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.