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 Not of This World 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 DLC.

You will receive an unintelligable radio transmission which will lead you to an Alien Crash Site.