The series follows the adventures of Taran the Assistant Pig-Keeper, a youth raised by Dallben the enchanter, as he nears manhood while helping to resist the forces of Arawn Death-Lord.

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.

Due to the threat posed by a warlord known as the Horned King, servant of the evil Arawn Death-Lord of Annuvin, Taran is forbidden from leaving the farm and charged with the care of Hen Wen, the oracular white pig.

He awakes to find his wound treated by Gwydion, the crown prince in Prydain's ruling House of Don, who has been travelling to Caer Dallben to consult Hen Wen.

Guided by Gurgi, a hairy humanoid living in the forest, they reach the Horned King's camp, and learn that his target will be Caer Dathyl, the home castle of the House of Don.

The sorceress asks Gwydion to help her to overthrow Arawn—her former apprentice and consort who usurped her throne and claimed the Iron Crown of Annuvin for his own—and to join her in ruling Prydain together.

Eilonwy has misunderstood Taran's request to free his companion, for the man waiting outside is not Gwydion, but another former prisoner of the castle: Fflewddur Fflam, a king by birth but a wandering bard by choice.

Rejoined by Gurgi, but pursued by the Cauldron-Born, the group is driven far east of their northward course, and ends up in the underground realm of the Fair Folk, who have rescued Hen Wen.

Eilonwy receives a ring made by the Fair Folk, Gurgi a wallet of food that cannot be depleted, Fflewddur a golden harp string that can never break, Doli the ability to turn invisible (which he unusually lacks).

[6] The School Library Journal called the novel, "one of the most iconic and influential works of middle-grade fiction from the 20th century", which "helped pave the way for countless fantasy adventures".