But at this time , Narnia is under the spell of a witch . The children then fulfill an old prophecy as they help Aslan ( a lion ) save Narnia from the evil White Witch , who had ruled over the kingdom for a hundred years . The Pevensie children help Aslan , a talking lion , save Narnia from the evil White Witch , who has reigned over the land of Narnia for a century of perpetual winter .