion.
Rewards for completing the quest: Experience points, caps.