This quest starts only if you failed Underground Undercover or if you Banished from the Institute.
Since you've lost your cover and contact with your allies in The Institute, talk to Desdemona to learn the next course of action. She'll tell you to seek from from the Minutemen since the Railroad’s current forces are not enough to take on the Institute. If you haven't encountered the Minutemen yet, Desdemona will refer you to Concord where the last remnant of the Minutemen are found. Otherwise, talk to Preston about this quest. Once done, the quest will be complete and you won't be able to complete any of the remaining main quests for the Railroad.