Taran, an assistant Pig-Keeper decides to pursue the oracular pig Hen Wen after she escapes her enclosure. On his way, he and Prince Gwydion are captured by an evil enchantress named Achren and are put into the dungeons of the Spiral Castle.
A girl named Eilonwy, who is the self-proclaimed “niece” of Achren, helps Taran escape after she drops her golden sphere into his cell. Eilonwy steals a magical sword called Dyrnwyn on their way out, and as they emerge into the woods, the castle collapses. It is revealed that the castle fell due to the loss of the sword. Eilonwy misunderstood Taran’s request and didn’t free Gwydion, who Taran assumed was in the next cell, but instead freed Fflewddur Fllam. They all mourn the death of the Prince.
Taran changes the plans of his quest and wants to warn Caer Dathyl of the Horned King’s armies, instead of searching for Hen Wen. They are guided by Gurgi, a so called “what is it”. The group encounters a few bumps on the way, but they manage to maneuver themselves out of the problems they face, though the help of people and creatures they meet on the way. They manage to find Hen Wen through the help of Gurgi, and they are accompanied by a dwarf guide called Doli. Taran saves the life of Gwythaint Fledgling, despite Doli’s warnings. The deadly bird escapes, and the Hen Wen vanishes.
The Horned King and his army approaches Caer Dathyl and Taran rushes to Caer, whilst catching the attention of the King. He tries to draw Dyrnwyn but is stopped by magic. Suddenly, a man appears and speaks a word that burns the king. When Taran wakes up, Eilonwy explains everything that has happened. The Prince, Gwydion, didn’t die, but had learnt to understand the heart of all creatures and had found out the name of the Horned King, and when he muttered his name, the king died. The group were rewarded for their efforts with gifts.
