Long Road Ahead Quest
To start this quest, talk to MacCready in The Third Rail bar in The Goodneighbor and hire him as your companion. Do some stuff together (afaik, he likes lockpicking & hacking). When your affinity level will be high enough, talk to him and he will ask you for help.
Related points of interest
Stages of "Long Road Ahead" quest
1. 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.
2. Kill Barnes
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.
3. Kill Winlock
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.
4. Speak to MacCready
Winlock and Barnes are both dead. They should no longer pose a threat to MacCready.
5. Continue Traveling with MacCready
You need to do a bit more stuff together and raise you affinity level. Then he will ask you for help again.
6. 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.
7. 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.
8. Locate the Executive Terminal
Room with the Executive Terminal is located on the top floor. When you reach it, MacCready will give you Jacob's Password.
9. 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.
10. Enter Med-Tek Sub-Level
Go back to the ground level and use the Airlock Terminal.
Go through the airlock, use another terminal to open more doors and find an elevator.
11. Retrieve cure for MacCready
Find Med-Tek Cell Block Terminal and use it to open cell doors.
Go through a hole located inside one of the cells.
In the end you will get to the Med-Tek Lab where the cure is.
12. Speak to MacCready
13. 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.