Thirteen-year-old Gorse is the youngest of 13 children. Her father is an elf, and her mother belongs to the Shouting Fey family — a family of fairies long exiled from the fairy courts, living among humans and under the authority of the royal human family. Their name, "Shouting Fey," refers to their "Shouts" — extremely loud and powerful spells.
The human king and queen have had no success in producing an heir, so the queen bids Gorse's great-aunt Gilda to grant her a wish guaranteeing a child. Princess Talia is born. The king issues another Royal Bidding commanding all of the Shouting Fey to come to Talia's christening bearing magical gifts. When Gorse questions why they must obey the king's Biddings, her father explains that after her great-grandparents Fergus and Maeve were exiled from the fairy courts long ago, they were taken in by the royal family — with the condition that the Shouting Fey swear an oath to fulfill the Royal Biddings. The oath meant that Fergus and Maeve and all of their decedents were tied to the land and the whims of the royal family. Though Gorse has been plagued with fevers and headaches throughout her childhood, her family holds out hope that she will be "the One" told of in a cryptic fairy prophecy who will free the Shouting Fey from their servitude to the humans.
When the day of Princess Talia's christening arrives, the Shouting Fey head to the castle, leaving Gorse sick in bed with another fever. Gorse awakens and realizes that if the king and queen notice that she is absent, the Bidding will not be fulfilled, and the entire Fey family will be in danger of bursting into a thousand stars — the consequence of a broken oath or unfulfilled Bidding. She rushes off to the castle, planning to give Talia a spindle with a charmed thread that will bestow long life upon the princess. En route to the castle, she falls into a magical trap and tumbles into an underground cave. Aware that the cave is teeming with old, powerful magic, Gorse surmises that she has fallen Under the Hill and is in the fairy courts. Two fairy men appear — Prince Orybon and Grey. Gorse learns that Orybon is in charge, and Grey is his loyal helper. The fairy men intimidate Gorse; they seem cold and manipulative.
Gorse learns that Orybon and Grey have been exiled from the fairy courts and cursed to remain in the cave. An impenetrable magical gate stands at the opening of the cave, preventing their escape. Orybon explains that Gorse is not subject to their curse and can therefore leave the cave. However, he refuses to tell her how to escape until she swears an oath. While Gorse is considering the offer, Grey divulges the details leading to their exile.
Grey tells Gorse that her great-grandfather Fergus is Orybon's brother, and Fergus' wife, Maeve, was once betrothed to Orybon. As a young man, Orybon became obsessed with his cursing ability and neglected Maeve. Ignored by Orybon, Maeve began a romantic relationship with Fergus. When Orybon surprised them in the midst of their first kiss, he was enraged and cursed them both, exiling them from the fairy courts forever. Fergus and Orybon's father, the king, then cursed Orybon for his actions, sending him into exile until Orybon became truly repentant of his actions. Grey swore an oath to accompany the prince into exile, believing at the time that Orybon would soon repent and free them. Many, many years later, still unrepentant, Orybon is still in exile and Grey with him.
While Grey is telling Orybon's story, Gorse realizes that while Orybon is just as cold and uncaring as she first guessed, she has misjudged Grey — he is bound to Orybon by an oath and by a deep sense of loyalty, but he does not love or respect the prince and was tricked into swearing his oath to Orybon.
Even after hearing Orybon's tale, Gorse still is unwilling to swear the oath to him, so Orybon tricks her into doing so. He also snaps the thread on the spindle meant for Princess Talia. Realizing the danger her family is in if she does not return for the christening, Gorse swears the oath — that she will approach the cave's gate, proclaim her repentance and command it to open; she will then track down Orybon's father and tell him that his son repents and then report back to Orybon in the cave.
After Gorse's first attempt to open the gate fails, she decides to try Shouting it down. While she is preparing to do just that, Orybon arrives with Gorse's brother Dusty, who fell into the cave while trying to find Gorse. Gorse decides that she and Dusty will attempt to Shout the gate down together. Their attempt fails; the gate remains standing, but their Shouts cause cracks in the rock around the gate. Grey, Dusty and Gorse decide that trying to knock down the gate itself is futile, and instead they will use explosives made from bat guano to demolish the rock around the gate. The explosion, along with a Wish and a Shout from Gorse, succeed in creating openings in the rock around the still-standing gate. Orybon runs straight for the fissures, hoping to finally escape the cave, and Grey, compelled by duty, runs with him. Reaching the gate still unrepentant, Orybon immediately bursts into a thousand stars, and Gorse and Dusty believe that Grey is gone, too. However, a teenage boy resembling Grey appears, and the Fey children realize that it is Grey, freed from his Oath in Orybon's death and returned to the age he was when the Oath was spoken. Grey has no recollection of the events that have unfolded since the original Oath was spoken.
Gorse, Dusty and Grey leave the cave and head to the castle to present their gifts to the princess and save their family. Gorse gives the baby princess the spindle, realizing too late that the thread broken by Orybon has not been repaired, meaning that she has just given the princess the gift of an early death instead of a long life. Everyone is horrified. The king orders the Fey to fix the error. Her mother ties a knot in the broken thread, explaining that instead of dying, the princess will fall asleep for a time, and the royal family with her. She says that this will happen when Talia is 15 years old. The king and queen, relieved that their daughter will not die young after all, are satisfied.
After Talia's christening, Grey is adopted into the family by great-aunt Gilda and bonds easily with the Shouting Fey. He becomes especially close to Gorse. Fourteen happy years pass, and Princess Talia's 15th birthday approaches. She plans a party for the occasion and issues a Bidding that the Fey attend. At the party, the Shouting Fey join together to cast the sleeping spell foretold at the christening, and every human in the castle falls asleep along with Talia. Mother orchestrates the spell to last 100 years, the longest duration a spell can last, knowing that in 100 years time, the Family's tie to the land and the royal family will be broken, and they will be free to travel at will and be free of the Royal Biddings, too.
Father tells Gorse that their new freedom is all because of her, because she fulfilled the ancient prophecy after all. She is stunned to realize that he is right. Grey tells Gorse that she is indeed the One and smiles knowingly at her. Gorse finishes the tale by saying that she knows the real power came not from being one but two — from the union of her power and Grey's — noting that she hopes they will be together forever.