After completing the Forest Temple and being given the Forest Medaillion by Saria, you and Link went back to the Temple of Time where Sheik is supposed to teach Link the Prelude of Light. But when you arrive, you're met with a dire sight: an entire horde of spies of Ganondorf are inside the temple, trying to destroying it, and one of them has Sheik pinned to a wall and grabbed in a chokehold.