Master Dallben has decided it is time for Eilonwy, daughter of the Royal House of Llyr, to learn to behave like a proper princess. He sends her away to the Isle of Mona but instead of training in the art of being a lady, Eilonwy falls into the hands of the evil enchantress, Achren, who wants to use Eilonwy's magical powers for her own ends. Taran and his companions must rescue the princess or the peaceful land of Prydain will face a fearful future.

Their quest is a perilous one and demands bravery and sacrifice, but it also promises adventure, excitement and a touch of magic.

The Castle of Llyr is the third book in Lloyd Alexander's classic fantasy epic The Chronicles of Prydain.