There is another world, not part of ours, though some people from our world can go there. Some call it Avrenes, and some call it Nomis, and some just call it "the other place." This world is not divided along geographic boundaries; it is not split up into good and evil - except for one man. Years ago, the Grey Man put himself outside the spheres of interest of the Powers of the other world and set himself up as a Power of his own, feeding on the despair and misery of those he traps. In all the time since, only two people working together have defeated him: Charlotte, the Apprentice, and Miri, the Maiden - two twelve-year-old girls. They have been his sworn enemies ever since.

But the other world, like all worlds, must change to keep living. The Thorn King, who controls the world's basic structure, is dying. His heir, the Green Knight, helps him to bear the burdens of the world. The Green Knight himself must have an heir to take up the burden when the Thorn King is gone. If the Lost Heir to the Green Knight is not found, the Thorn King's burdens and his power will be spread out to the rest of the ruling Powers, including the Grey Man. Charlotte and Miri are willing to do whatever it takes to stop the Grey Man from gaining power in the world, but not just anyone can be the heir to the Green Knight. If the wrong person attempts to become the Green Knight's heir, the structure of the world will collapse when he tries to take it, and the world itself will fall apart.

While Charlotte and Miri are looking for the Lost Heir and testing their own powers, their friend Megan's older brother, Nate, comes to the other place as the Baker's Boy. Nate finds that the problems of his home world - the death of his father, his arguments with his mother, trying to believe in himself - don't go away when he retreats into the other place. He has to learn to accept himself and his powers while trying to learn the rules of the other world. When they find out Nate has access to the other world, too, Charlotte and Miri become convinced that he is the Lost Heir. Nate refuses to step forward, fearing that he will end the other world just as he is learning to enjoy it. With every step Charlotte and Miri try to take, they find themselves shunted aside, thwarted, or outright attacked by the minions of the Cat Prince and the Sun King, two rulers who are hungry for some of the Thorn King's power. The Cat Prince and the Sun King have formed a temporary truce with the Grey Man while they make their power grab.

Just as they are starting to make progress, Miri loses her ability to go to the other world at all. Charlotte reconstructs a lost spell that allows her to bring Miri into the other place again, in a new role - she is now the Wanderer. Charlotte creates the first spell ever to bridge the two worlds, bringing a snowstorm from the other world to ensure that she and Miri can work their spells uninterrupted by grown-ups. They force Nate into the other place with them and must fight off the Sun King, the Cat Prince, and their minions to get Nate to the Thorn King's Fortress. As the Thorn King is dying, Nate has to decide for himself to take on the powers and responsibilities of the Lost Heir.