To trigger this quest, head to Diamond Market and enter the Dugout Inn. Look for a friendly Ghoul named Edward Deegan. If he's not there, check Colonial Taphouse in Diamond City, The Third Rail bar in Goodneighbor or the market in Bunker Hill.
Deegan will give you details about a mission given by a man named Jack Cabot. This conversation will reveal the location of the Cabot house on your waypoint indicator. You'll find it in northeast Boston, in the Beacon Hill Neighborhood.
When you reach the house, activate the intercom and mention Jack Cabot. Enter the house and agree to the mission, after which you'll receive a key.
Travel to the north side of The Commonwealth and look for Parsons State Insane Asylum. Greet the guard captain named Maria at the front entrance and mention Edward. Now head to the nearby creamery and take out the Raiders residing there. Be sure to kill the Raider marked on your waypoint indicator — and to pick up the serum that the Raider was carrying.
Head back to Cabot House and speak with Edward. You can either lie and tell him that you couldn't find the serum, or you can give him the serum for a reward. This step will complete the quest.