Fallout 4 created by Bethesda Game Studios.
Images used for educational purposes only.
Main Walkthrough

Prerequisites: Complete Mankind - Redefined
Trigger: Speak to Allie Filmore
Reward: 600 XP

Redirection Note: If you're playing as a Brotherhood of Steel loyalist you'll get a quest called Spoils of War that's functionally identical to this one, the primary difference being that you'll be fighting the Institute, not the Brotherhood. The strategies suggested for getting through the Mass Fusion Building are nevertheless basically the same - indeed, it's probably an easier mission, as you won't have to fight any soldiers in Power Armor.

- You’ll get this quest immediately after finishing up Mankind - Redefined and speaking with Father. Once that’s done, head down to the central hub of the Institute and look for Allie Filmore. She likes to explore the ground floor and the western staircase, and shouldn’t be too difficult to find.

- Allie describes the next job as having to enter the Mass Fusion building, and it’ll be an escort mission. Oh joy. You’ll need to teleport there from the pad that you used to enter the Institute in the first place, so use the central elevator to ascend to the room.

- Upon touching the pad you will receive a dire warning that you are about to make yourself an enemy of the Brotherhood of Steel. You aren’t yet committed to an Institute-centric ending, but you’re well on the way there. If you continue this quest the Brotherhood will always become hostile on sight, you’ll lose any Brotherhood settlements and access to the Prydwen, you’ll no longer have Danse as a companion, and any quests given by the Brotherhood will be lost. At the very least you may want to travel to the Prydwen, assuming you have access, and grab your suit of Power Armor. Just sayin’.

Mass Fusion

- You’ll appear atop the Mass Fusion building, and pretty much immediately you’ll come under assault from two sides. Get behind cover on your right and fire back alongside Dr. Filmore. The fighting won’t last long, but it can be fairly brutal if you stretch it out too much. Try to take out the Knight with Power Armor first, then go for the weaker, more fragile Scribes. You’ll have to take down five or six BoS troopers to clear the area.

- Climb to the second floor once the troopers are dead. There’s a desk in the northeast that contains a Mass Fusion Executive Lab Password and a Mass Fusion Executive ID. The Password can be used on a large computer bank in a nearby room on this floor. The first entry in the message database will ID your next destination. Check the desk to the right of the computer for the Oslow’s Office Recording Holotape.

- If you check the third floor you can find an Advanced locked room with some goodies inside. Otherwise, hop on the elevator on the lowest floor. You can use the ID to ride it down… though along the way you’ll come under fire by more BoS dudes. There’s not nearly enough room to hide, so try your best to V.A.T.S. them to death before they can peg you too many times. Aiming manually’s a bit too tricky given all the things in the way.

- When you hit the bottom you’ll come under assault by more BoS dudes, including one in Power Armor. You’re horribly exposed, so unless you like fighting in the open, immediately move to the right side of the room and look for a doorway. You can use this to avoid being flanked and fire back at your attackers.

The reactor core of Mass Fusion in Fallout 4.
You get to go here! Radical!
- Poke around the adjoining rooms once the attack is over. The first floor isn’t terribly interesting, though you can find an Armor Workbench in the longer southern room, and if you check the first desk on your left in the northwest room you’ll find a Mass Fusion Labs Key. 

Climb to the second floor via the southern room and you can find a Circuit Breaker to hit in the north. This restores power to the elevator, which will take you down to the ground floor - and more BoS dudes. A firefight ensues. If, on the other hand, you want a more controlled approach, you can pick through an Expert door in the northeast to find a small storage room with a big hole in the floor. This hole will allow you to descend from one busted-up floor to the next, and, potentially, kill the enemies on the way down in a more stealthy manner. Grab the Stealth Boy from the shelf near the hole, then hop down. You’ll find the following on the subsequent floors, in this order:
More enemies, as well as a Mini Nuke in a side office and a Master-locked wall safe full of goodies
Yet more enemies - on a lower level, making an ambush easy - as well as lots of ‘meh’ lootable stuff and a terminal with an impatient paramour’s letters
Other than a lot of great spots for sniping people quietly, not a hell of a lot… though you should watch out for the first closed bathroom stall you see, as there’s a grenade trap inside
Aside from an Advanced locked safe in the western room and a security terminal in the eastern rooms that will activate some Protectrons and Turrets, there’s not a hell of a lot on the ground floor. Hop in the elevator across from the latter security room and you can ride it down to the sub level.

- You’re now just outside Mass Fusion’s main reactor room. Start by taking a left from the elevator. There’s a small room over here with another Novice security terminal that you can pop open to activate a nearby Protectron unit. Leave this robot turned off, for reasons.

- Take a right from the elevator to find stairs. There are several connected rooms of little use on this floor (an Advanced terminal might be of use, but I suck too much), but you’re more or less forced to head towards the reactor. Check the side catwalks for gear, then wander down. 

- You’ll start getting hit by Rads right away, and it only gets worse when you go through the door on the left. Check the offices around the base of the stairs and, in addition to another Protectron, you’ll find a terminal that will activate said Protectron, unlock a nearby safe, and provide some details on a bunch of RPing dead people. Again, don’t turn on the Protectron. Sitting atop the safe is a Hazmat Suit which you can wear to offset the Rads and enter the reactor, where the Berylium Agitator awaits. Grabbing it starts quite the battle, however, and you’ll want to get your gear back on as soon as you leave the reactor room.

A Sentry Bot in Fallout 4. Either go for the weapons
or wait for the Fusion Core to pop out of its back.
Sentry Bot and Company

Harsh. This battle takes place in roughly three phases, though two of them overlap:
  • The first has any Protectrons you activated becoming hostile and attacking anyone back in the control chamber of the reactor core. Hopefully that number is zero. Accompanying them are Turrets on the ceiling. Destroy these as quickly as you can, because within about half a minute…
  • … a Sentry Bot shows up. This thing is harsh in close quarters, and can rock you badly if you come out in the open. Use the stairs to hide from it and duck out to deliver V.A.T.S. assaults every now and then, preferably to take out its arms. Be sure to get far away from it when its health drops to zero, as the resulting explosion will wipe you out. Be ready, too, for more trouble…
  • … ‘cause another door will open, releasing two Assaultrons into the room. Ouchies. They’re much easier to kill than the Sentry Bot if you aim for the torso, but they can really mess you up at close range, and they start charging their visor laser attacks immediately. Run from them until at least one has engaged Allie or a companion, take out the other, and then team attack the second Assaultron.
- Return to the elevator after wiping out the ‘bots and ride it up to the main level. There’s another battle in progress, but Institute Synths are there to help you out of it. Flee for the front door, then, once outside, jump back to the Institute. Allie is in Institute Advanced Systems; speak to her there to finish the mission and trigger a new one, Pinned.

- Don't forget! If the Brotherhood of Steel didn't hate you before, they sure as shit hate you now. Avoid anything Brotherhood related unless you want to get into a fight. They'll be packin'.

Main Walkthrough