Fallout 4 Mass Fusion Guide

Fallout 4 Mass Fusion Guide


Mass Fusion is Fallout 4‘s point of no return between the Institute and the Brotherhood of Steel. You’ll have to pick a side here, so make sure you give it some thought before you proceed, especially if you’re trying to do the sabotage quests for the Railroad, as you’ll be locked out of them if you’re banished from the Institute.

I’m going to assume that you’re playing as an Institute loyalist, though, in which case feel free to plow on ahead with no cares in the world.

Mass Fusion isn’t the hardest quest in the Institute main questline, but it’s pretty close. You’ll be fighting a lot of BoS soldiers, and with little to no cover for a decent chunk of the mission. Plus, there’s a nightmare of a boss encounter with a Sentry Bot at the end of the mission. So, come prepared. If possible, bring some power armor with a jetpack.

Why a jetpack? Well, you’ll have to read my Fallout 4 Mass Fusion guide to find out, won’t you? (It involves a hidden piece of unique legendary armor).

Quest Details Up Front

  • Quest Location: Mass Fusion Building.
  • Quest Giver: Father.
  • Requirements: Completion of Mankind – Redefined.
  • Length of Quest: Short.
  • Difficulty: Very Hard.

Quest Synopsis

Phase three of the Institute’s long-term plan involves firing up a new reactor capable of solving the Institute’s power troubles. A Beryllium Agitator is needed to kickstart the new reactor, and Institute scientist Allie Filmore knows exactly where to find one.

Brief Walkthrough

Mass Fusion

Speak with Doctor Filmore

Upon starting Mass Fusion, Father will send you to speak with Allie Filmore regarding a Beryllium Agitator needed to power up the Institute’s new reactor. Allie will divulge that the Agitator is located in the Mass Fusion Building and that the Brotherhood of Steel is also looking for it. In other words, you’ll be fighting the Brotherhood if you proceed with the quest.

This marks the point of no return between the Brotherhood and the Institute. If you proceed with Mass Fusion, you will become an enemy of the BoS. However, if you inform the Brotherhood of the Institute’s plans and complete Spoils of War, you will be banished from the Institute.

Loot the Keycard and Password, Read the Terminal

Once you’re finished speaking with Doctor Filmore, take the elevator up to the Relay Room. You will be teleported to the roof of the Mass Fusion building, where a Brotherhood of Steel squad is waiting for you.

Clear out the BoS and loot a keycard and terminal password from a desk on the second floor of the roof. There is a terminal in a room near the desk. Unlock the terminal using the password and read the first entry to learn the location of the Beryllium Agitator.

Take the Elevators Down to the Reactor Level

Using the keycard, activate the elevator to proceed down through the Mass Fusion building. You will be fighting various members of the Brotherhood as the elevator descends.

Eventually, the Brotherhood will cut power to the elevator. This forces you to either restore power or proceed to the ground floor on foot. Either approach will work.

Once on the ground floor, wipe out the BoS forces and take a second elevator down to the reactor level.

Retrieve the Beryllium Agitator

Make your way through the reactor level until you reach the reactor core. The inside of the room will be flooded with radiation, and you will not be able to retrieve the Agitator without either power armor or a Hazmat Suit. There are two Hazmat Suits located in the area around the core.

Enter the reactor chamber and retrieve the Beryllium Agitator. This will trigger a security lockdown. Destroy the resulting Sentry Bot and other robots before proceeding back up to the ground floor.

Return to the Institute

Leave the Mass Fusion building and return to the Institute. Speak with Allie Filmore to complete the quest.

Extended Walkthrough

Speak with Allie Filmore in the Institue

Speaking with Father will trigger the Mass Fusion quest to start, although you’re going to be dealing with Allie Filmore for the majority of it.

When you speak to her, she’ll inform you that the Institute needs a Beryllium Agitator to help power its new reactor. She believes that an Agitator is located in the Mass Fusion Building in Downtown Boston, but the Brotherhood of Steel has secured the area, likely in search of it themselves.

She will then insist on accompanying you to the building to secure the Agitator. You can choose to let her go, or have her stay in the Institute. Allie is a protected NPC and cannot be killed, so there is no incentive to not bring her along as she acts as an additional companion.

Regardless, take the central elevator up to the Relay Room and enter it to teleport to the roof of the Mass Fusion Building.

This particular point is one of the points of no return in Fallout 4. Specifically, between the Institue and the Brotherhood of Steel. Teleporting to Mass Fusion from the Institute will make you an enemy of the Brotherhood, locking you out of the faction’s content.

On the other hand, you can inform the Brotherhood of the Institute’s Mass Fusion plans. This will make you an enemy of the Institute and lock you out of that faction’s content, including the sabotage missions for the Railroad. This also precedes the Brotherhood’s Mass Fusion equivalent, Spoils of War, in which you do the same thing except you’re fighting Synths, instead.

Teleport to the Roof of the Mass Fusion Building and Locate the Beryllium Agitator

Once you teleport to the roof of the Mass Fusion Building, you’ll be greeted by a heavily armed Brotherhood of Steel squad. While not all of the BoS members you fight here are wearing power armor, they are all capable of dealing large amounts of damage. Therefore, it’s recommended that you bring some power armor of your own with you. Specifically, power armor with a jetpack. Not only will this help you in combat, but it protects you from the radiation in the reactor room and will allow you to access a piece of unique armor.

After you’ve dealt with the Brotherhood forces on the roof, take the delipidated staircase up to the second floor. There will be an office, with a desk sitting just outside of it. Inside the desk, you’ll find both a keycard and terminal password. Loot both.

Inside the office, use the password to unlock the terminal. You’ll find a number of different entries, with the first one informing you of where the Beryllium Agitator is located. You can read the rest of the entries for additional backstory, as well as listen to a holotape on a table next to the terminal.

Once your quest objective has been updated with the location of the Agitator, take the elevator down to the ground floor using the keycard you picked up.

Take the Elevator Down to the Ground Floor

Once inside the elevator, you’re going to descend down several floors before you come to a stop. During this time, there will be Brotherhood combatants shooting at you from outside of the elevator. You’ve got little to no cover here, making power armor almost mandatory on higher difficulties. This will also prevent you from using melee weapons for the short duration of the elevator trip.

Eventually, the Brotherhood will turn off the power to the elevator and you’ll be forced to exit it. After killing all of the BoS members, you’ll have two choices.

First, you can restore power to the elevator by following your quest marker to the power conduit. However, I recommend your section option which is continuing on to the ground floor on foot.

Going on foot will give you access to the two notable pieces of loot in the area: the Strength Bobblehead, and the Freefall Legs unique armor piece. There is also an issue of Tesla Science on a console on the second level of the floor that the elevator stops on.

To proceed on the elevator, simply proceed up to the floor above you and flip the switch before returning to the elevator. However, if you want to continue on foot, you’ll need to pick up the Mass Fusions Labs Key from the desk on the same level as the elevator.

Before you continue downwards, make sure you grab the Freefall Legs. These are unique Combat Leg pieces that completely negate all fall damage.

Getting your hands on the Freefall legs is its own challenge. If you look up, you’ll be able to see all of the floors that you passed on the elevator. You need to get up to the very top one. This is impossible to do without a jetpack on your power armor, although you can glitch your way up by abusing Fallout 4’s physics system (something that takes a hell of a lot of time to learn).

With the Freefall Legs in your possession, continue downwards until you reach a big silver statue. The Strength Bobblehead will be on top of the statue’s head.

Continue fighting your way down to the ground floor, and clear the area to proceed down to the reactor level.

Proceed Down to the Reactor Level

There is a separate elevator on the ground floor that will take you down to the reactor level. Once down there, you’ll be done with the Brotherhood for now. Instead, you’ll end up facing a horde of high-level robots.

Make your way through the floor until you reach the reactor chamber. It’ll be fairly obvious when you find it. If your Science skill is high enough, you can hack a nearby security terminal to gain access to the room with the currently powered down Sentry Bot. By using the terminal next to the bot, you’ll be able to disable the security system, preventing a very difficult fight later on in the mission.

Back in the reactor chamber, you’re going to need a way to absorb a whole bunch of radiation. If you allowed Allie to come along for the ride, she’ll give you some Rad-X, but that’s not going to be enough. You’re going to be dealing with radiation levels similar to the GECK back in Fallout 3.

If you haven’t come prepared and have no power armor, you can loot one of two Hazmat suits on this floor. One is in the locker room, and the other is located in an office next to the reactor chamber entrance.

Retrieve the Beryllium Agitator

Once you’re suited up, head into the airlock and hit the big button to enter into the reactor. Avoid the big blue beam, and make sure you don’t drink the water. Both of these will kill you instantly.

Make your way up the scaffolding and retrieve the Beryllium Agitator. If you haven’t disabled the security system, several alarms will go off and the Sentry Bot will become active, along with a host of other robots.

While Doctor Filmore can’t be killed during this fight, you can, so try to deal with the Sentry beforehand, if possible. Otherwise, you’re in for an incredibly difficult firefight.

Either way, take the elevator back up to the ground floor of the Mass Fusion Building.

Assist the Synth Forces and Return to the Institute

When you’re back on the ground floor, the Brotherhood will be attempting one final push. At this point, Allie will stay behind to buy you some time. You can either head straight to the Institute now or stay behind to help the Synth forces repel the Brotherhood. There are no story implications for either choice, so it’s entirely up to you.

Regardless of whether or not you stay to fight the BoS, exit the Mass Fusion Building and teleport back to the Institute. Allie Filmore will be waiting for you there. Report to her to finish Mass Fusion.

Quest Rewards

All you get for completing Mass Fusion is a handful of XP. You’re loyal to the Institue at this point, so don’t expect to be getting paid for any of the work you put in. You’re doing it for the cause and all that.


Question: What happens if I inform the Brotherhood about Mass Fusion?

Answer: If you inform the Brotherhood of Steel about the Institute’s plan during Mass Fusion, you will be banished from the Institute and instantly fail any related quests. You will no longer be able to peruse the Institute’s main questline, as well as any of the sabotage missions for the Railroad.

Question: Where is the ID card for Mass Fusion?

Answer: The ID card for Mass Fusion is located in a desk on the roof, along with the password for the nearby terminal. Both of these items are only available during either Mass Fusion or Spoils of War.

Question: Can I betray the Institute after Mass Fusion?

Answer: Yes, you can betray the Institute after Mass Fusion. While this quest locks you out of working for the Brotherhood, you can still move forward with both the Minutemen and Railroad main questlines.


Hopefully, you’ve found this Mass Fusion guide helpful. It’s one of the more difficult quests in Fallout 4, purely because you’re fighting a bunch of BoS soldiers, and those guys put up a serious fight.

If you decide to go the Brotherhood route and do Spoils of War, the process is the exact same. You’ll start on the roof of the Mass Fusion Building, loot the keycard, take the elevator down, and retrieve the Agitator. However, it’s notably easier because instead of fighting Knights and Paladins, you’re fighting Synths along with the help of power-armored soldiers.

Either way, just remember that this is a point of no return quest, so make sure all of your loose ends are tied up before you finish it.

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