Not of This World

 is a Fallout 3 quest and achievement which will be added to the game in the Mothership Zeta add-on.

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 will 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 Northeast. If you haven't found Greener Pastures travel to MDPL-13 power station and head DIRECTLY NORTH (and I mean DIRECTLY). Also you should obtain significantly less radiation by traveling from MDPL-13.