Not of This World

Not of This World is a main quest and achievement/trophy in the Fallout 3 add-on Mothership Zeta.

Summary
It started innocently enough.... a simple case of curiosity. While travelling an unintelligible radio broadcast got picked up by my PIP-Boy 3000. I decided to follow it. When I got there... I was abducted by aliens.

Quick walkthrough

 * 1) Go to the alien crash site.
 * 2) Escape your cell with Somah.
 * 3) Work your way through the holding cells and steamworks
 * 4) Enter the engineering core.

Abduction
The quest can be obtained at any point of the game, the same as all other Fallout 3 add-ons.

You receive an unintelligible radio transmission which will lead you to the alien crash site, where you will be abducted by the aliens. The fastest way to get to the crash site is travel to Greener Pastures Disposal Site, and head northwest. If you haven't found Greener Pastures, travel to MDPL-13 Power Station and head directly north. You should be exposed to significantly less radiation when traveling from MDPL-13.

After being abducted, you will be probed by the aliens. You will wake up in a cell, stripped of all your gear, with a woman from the Capital Wasteland named Somah. She will suggest staging a fist-fight as a ruse to attract the attention of the guards, so that you can overpower them and escape. Once you and Somah have exchanged a few punches, two aliens will enter the cell and she will help you beat them up. You may wish to pick up one of their shock batons to arm yourself.

After you and Somah have escaped, be sure to check out the contents of the other cells: a dead Enclave officer and a confused Rivet City security guard who is trying to convince herself she's just dreaming.

The escape
Not long after you and Somah escape, you'll come across a caged cell holding a young girl named Sally. She requests that you free her by overloading and overheating the module near her cell. To do this, you need to activate the three cylinders surrounding the module. Once it explodes, Sally will open the previously sealed door by climbing through small ducts in the walls. She will then offer herself as a guide and help lead you throughout the ship. Somah says she will be staying behind to "watch your back" and that she'll catch up later. Your gear may be retrieved from a container located in a storage room to your left down the hall.

You must now make your way through the fairly linear path with Sally helping you along the way. Not long afterwards you both alert some alien guards (which appear as friendly dots on the compass, for now), who will spawn nearby and Sally tells you to wait. You have the option of asking Sally to assist you by giving her a grenade (only frag or plasma can be given) so that she may sneak up on them from behind the walls and hit them with the grenade after you distract them. Otherwise, you can deal with them on your own and she'll crawl anyway behind the walls. It may be a good idea to let her assist you, as the three aliens will all die immediately from her grenade as soon as you approach them, saving you the trouble.

Sally will continue offering you assistance as you continue through the ship, such as unlocking doors or creating distractions to make your trip easier. You don't always have to accept her help, though. You may opt to just cut a bloody path during your escape, which will also provide a lot of alien loot and weaponry, such as alien epoxy, alien power modules, and interesting new weapons.

During your escape you'll eventually encounter alien workers. Sally informs you that it would be better to leave the harmless aliens alone. Killing them earns you negative karma.

The quest ends when you make your way through the holding cells and steamworks and enter the engineering core.

Bugs

 * On rare occasions the signal will be picked up, but upon approaching the ship nothing ever happens. If this happens the only way to gain the quest is through the console commands.
 * Sometimes, if you reload a previous save while being examined by the aliens, your armor (if in first-person view and depending on which weapon you're using) and Pip-Boy 3000 will glow blue, as seen from the blue abduction ray. This effect is sometimes permanent. Restarting and reloading may fix this.
 * Somah may hang in the decision of whether to fight her or not, even after you've killed the first two alien guards.
 * There is in fact another way out of the cell besides fighting Somah. Keep in mind that you'll have to reload an older save. By using the camera glitch in which you hold LB and move the camera around you can reach things that you normally wouldn't be able to (e.x. The force fields in Raven Rock). The release mechanism is on the left side of the door if you're in the cell. You still have to return to Somah and initiate the fight or else she'll stand in the cell even though the door is open. When the aliens come to break it up, instead of deactivating it they reactivate it (you cannot deactivate the door a second time and the alien peacekeepers just run into the barrier endlessly) leaving you in the cell with Somah and no way to break up the fight. She will attack you until you kill her. If you leave the cell this way the aliens that break up the fight are NOT hostile until you attack one of them. Needless to say if you get out of the cell using this method Somah will not follow you forcing you to return to her and fight her either trapping you in the cell with a hostile Somah or if you kill the aliens they can't break the fight up and She'll remain hostile indefinitely.
 * Sometimes when being probed by the aliens immediately after abduction the game will freeze. It is possible to bring up the pause screen and pushing buttons makes a metal clanking noise trigger within the game.