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. Also you should obtain significantly less radiation by traveling from MDPL-13.

After being abducted, you will see a scene of your character being 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 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 under the holes in the walls. She will then offer herself as a guide and help guide 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 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 after you both set off some Alien Security guards (which appear as yellow dots on the compass, for now) will spawn nearby and Sally tells you to wait. You then have the option of having her assist you by giving her a grenade (Only Frag or Plasma can be given) so that she may sneak up on them via 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 her way behind the walls anyway. It may be a good idea to have her assist you, as the three enemy aliens will all die immediately from her grenade as soon as you approach them, saving you the trouble.

She'll continue offering you assistance as you continue through the ship, like unlocking doors or causing distractions to make your trip easier. You don't always have to accept her help though if you opt to just cut a bloody path during your escape, which will also provide a lot of Alien loot and weapons too, namely epoxies, Alien Power Modules for ammo, and weapons.

During your escape you'll eventually encounter Alien Workers. Sally informs you it'd be better to leave the harmless Alien Workers alone. Killing them will get you negative karma as well.