Not of This World

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

The abduction
The quest can be obtained at any point of the game from the time the Lone Wanderer leaves Vault 101, the same as all other Fallout 3 add-ons except for Broken Steel.

The player character will receive an unintelligible radio transmission which will lead them to the alien crash site, where they 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 one has not found Greener Pastures, travel to MDPL-13 power station and head directly north. There should be a significantly lower risk of radiation exposure when traveling from MDPL-13.

After being abducted, the player character will be probed by the aliens, who then direct a robotic arm equipped with a laser-like device to start moving down the table towards them. The screen flashes red and one drift into unconsciousness. The player character will wake up in a cell, stripped of all their 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 one can overpower them and escape. After exchanging a few punches with Somah, two aliens will enter the cell which will need to be killed. One may wish to pick up one of their shock batons to arm themselves.

After escaping with Somah, 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 is just dreaming.

The escape
Not long after the escape, one will come across a caged cell holding a young girl named Sally. She requests to be freed by overloading and overheating the module near her cell. To do this, one will need to activate the three cylinders surrounding the module down the hall. 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 the player character throughout the ship. Somah says she will be staying behind to "watch your back" and that she will catch up later. One's gear may be retrieved from a container located in a storage room to your left down the hall.

Head through the fairly linear path with Sally. Not long afterwards, some some alien guards (which appear as friendly dots on the compass, for now) will be alerted, who will spawn nearby and Sally tells the player character to wait. One has the option of asking Sally to assist them 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 one distracts distract them. Otherwise, one must deal with them on their own and she will crawl anyway behind the walls. It may be a good idea to let her assist, as the three aliens will all die immediately from her grenade as soon as they are approached.

Sally will continue offering assistance as one continues through the ship, such as unlocking doors or creating distractions. Her help does not always need to be accepted though. One may opt to just cut a bloody path during the escape, which will also provide a lot of alien loot such as weaponry, alien epoxy, and alien power modules.

During the escape one will eventually encounter alien workers. Sally informs the player character that it would be better to leave the harmless aliens alone. Killing them results in negative Karma.

Reward

 * 300 XP

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 one has killed the first two alien guards.
 * A way to avoid this is to not punch Somah and rather let her punch the player character. Alternatively, when the option to fight appears, choose to not fight and then choose to fight afterwards.
 * There is in fact another way out of the cell besides fighting Somah. Keep in mind that you will have to reload an older save. By using the camera glitch in which you hold LB and move the camera around one 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 the player character is in the cell. One will still have to return to Somah and initiate the fight or else she will 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 (one cannot deactivate the door a second time and the alien peacekeepers just run into the barrier endlessly) leaving the player character in the cell with Somah and no way to break up the fight. She will attack the player character until one kills her. If one leaves the cell this way the aliens that break up the fight are NOT hostile unless attacked. Needless to say if one gets out of the cell using this method Somah will not follow them, forcing one to return to return and fight her, either trapping them in the cell with a hostile Somah or if one kills the aliens they cannot break the fight up and she will remain hostile indefinitely.
 * Sometimes, when the payer character retrieves their items from storage the "Take All" option does not actually take all of them items from the container.
 * Upon reaching the crashed alien ship and being taken into the dlc the cut-scene where the aliens are probing the player character will glitch, causing the cut-scene to freeze without continuing into the download.
 * Upon being abducted while holding the Shishkebab, the player character will indefinitely have the oven mitt on their right hand as if the Shishkebab's still equipped. Re-equipping and un-equipping the Shishkebab or reloading a previous save before starting the DLC will fix this.
 * Somah may not appear in the prison cell at the start of the DLC. To remedy this use the console and type 'Prid xx000988' and then 'enable' before approaching the crashed spaceship. Somah should then appear in the prison cell as normal.