It is centuries after the events of "The Silver Chair", Aslan has not been seen in many years, and Narnia faces a threat from Calormen - the country to the south, whose people worship a malevolent demon named "Tash". Trouble begins when the talking gorilla Shift deceives the people and the talking beasts by falsely claiming that he has been directly communicating with Aslan.
The book contrasts those who worship Tash out of fear and those who worship Aslan out of love. Shift insists that Aslan and Tash are two names for the same being - an absurd notion given their opposite ways of dealing with their disciples. It is a commentary on those who perceive God as a judge whose main purpose is to exact vengeance on man. My perception of God is that He is filled with love for man and wants to save us all.
This book may be the most religious in the series. Lewis explores themes such as loss of faith, false prophets, the afterlife, and end times.
It is obvious that Lewis intended Aslan as a version of Christ in this alternate universe, but Tash's counterpart in our world is less obvious. When I first read this years ago, I wondered if Lewis intended Tash to represent Allah, as the color and diet of the Calormenes suggested they might be Arabs. But, with this reading, I believe that Tash is a stand-in for Satan.
Halfway through the story, Eustace and Jill and called back to Narnia to help right the wrongs committed there. Eventually, the book reunites us with many of the characters from past stories - a delight for readers.
This story is darker than its predecessors, but it still conveys the optimism and hope that permeates all these novels. It was a very good conclusion to an excellent series.