This quest becomes available after you've fulfilled the following conditions:
You must have at least four settlements aligned with The Minutemen.
You must have completed at least a dozen Minutemen Radiant quests.
You must have completed the Institutionalized main quest.
The Institute still exists.
Eventually, Preston Garvey will assign this quest to you. You can also increase your chances of getting this quest if you have Radio Freedom active on your Pip-Boy. When you do get the quest, travel to the randomly designated waypoint and speak to the settlers.
They've been alerted that Synth has infiltrated the settlement. Speak to the settlers to decide the Synths fate. You can either banish the Synth or kill it.