The third volume of The Chronicles of Prydain begins with the hot-headed sword-maiden Eilonwy sent off to the royal house on the Isle of Mona to learn how to be a princess and a lady, two things she has zero interest in. Taran and Gurgi accompany her on the sea voyage with Prince Rhun, a clumsy young man Taran dislikes from the first. Eilonwy thinks her friend is just jealous.
The unexpected appearance of Fflewddur Fflam thrills Taran and Gurgi, as does a shoemaker, who is Gwydion in disguise. Gwydion warns Taran he and Eilonwy are in peril. An old enemy thought dead threatens them anew: Achren, the evil enchantress who the princess had apprenticed under as a child, yet remained uncorrupted by. The man admits he fears this threat as much as the greater evil of Arawn. Gwydion swears Taran to secrecy and then disappears.
After a feast to welcome Taran and his companions, the young man seeks out Gwydion. He happens to see the Chief Steward sneak out too and travel to the harbor where he signals a ship at anchor. Gwydion reveals the dread news their enemy is abroad it.
Taran and Gurgi take turns standing guard at Eilonwy's chamber, but still the evil Steward kidnaps the princess after Prince Rhun waylays Taran. During the search that follows, the king asks Taran to watch over Rhun, who he wishes Eilonwy to marry. This does not sit well with Taran, but he does as the king wishes.
Rhun goes off on his own one night. Taran finds him the next morning in an abandoned hut. Abandoned that is, expect for the oversize mountain lion who comes to call while they are inside. This and other desperate perils assail Taran and his friends in their search for Eilonwy, who is in ever deeper danger than they.