So that Zelda can fight the Guardians since she made them, Impa pretends to be Zelda inside the castle. (I'm not sure how they survive for 100 years, perhaps with the ocarina of time.) When Ganon is about to break free, Sheik/Zelda calls on Link. Unfortunately it's to late, and Ganon encircles the castle in a mist form with Impa inside. Link and Zelda must now rescue Impa.