Mass Fusion (called Looking to the Future in certain game files) is a quest that can be obtained in the Commonwealth in Fallout 4.
The fate of the Commonwealth is now in the hands of the Sole Survivor, and an important decision needs to be made. Is the Institute really the right choice?
- After returning to the Institute, Father will refer the Sole Survivor back to Allie Filmore, who has agreed to help find a device at Mass Fusion, called the Beryllium agitator, that is to be used to kickstart the Institute's new nuclear reactor, which will ensure their survival and scientific pursuits for the foreseeable future.
- Decide right now as to whether or not she will join the Sole Survivor at Mass Fusion, although there are no downsides of bringing her along.
- Once the final decisions have been made, and the Sole Survivor is willing to become permanent enemies with the Brotherhood of Steel, the Sole Survivor will need to travel back to the teleporter room, which will instantly take them to Mass Fusion.
- Upon arrival, it is discovered that the BoS have already beaten the Institute to the agitator, and are seen to be immediately hostile to the Sole Survivor, as they have discovered their ulterior motives.
- The BoS will need to be fought through, until finding the Mass Fusion executive lab password and then the Mass Fusion executive ID that can access the elevator. Traveling down in the elevator, the BoS will blow the conduit powering it, requiring for the Sole Survivor to either bring the power back on, or to just jump down from floor to floor until reaching the reactor level.
- Find the Mass Fusion labs key, which will give access to the reactor room, where the Sole Survivor will need to obtain the Beryllium agitator.
- After retrieving the agitator, the Mass Fusion security system will activate, requiring the Sole Survivor to fight through turrets, protectrons, assaultrons, and an influx of BoS backup that has now arrived to kill the Sole Survivor.
- Upon reaching the reception area, a group of Institute synths will be seen fighting against the newest wave of BoS, and an optional objective will open up, where the Sole Survivor can help the synths finish them off.
Should the Institute be betrayed at this point, and their intentions revealed to Elder Maxson, instead, the Institute will become permanent enemies at this point, and the quest Spoils of War will commence.