When Endless Horizons enters the battlefield, search your library for any number of Plains cards and exile them. Then shuffle your library.
At the beginning of your upkeep, you may put a card you own exiled with Endless Horizons into your hand.
8/1/2008 If Endless Horizons leaves the battlefield, the remaining Plains cards stay exiled. If Endless Horizons returns to the battlefield, it’s a new object with no relation to its previous existence. The previously exiled Plains cards will still stay exiled.
8/1/2008 If a different player gains control of Endless Horizons, its ability will trigger at the beginning of that player’s upkeep, but it won’t do anything when it resolves.