The Riddle-Master of the title is Morgon, the Prince of Hed, a small, simple island populated by farmers and swineherds. He has three stars on his forehead, which no one has been able to explain. As the book opens, his sister, Tristan, discovers that he is keeping a crown hidden under his bed, and he must explain that he won it in a riddle-game with a ghost, the cursed king Peven of Aum.
When Deth, the High One's harpist, finds out, he explains that another king, Mathom of An, has pledged to marry his daughter Raederle to the man who wins that crown from the ghost.
Morgon sets forth to claim his bride accompanied by Deth, but while at sea, his ship is sunk by mysterious shapechangers. Shipwrecked, Morgon loses his memory and the power of speech. When Deth finally finds him again, and he recovers his memory and speaking ability, he resolves to travel to ask the High One about the shapechangers. The High One's home, located in the far north on Erlenstar Mountain, is seldom visited. As Morgon and Deth travel the length of the realm, they are repeatedly attacked by the shapechangers, and Morgon learns more and more dangerous knowledge about his three stars and the great powers that come with them.