As Sarah and her cousins Philip and Michael eavesdrop on their parents (discussing a time in their own childhood when they travelled – or pretended to travel - to a world called “Within”), an old man (the Sage) appears and leads them to Within through a mysterious tunnel connecting the two realities. En route they are waylaid by the Envoy. While the Sage and the Envoy fight, the children are separated and tumble into Within.
Within is so called because it exists on the inside of a sphere: there are a number of reflections in the book on its strange topography. Part of Within is fenced off by a magical Barrier keeping hordes of mindless half-men (halfen) away from the free peoples.
Sarah staggers to Arnwarth, a natural fortress occupied by women learning to use mystic powers. During that first night, the Envoy bends over her as she sleeps and in the morning, she cannot remember her name or how she came to Arnwarth. Philip and Michael come to in a rocky place on the wrong side of the Barrier. The Envoy also causes them to forget their names and their past lives. After being chased by nightmarish halfen and a monster called Groat, they pass through the Barrier and join a band of mercenaries.
Years pass. Sarah is taken in as a servant at Arnwath and eavesdrops on lessons. The boys take commissions as hired swords with the mercenaries, and grow to be dangerous warriors. The Sage desperately tries to find them all. He knows that a giant halfen (the White Beast) has gained rudimentary intelligence and has found a Key that can lower the Barrier, releasing hordes of lesser halfen. He needs Sarah, Philip and Michael with their growing powers and their ability to pass through the Barrier, to fight the Beast for possession of the Key. He cannot understand why the Beast withholds using the Key, and despises his distant relative the Envoy for being in league with the creature.
Sarah, now a young woman, thwarts a magical attempt by the White Beast to invade Arnwath. The teachers realise something is amiss and summon the Sage. As soon as she sees him, Sarah’s memory returns. She finds that she has access to enormous reservoirs of mystic power, and remembers that she saw her cousins with the mercenaries many months ago. This helps the Sage to track down first Michael and then Philip, who have been separated following a squabble among the mercenaries. Now that all three “strangers” have regained their memories, events are set in motion over which they will have enormous influence.
The White Beast uses the Key to lower the barrier, releasing the halfen flood to wreak horror and destruction on the free peoples. Groat is also released and by chance turns an escaping youth into a deranged copy of himself. Sarah uses her new-found power to trick one group of halfen into the ‘whispering trees’ (which she had dreamed about back on Earth), and the trees destroy the creatures. Philip and Michael, with the Sage, do battle with an ancient witch cast out of Arnwarth centuries ago.
During a bloody battle outside the walls of Arnwarth, the Envoy reveals to the Sage that he delayed the Beast’s use of the Key to give Philip, Michael and Sarah time to grow older and much stronger. But the Sage is knocked unconscious before he can tell this to the others. He finds himself hanging in a void between worlds. After a conference in the temporary safety of Arnwath, the mercenaries mount a diversion, and when the halfen are drawn there, Philip, Michael and Sarah slip out the other way and head for the Barrier.
On the final, desperate journey, Philip, Michael and Sarah are alternately hindered and assisted by forces both in Within and back on Earth. When they disturb an ancient monster, the Envoy appears and gives up his life to save them. Sarah is almost overmatched during the confrontation with the Beast, and only prevails by the fortunate intervention of the insane creature made by Groat. With the Beast overcome, Sarah uses power from the Key and Earth to destroy the halfen. The Sage, acting as a conduit for the power from Earth, is killed during this episode. Philip and Michael are whirled back to Earth. Sarah, however, uses her new powers to resist – she wants to stay behind to enjoy the powers she has discovered.
A final interlude includes a section set in the future, with Sarah (now the Sage) returning to ask Dylan, her great-nephew, to come to Within because another crisis has arisen – thereby setting up the possibility of a sequel.