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.

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 itself is fairly weak, but it 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 Atomizers. 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 has finished talking, another smaller 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:


 * Full Shields: Most of the ship's energy is routed to the defensive shields, giving extra protection but weakening the Death Ray. This is the leftmost button.
 * Intermediate: The ship's energy is divided equally between the shields and the Death Ray. This is the initial condition.
 * Low Shields: The ship's energy goes mostly to the Death Ray, severely weakening shields but giving the Death Ray much more power. This is the rightmost button.
 * Fire Death Ray: This is the fourth button, to the right of the three shield buttons.

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.

After the hostile ship has been destroyed and the fireworks have ended, speak with Sally to finish the quest. One may now use the still-accessible parts of the ship (mostly the bridge, engineering core, holding cells and steamworks) as their own home.

Reward

 * 600 XP
 * Captain's Sidearm

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..."
 * It is possible to fire the Death Ray at Earth. It creates a giant mushroom cloud and a crater on the Earth's surface, around the real world Kawartha Highlands Provincial Park. If the Lone Wanderer watches the second ship after they have destroyed it, it will eventually fall away from view and down to Earth, crashing roughly around the real world Gulf of Mexico.

Bugs

 * If you still have the Picking Up the Trail quest active, sometimes it will be completed automatically for no reason while this quest is active, usually, in conjunction with the auto-completion of this quest, see below.
 * Similarly to the Rescue from Paradise main quest. This quest may randomly auto-complete up to the 'kill the captain' stage after activating the engineering core teleporter, causing Sally and the other non-player characters to port back into the engineering core and make a beeline for a teleporter to the bridge located behind an inaccessible door. There is presently no simple fix for this. The easiest way to continue is not to activate the first teleporter. Then, use the no clip console command (tcl) to go through the doors Sally normally opens and then go to the weapons lab as normal. This way you can collect the alien captive logs and the Xenotech perk. Proceed onward until you get to the bridge, kill the aliens, then use the setstage command to complete the quest up to stage 40. Then, your companions will teleport to the bridge and you can complete the quest normally. You should get quest credit when the enemy alien ship blows up, even if all the quest objectives aren't shown as complete on your Pip-Boy.
 * Or right after activating the first teleporter, run straight to the door (or the small vent she would get into) and Sally will open the first door for you, once that's opened, quickly run towards the next door (or the small vent she would get into), ignoring the big open space in the middle and stay there. Sally and the others will comment on the view about Earth from up here, just patiently wait. Then she will say oh I should go open the door for you and goes to do that. Keep waiting here at the door or the vents she got into. An angry alien hologram will project onto the area where the people commented about the view and a shot is fired from the ship's death laser cannon. Then Somah will slowly walk towards you. It will of course take some time since you're all the way at the door and not near her unless you increase walking speed through a console command or some such. She will make her comment about the scene. Then the second door opens. Sally will then tell you that the teleporter is not working and that you have to take the long route to enable it and that they reckon you should probably do it alone. At this make a sprint to the Weapons Lab and proceed as normal from here, the quest may automatically skip a couple or so quest markers and jump to "kill the captain" and there's nothing you can do about it except reloading from a save that's outside the ship, just before you're tractor beamed back into the ship.
 * Make sure you have had a save prior to starting this quest because you'll need to reload every time if it doesn't pan out...
 * This method may help to get through the first half of the quest without using the console and skipping some dialog that you may want to participate in, just so you can feel a part of the story, but it may sometimes still skip stage 20 and assume you've passed 25 which the quest log will then tell you to kill the captain part and securing the bridge, in which case you'll have reloaded again. It is frustrating just to participate in their dialogs, with constant reloading.
 * After doing the above point and proceeding to the bridge the long way, it skips stages 50 and 60 and jumps to 200 and finishes there, but you won't get a filled box to say you've destroyed the other ship in the quest log, you might have gotten additional ending dialog here but since the quest never completes correctly, it never happens.
 * When entering the observation area, Sally sometimes will go to the large window in the middle of the room and stay there. Unable to talk with her, she'll stay there and won't open the door for you, making it impossible to continue. A way to fix this is to "push" her near the door, and she will un glitch and open the door. Alternatively, use the teleporter to travel away and then come back through it, causing an autosave, then load from this point. But if to no avail, walk up to the door, bring up the console (`) click the door, and then type "unlock" which will unlock the door.
 * Sometimes, after you have enjoyed the view of Earth, Sally should go to open another door. But instead of opening the door, she'll wait for you at the teleporter behind the "key locked" door. Using the console command "unlock" on the PC will fix this problem. Also, using the teleporter to the engineering core and teleporting back again to the Observation Deck may fix this problem as well.
 * When trying to access the teleport to the Death Ray (that was the only one active out of the three, one of which is the bridge) it does nothing and when pressed a buzzing noise is produced (as if it is inaccessible). The way to easily fix this is to make a save in the room with the teleporters, before you hit the activation switch, turn off your console, and load the save.
 * When trying to access the teleportation matrix in the room after the decompression chamber, the characters won't appear. This can be fixed by reinstalling the DLC, restarting your console, or restarting the game and loading a slightly earlier save game.
 * When in the experimentation lab, sometimes the objectives 'Reach the bridge of the alien ship' and 'Disable the Death Ray' complete without doing anything.
 * Elliott may flee during the final battle, making use of the teleporter and never returning. The player will not receive a message that he has died.