Original: 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.
Simplified: Jill and Eustace return to save Narnia from Shift, an ape, who tricks Puzzle, a donkey, into impersonating the lion Aslan.