Main mission, however i will be giving a rundown of the mission then giving you some info to prevent having to multiple play through for those that don't want to do that.
The Molecular Level
Speak to Dr. Amari in Goodneighbor (Memory Den, From mission Dangerous Minds)
Okay now this will save you hours on hour of your life if you want the whole game done.
Since there has been some confusion. These are the quests for each faction the go with this quest which will help you get the faction related achievements
1. BoS - Outside The Wire
2. Railroad - Underground Undercover
3. Minutemen - Inside Job (don't even bother you're already done with all their quests)