Eustace Scrubb (last seen in "The Voyage of the Dawn Treader") returns to Narnia, this time with his schoolmate Jill Pole. They are tasked by Aslan with finding Prince Rilian, the son of King Caspian - Rilian vanished a decade ago. Unfortunately, all Aslan gives to guide them is a set of four cryptic signs which Pole soon forgets ... They're introduced to Puddleglum the Marsh-wiggle, a depressive creature who is nevertheless reported to be too high-spirited for his fellow Marsh-wiggles. He is always envisioning some horror happening in the world or to himself or his companions. Despite being annoying to listen to, he's reliable and very brave. They fall off cliffs, fly on owls, have a not very pleasant visit with some giants, and spend a lot of time underground. You'll be unsurprised to find that it all comes out well in the end.

The list of modern-day complaints about the writing in Dawn Treader still apply here:

"Where I come from," said Jill, who was disliking
him more every minute, "they don't think much of men
who are bossed about by their wives."

I'm interested to find that he wrote this before he was married (although he was about 55 when this book was published).