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.