Amazingly enough, even back in the 1880s you could look at a sequel and say "Dude, the original was better." "The Princess and Curdie" is the sequel to "The Princess and the Goblin" [link]. It comes across as more moralistic and heavy-handed than the original, and lacks some of its charm. Continuing on to MacDonald's final piece of adult fantasy, "Lilith" (1895) I have come to the conclusion that his genre work took a downhill turn after "The Princess and the Goblin" and never really recovered.
"Curdie" starts out one year further on from "Goblin." Curdie is 15 now and turning into a teenager, with a bit of the angst that that entails. However, after shooting a pigeon and realizing that it belonged to the Galadriel/Grandmother figure from the first book, he sets his life back on the right path. The fairy godmother sends him on a journey to the capital of the kingdom. She grants him a few boons: the ability to tell good people from bad by holding their hands, and a big ugly monster, Lina, who is really a good person inside. As he travels to the capital he and Lina recruit more unique and ugly monsters, which will of course come in handy later.
Curdie gets to the capital, and is immediately treated badly by the corrupt and petty townsfolk. Only one old woman and her granddaughter are nice to him. He is arrested and led off to jail in a moment rather strongly recalling Christ's journey to Gethsemene. Lina finds him and they bust out of jail and into the castle. It turns out that the King is being poisoned slowly by his staff, especially the Lord Chancellor and the doctor. Princess Irene from the original book has been too naive to see any of this (which I found a bit hard to swallow, given how with-it she was back then) but is immediately convinced by Curdie's testimony.
So they (mostly Curdie) seperate out the bad people (lots) from the good people (few) in the castle, and all the ugly monsters come in and drive out or capture the bad people. That's all well and good, but the King isn't back to being an effective King yet, and the townsfolk are conspiring against him. They call over to the neighboring kingdom, offering to sell out their kingdom for good treatment. Eventually the King, Curdie, Irene, the lone good soldier, a page, a handmaid, and all the ugly monsters go out to confront the invading army. It predictably doesn't go well until the fairy godmother saves the day.
MacDonald can't contain his cynicism even at the end. The denoument mentions that the King gets better and reforms the kingdom. Irene and Curdie marry and are a great King and Queen. But they don't have any children and the next king is so greedy that he mines all the minerals (mostly gold) out from under the castle, collapsing it and leading to the collapse and erasure of the kingdom as a whole.
So basically, this book is all about the divine right of kings and how the awful urban merchant and middle classes are corrupt and venal. It's even mentioned that...