Gandalf: For sixty years, the Ring lay quiet in Bilbo's keeping, prolonging his life, delaying old age. But no longer, Frodo. Evil is stirring in Mordor. The Ring has awoken. It's heard its Master's call.
Frodo: But he was destroyed. Sauron was destroyed.
Gandalf: No, Frodo. The spirit of Sauron endured. His life force is bound to the Ring and the Ring survived. Sauron has returned. His Orcs have multiplied. His fortress of Barad-Dur is rebuilt in the land of Mordor. Sauron needs only this Ring to cover all the lands with a second darkness. He is seeking it, seeking it, all his thought is bent on it. The Ring yearns to go home, to return to the hand of its Master. They are one, the Ring and the Dark Lord. Frodo, he must never find it.