Princess Eilonwy, the latest enchantress to be born of the House of Llyr, "faces the unavoidable (and in her view absolutely unnecessary) ordeal of becoming a young lady.

"[1] The young hero Taran accompanies her on her journey to the royal court of the Isle of Mona, the same island where her ancestral home was located, and where she will continue her education as a princess.

The series was inspired by Welsh mythology and by the castles, scenery, and language of Wales, which the author experienced during World War II army combat intelligence training.

Like the other books in the series, The Castle of Llyr takes loose inspirations from Welsh folklore, but the stories are not meant to be retellings.

"[4] Eighteen months after the destruction of the Black Cauldron, Dallben the enchanter has decided that Eilonwy, as a princess and last of the line of the House of Llyr, needs a proper royal lady's education that he cannot provide.

While Eilonwy is introduced to the tedium of life at court, Taran encounters his old companion Fflewddur Fflam—a minor king who lives as a wandering bard—and a shoemaker who turns out to be Prince Gwydion, traveling incognito.

Inside, they find a small book of blank pages that Rhun keeps for himself, along with a sheaf of notes belonging to the former resident, Glew, a man who experimented with size-enhancement potions.

Taran discovers that under the light of the bauble, Rhun's book of blank pages is revealed to be filled with writing, though none of them can read the language.

Reaching the mouth of Alaw on the reconstructed raft, the companions reunite with Gwydion, who reveals that he has visited the northeast offshore ruin of Caer Colur, the ancestral home of the House of Llyr, where Eilonwy's grandmother Queen Regat was the last in the line of women to reign.

Against Regat's wishes, Eilonwy's mother, Angharad, married the common man Geraint and left Caer Colur, taking a book of the House of Llyr's most powerful enchantments, as well as the Golden Pelydryn necessary to read them.

Achren hopes to rule Prydain by controlling Eilonwy's mind while also awakening her full ancestral magical power.

That night, Gwydion rows their raft to a point of land below the seaward walls that protect the ruins of Caer Colur from the being flooded by the ocean, hiding the book and bauble before they begin their search for the princess.

Achren needs the spell book to master her control of the House of Llyr's magic, and Rhun stupidly reveals that he and the companions know its location.

Achren turns to Taran and offers a bargain: she will restore Eilonwy's memories of him and allow them to wed if the young man helps her acquire the book and bauble.