This will start Long Road ahead, and Mass Pike Interchange will be marked on the map (MacCready's affection gains will cap at 499 until the Gunners portion of this quest is completed).
When traveling with MacCready long enough for his affinity rating to reach between 250 and 500, MacCready will talk to the Sole Survivor and tell them about Winlock and Barnes, and how he wants the Sole Survivor to go to the Mass Pike Interchange and kill them.
