Not of This World

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

Walkthrough
The quest is received several minutes 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 unintelligable radio transmission which will lead you to an Alien Crash Site. Where you will then 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 with a woman from the Capital Wasteland named Somah. She will suggest a plan to escape the cell where you have to stage a fight with her in order to trigger the aliens attention, which leads to the two of you escaping from the cell.

After you and Somah have escaped be sure to look at the other cells. A dead enclave officer, a dead wastelander, and a confused Rivet City security guard (that is trying to convince herself that she is in a dream) can be found.

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.

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 net you negative karma as well.

Notable Loot
-Before meeting Sally, there are some Recording modules you can activate in the control rooms that will add Holotape notes to your Pipboy. The recordings contain excerpts from the many victims of the Alien's Abductions, who were all forced to speak for the Aliens after being kidnapped, before being subjected to experiments.

-The are a variety of Alien Gems on the many shelves you'll come across, and a few Alien Power Modules on the first set of shelves you come across.

-There are two other Containers in the same room as the Container with all of your Gear, filled with minor loot from the Capital Wasteland, as well as a Enclave Power Helmet on the floor between them.

-Many containers with minor Alien loot can be found in small rooms you come across during your linear trek through the mothership. Taking minor detours to open up every door you find is advised.

-The room which Sally finally reappears from in the end contains 4 Alien Power Cells (x12 each), Alien Epoxies and a container with minor Alien Loot.

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