Not of This World

 is a Fallout 3 quest and achievement in the Mothership Zeta add-on.

The abduction
The quest is received shortly after leaving Vault 101, the same as Aiding the Outcasts for Operation: Anchorage, Into the Pitt for The Pitt, and The Local Flavor for the Point Lookout add-on.

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 the module nearby via overheating by activating the three cylinders surrounding it. 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 behind the walls anyway. 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.

Bugs

 * People at Rivet City will instantly know if you kill the captured RC security guard. This is due to her using the normal RC guard script and belonging to the crime-tracking RC factions.
 * 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. On the PS3, the effect would not go away even after loading a save from before MZ was installed, but did go away after re-downloading and reinstalling MZ.(Confirmed PS3, PC GOTY.)
 * Game may lag or hang during the abduction/probing scenes (PS3, PC GOTY)