This is MacCready's companion side quest, which is obtained after getting enough affinity with him. There are two phases of the quest, and each are obtained after reaching a certain level of affinity with him.

Phase I: Barnes and Winlock

For the first phase of the quest, MacCready will give you a map marker Mass Pike Interchange, and ask you to meet him there.

Head to Mass Pike Interchange. Once there, MacCready will tell you to kill Barnes and Winlock.

There will a low number Gunners defending the lower area. Once dealing with them, search the area to find a shack with a button that lowers a lift.

If you want MacCready to help kill the enemies you will encounter above, wait till he gets on the lift, or command him to move on it. Push the button to raise the lift.

At the top, there will be lots of Gunners, which includes an Assaultron and a Gunner in Power Armor. You will also find a Grognak the Barbarian - Fatherless Cur (Magazine) up on this level. After Winlock and Barnes are dead, talk to MacCready to complete this phase of the quest.

Phase II: Med-Tek Research

Once gaining enough affinity with MacCready, and talking with him, the second phase of the quest begins. MacCready will give you the Med-Tek Research marker on your map if you don't already have it.

Head to Med-Tek Research. You will encounter many Feral Ghouls outside and inside.

Once inside, MacCready will say you need to find the Executive Terminal to override the facility's lockdown.

Head forwards, and then through a door on your left. You'll encounter the Airlock Terminal, that won't do anything at the moment, but which you will need to come back to after the lockdown has been lifted. From here, take the door to your right.

Head up the stairs. To your left, there will be a ramp that leads to the next level. Once you are on the second level, follow the map marker to find the executive terminal.

MacCready will give you the password for this terminal. If MacCready doesn't give you the password, try commanding him to move closer to the terminal and then talk to him. Use the terminal to override the containment lockdown.

Now on to that earlier Airlock Terminal we mentioned on the ground level. In this room there are holes in the floor, that will help you reach ground level faster.

Use the Airlock Terminal to open airlock doors. Go through the newly opened doors, and you'll come across a Machine Gun Turret in the next room. To your left there is another terminal. use this terminal to open some security doors for the rooms ahead. You can also deactivate the turrets here.

Head through the newly open door that is straight ahead from this terminal. Take a left, then through the next door, then up some stairs.

Once up the stairs, keep going straight, then through the set of doors ahead, then take a left. There will an elevator that will take you the Med-Tek Sub-Level.

On arriving on the sub-level, take the path to your right. In the next room there will be two Machine Gun Turrets. Head to the opposite end of the room to find a terminal. Use the terminal to open the security doors in the area.

Head down to the lower level, in one of the middle rooms, there is a ramp leading further down. In the next room, there is a Fusion Core to your right. Take the path to your right.

Keep going until you find more stairs leading down. Follow your map marker until you find a Med-Tek Lab Terminal that will unlock the final security doors. Open the doors, and the cure, Prevent, will be in the room straight ahead. There is also Massachusetts Surgical Journal - Raised By Robots (Magazine) on an adjacent table.

Exit the building, and talk to Daisy in Goodneighbor. This will complete the quest.