This Galaxy Ain ' t Big Enough . . . This Galaxy Ain ' t Big Enough . . . is the final " Fallout 3 " main quest and achievement in the " Mothership Zeta " add - on . Detailed walkthrough . Reunite with the others . Exit the space walk in an airlock on the observation deck . Activate the switch to repressurize the airlock ; the Lone Wanderer can now remove their spacesuit . In the next room , there is a teleporter back to the observation deck ; activating this allows Sally and any surviving abductees to join the player character . Sally then crawls through a duct to power down the energy gate leading to the next part of the level . Disable the Death Ray and head to the Bridge . After the door is open , head down the hallway into the room with the glass floor . Enjoy the view of Earth . After a short dialogue , Sally will go to open another door . Immediately afterward , a hologram of an alien will appear over the glass floor , and the player character will learn about the death ray . Once Sally returns , proceed through the newly opened door . One will find a deactivated teleporter and be tasked with finding the other end and turning it on . Upon entering the weapons lab , the player character will face aliens , turrets , and robots ( one needs to obtain the Xenotech Expert perk before leaving ) . After defeating them , turn left and go down the hallway on the far side . In a room to the right , one will find a drone pod with a guardian drone and a fair amount of loot , including power modules , several alien epoxies , biogels , the atomic pulverizer ( a unique alien atomizer ) , along with a normal alien atomizer and a Buttercup toy . Once the player character reaches the T - intersection , turn right . Follow the hallway to the second floor of the weapons lab . Go down the stairs and through the door in the middle . Once in another room , turn left and go upstairs . Go through the circular door and follow the room to the left . Go into the hallway to the northeast and up the stairs into the experimentation lab . Once in the lab , veer right into another room and go down the hallway before the player character . Follow the hallway until reaching a large room with a force field covering a door on the other side . Activate and destroy the power generator in the middle of the room to disable the force field . Head down the hallway and enter the biological research lab . Head up the stairs on the opposite side of the room . Turn right and go across the bridge . One will find a shielded door and switch to the right . Go through the door and turn right . Walk straight across the platform to the teleporters . Once one uses the teleporters to the Death Ray hub , they cannot go back until after the quest is over . If one is going for the Alien Archivist achievement , make sure to have all the previous recordings . There should only be one more to get after this point if one has been doing it correctly . Fight through the Death Ray Hub to the Death Ray control , where the player character will face a room full of hostile aliens . Once they are defeated , press the button with the cylinder graphic above it four times , causing the four generators to rise . Repeat the same pattern to overload each one . After destroying the second generator , another wave of several aliens will attack . If one desires , they can redirect the death ray from this room as well . Once all four generators have been destroyed , the door across the way opens to the living quarters . The player character will encounter portable shields here for the first time , which can be destroyed by several hits . Fight past turrets , aliens and the occasional abomination to reach the final destination : the teleporter to the bridge . Kill the alien captain . Once entering the bridge , the player character is committed to finishing This Galaxy Ain ' t Big Enough . . . and will not be able to re - enter most areas of the ship . Once arriving on the bridge , the player character ' s next objective is to kill the alien captain . The captain himself is fairly weak , but he does carry a powerful weapon , the Captain ' s Sidearm , which fires multiple shots at once . There are also several other aliens in the room , armed with alien atomizer s . The player character ' s allies ( if any ) may kill the alien captain without them . Once he is dead , a large holographic alien head will speak to the player character , but as always , it is impossible to understand . Once he ' s done talking , another alien ship passes by and attacks the Mothership with its Death Ray . Destroy the attacking alien ship . The player character ' s allies ( if they have survived ) will come to the bridge . Sally remarks that the control room looks exactly like the one from Captain Cosmos , and that she knows exactly what to do . She orders everyone to take a specific position on the bridge . Paulson , however , is asked to " guard our backs " ( specifically , Sally tells him he can be the " Moon Monkey , " to which he replies " I ain ' t no goddamned space monkey " ) ; he remains in the area with the teleporter . Now the player character must destroy the attacking alien ship . One can command the ship ' s Death Ray and shields , and determine how the power in the ship should be divided between them : Occasionally a hit from the attacking ship will cause the ship ' s generators to go offline . One must reactivate them via the four buttons located on the east and west walls of the bridge . While the generators are offline , Elliot will not be able to use the teleport jammer . This will result in aliens teleporting into the ship and attacking . If one wants to make sure everyone survives , one may need to kill the aliens before going back to restart the generators . Once the player character damages the attacking ship halfway , Somah will say that the shields and Elliott are down ( the shields will come back almost immediately , and Elliott just gets knocked to the floor when the panel explodes ) . At this point , the teleport jammer will no longer be active at all , and aliens will teleport into the ship more frequently whether the generators are online or not . Elliott will then head through the teleporter . He will then return after the battle is over . After the player character leaves the mothership , Somah and Paulson are removed from the game . Behind the scenes . The name of this quest is titled after a common phrase spoken in classic American western movies , usually before the start of a duel to explain that only one will survive . The full phrase is , generally , " This town ain ' t big enough for the both of us . . . "