Once he trusts you enough he will confide in the Sole Survivor that the reason that he is in the Commonwealth; to find a cure for his son Duncan. His only lead is a rumored cure in the Med-Tek Research building. After wading through ferals and automated security, the pair obtain the cure; Prevent. Once obtained the cure must be transported via Daisy's caravan connections back to MacCready's family in the Capital Wasteland.
Travel there and enter the upper levels of the interchange using either the north or south gantry elevators. Then simply find and kill both. This will remove them as a threat to MacCready and convince the Gunners to back off.
Continue traveling with MacCready.
Next, MacCready will confide that he needs to find a cure for his son's illness. He will want to travel to Med-Tek Research. Head there.
The cure can be obtained by clearing the location. First, find the executive terminal to unlock the containment. It's located on the highest level of the first part of the building, accessible through the rear stairwell and then the collapsed floor. MacCready will hand over the password for the executive terminal after entering the room.
After turning off the containment on the terminal, the secure part of the facility will become accessible. Open the airlock, then enter the secure section and use the security terminal to open the security doors. The cure is located in the lower levels. The elevator connecting to them is in the back of the secure section.
The cure is in the laboratory at the end of the level. Simply navigating through the maze-like corridors will lead to it. Open it with the terminal on the wall and collect the cure from the table in the center.
Talk to MacCready with the cure in inventory, then head to Goodneighbor to hand over the cure to Daisy and complete the quest.
Travel to the Mass Pike Interchange
MacCready has asked me to travel to the Mass Pike Interchange to hunt down and kill Winlock and Barnes, the two Gunners that were threatening him in Goodneighbor.
I've arrived at the Mass Pike Interchange to hunt down and kill Winlock and Barnes, the two Gunners that were threatening MacCready in Goodneighbor.
Speak to MacCready
Continue Traveling with MacCready
Winlock and Barnes are both dead. They should no longer pose a threat to MacCready.
Bring MacCready to Med-Tek Research
MacCready confided in me that his son is extremely ill and needs a cure that can only be found within a place called Med-Tek Research. I should bring him there as soon as possible.
Enter Med-Tek Research
MacCready and I have arrived at Med-Tek Research. I should proceed inside and help him locate the cure for his son's disease.
Locate the Executive Terminal
Override Containment Lockdown
MacCready has given me the password to the Executive Terminal inside of Med-Tek Research. I should use it to cancel the facility's containment alert.
Enter Med-Tek Sub-Level
Retrieve cure for MacCready
Bring cure to Daisy in Goodneighbor
MacCready and I have recovered the cure from Med-Tek Research. The final step is for both of us to bring it to Daisy in Goodneighbor so she can pass it along to MacCready's son.
When the player shows up to Med-Tek with MacCready, the game will occasionally not trigger the script when the player reaches the 3rd floor executive terminal and the quest will not update to recognize that. Reloading usually works.