A savage plague is sweeping through Faerie, the Immortal Realm that has for thousands of years been free of illness. Suspicion over the cause of the outbreak falls on Tania’s mortal parents, Clive and Mary Palmer. It’s believed that they brought the deadly illness through from the Mortal World. Clive and Mary Palmer are banished, and half-Mortal Tania is put under close guard. The Conclave of Earls commands that the ways between the worlds be closed forever.
Before that can happen, and against the wishes of the King, Tania steps secretly into the Mortal World with her sister Rathina at her side. Tania’s intention is to bring family friend Connor Estabrook, who is studying to be a doctor, back into Faerie in the hope that he can help her defeat the disease with his knowledge of modern medicine.
Upon her return, Lord Aldritch of Weir accuses Tania of bringing doom to Faerie. When Princess Eden learns that the sickness comes from Faerie itself, suspicions toward the Mortal World ease and Tania is free to find a cure.
But her relationship with her beloved Edric is under severe strain. He is meddling with the Dark Arts, and when she rejects his unexpected proposal of marriage, he tells Tania that he intends to return to his childhood home of Weir as a captain of Lord Aldritch’s court.
Brokenhearted, Tania begins her quest without him, and she, Connor, and Rathina seek the submerged Lost Caer, a sunken fortress off the coast of Weir, where they hope to find word of a cure for the plague. Here, Tania and her companions learn that in the long-ago times, the people of Faerie were Mortal and winged. They also discover that this is not the first time the plague has fallen upon them.
A mysterious entity calling herself the Dream Weaver tells them of a time when Oberon traveled to Tirnanog and made a covenant with the Divine Harper that gave the folk of Faerie the gift of Immortality in exchange for their wings. This covenant was broken when the Sorcerer King of Lyonesse temporarily took the throne of Faerie from Oberon. The breaking of the covenant has allowed an ancient enemy known only by the name Nargostrond to return. Nargostrond is working his evil again in the land, and now Tania, Rathina, and Connor must travel across the Western Ocean and seek the mythical land of Tirnanog to renew the covenant of Immortality and to bring Nargostrond’s plague to an end.
Tania communicates with Queen Titania through a water-mirror and learns that a Great Spell is being cast that will cover all of Faerie in a shield of Gildensleep. Faerie will slumber while Tania quests for the Divine Harper, and the plague will not prevail while the Royal Family can keep the mystic shield active.
The travelers take a boat across the Western Ocean. The Dream Weaver speaks through Rathina again, telling Tania that now that she is beyond the shores of Faerie, her power to walk between the worlds has returned to her. They sail on, but before they make landfall in Alba, they are accosted by the pirate Lord Balor. Tania and Rathina make their escape when Tania sidesteps into the Mortal World, but Connor is left behind, a captive of the pirates.
In the Mortal World Tania and Rathina meet a strange man and woman who seem to have some sense of who they are and what they mean to do, and who offer them help. Tania sidesteps back into Alba, and the sisters manage to penetrate the dark castle of Lord Balor. Connor is in a dungeon, being threatened by Lord Balor, who is accompanied by a great white salamander. The sisters overhear Connor agree to betray them and help Lord Balor discover the secret of Immortality, but they are sure he does this only to stave off torture.
They rescue Connor from the dark castle and flee across Alba to a safe haven: the palace of Caiseal an Fenodree, the home of Queen Titania’s ancient Mortal family. Here they are shocked to encounter Edric, who has tracked Tania by the use of the Dark Arts. He tells her he knows how to make her whole again: either entirely Mortal or entirely Faerie. But Tania cannot accept this offer; she needs both of her sundered selves to do her duty and finish the quest.
Learning that Lord Balor is in close pursuit, the travelers flee the castle of Fenodree, accompanied by Edric, who has promised Tania never to use the Dark Arts again. There is distrust between Connor and Edric, and Tania is stretched to prevent conflict from erupting as they enter the enchanted realm of Erin.
They find that Erin is a land without rhyme or reason. Insane sorceries swirl around them, and they can find no way out of the realm until Edric seeks a path by using the deadly Dark Arts once more. But his Arts draw the Green Lady to them: the Enchantress who holds this entire realm in thrall. Edric is stolen away by the Green Lady, and it is up to Tania to find and rescue him.
Tania survives the trials and magicks of the Green Lady, and upon mystical horses the travelers make their way across dragon-haunted Hy Brassail to the far coast, where they expect to find the land of Tirnanog.
Instead, they are ambushed by Lord Balor. Tania learns that Connor has been working for the sinister lord all along—keeping in contact with him and guiding him to them in exchange for a promise of wealth and fame in the Mortal World.
Tania overpowers Lord Balor and the Great Salamander is set free. The liberated Salamander tears Tania’s back open. Wings grow from her wounds, and she flies to Tirnanog. She meets the man and woman who befriended her in the Mortal World, and they take her to the Divine Harper.
But the Divine Harper asks a great price for the information Tania seeks: She must give up the deepest wish of her heart. Tania agrees, although she is sure this will mean the loss of Edric’s love. Now she learns some of the truth. Nargostrond, the plague-bringer, is King Oberon’s older brother, Prince Lear, banished thousands of years ago for his evil ways and his desires of conquest, both of Faerie and the Mortal World.
But even while Tania is trying to grasp the full dread of this revelation, the Divine Harper exacts the price for her enlightenment and Tania falls into deep unconsciousness.