Narnia is ending. All worlds must end. But no one expected it to end this way.
Shift is a clever ape, but not as clever as he thinks he is. He only thinks he's clever because he's smarter than Puzzle, his donkey friend who does everything he wants. When Shift sees a lion skin floating in Caldron Pool, he has a grand idea. When he fits it over Puzzle, Puzzle looks an awful lot like Aslan. Especially since no one in the current generation of Narnians has seen him before. With the fake Aslan, Shift has unlimited power of the Narnians. But soon, things begin to spin out of control.
He sets up shop in front of a stable, in which he hides the new Aslan. Shift then pretends to be man, the mouthpiece of Aslan. But he begins to invite trouble on himself when he signs a treaty with the Calormen.
King Tirian, the grandson of Rilian, knows nothing about this until a dying dryad comes to him and tells him how Aslan is ordering the animals to cut down trees to sell to the Calormen. After letting his anger get the best of him, Tirian is captured, along with his trusted friend Jewel the unicorn, and tied to a tree, by orders of Aslan.
But when Eustace and Jill appear in front of him at let him go, he knows all hope isn't lost.
They proceed to try to win Narnia back and find themselves in the midst of the last battle as things spin out of control for Narnia.
This was the bestselling book of the whole series for a good reason. It was mostly because by then, everyone had heard of the series and wanted to see the end. This book is probably the best series ender ever written.
Many other alternate world series have tried to copy this end but have fallen short. I believe what makes this series unique is that each book is different, as I've said before, and also, C.S. Lewis didn't try to make the books too long. Many fantasy authors like to cram all kinds of far out ideas into one to three books and it doesn't end up working because they don't know where to stop. People think that lots of original ideas make a good book, but it's really the plot that makes the book good. That's why the Chronicles of Narnia is the only Elite Series ever.