This quest is similar to the other Minutemen Radiant quests involving kidnapping, except that this involves a settlement that is already aligned with The Minutemen. You can either receive this quest from Preston Garvey (after completing at least a dozen other Minutemen Radiant quests) or by listening to Radio Freedom. Travel to the randomly designated waypoint to learn more about the kidnapping.
After you've spoken to the marked settler, you can choose to rescue the hostage or pay the ransom. If you have the bottle caps, you can save yourself the hassle of travelling to the kidnappers. Then again, confronting the kidnappers would give you loot and experience. The choice is yours. If you decide to rescue the hostage, report to the settlers after you complete the quest. The final step is to talk to Preston Garvey.