Jill and Eustace return to save Narnia from Shift , an ape , who tricks Puzzle , a donkey , into pretending to be the lion , Aslan . Jill and Eustace return to save Narnia from Shift , an ape , who tricks Puzzle , a donkey , into impersonating the lion Aslan , precipitating a showdown between the Calormenes and King Tirian .