The Book of Three opens at Caer Dallben, a farm run by the enchanter Dallben, whose animals include an acclaimed oracular pig, named Hen Wen.

When Hen Wen disappears, Taran defies Dallben's orders and leaves the farm to search for him. Taran's quest leads him to Spiral Castle, the home of the evil Achren; to the underground Kingdom of Tylwyth Teg; to Caer Dathyl, home of the heroic Lord Gwydion; and eventually back home. The book's only illustration is a map of the Land of Prydain. Alexander is very self-conscious of his setting, and his characters are constantly drawing maps or discussing their whereabouts.