You will recieve an unintelligable radio transmission which will lead you to the Alien Crash Site. The player will then be "beamed up" and end up on the alien mothership, Mothership Zeta, where you will meet other prisoners and plot their escape. The other prisoners originate both from the Capital Wasteland and from Earth s past.
In Mothership Zeta the aliens have returned and they are not happy. The mothership answers the distress call sent out by the alien ship at the Alien Crash Site and abducts the player, making the whole add-on take place onboard of the alien ship. Mothership Zeta is in orbit around Earth and the aliens have been watching Earth for quite some time.
Onboard the Mothership, the questline won't be completely linear and will feature some exploration. You will be able to take a spacewalk. There will be Alien Power Cells but still in a limited quantity. You will encounter other abductees, who are all fictional characters from the Wasteland and Earth's past.
"Having a high Science skill will absolutely be to the player's advantage. Skills like Speech, and even a few of the perks, won't be necessary on the quests, but will provide some extra ways to explore the environment of the add-on".
The environment will be artistically different from the rest of Fallout 3, with nearly no assets reused.
The player will be able to return to the ship once the main quest is over in order to use the Alien Epoxy and to use the ship as a "home-base of sorts".