Thirteen-year-old Jason Walker is volunteering at the local zoo when he hears music that seems to be coming from inside Hank the hippo. As he tries to listen to the music, Jason falls over the railing and lands in Hank's mouth. Instead of being crushed by the hippo's jaws, Jason finds himself transported to a fantasy world and ends up standing beside a large river.
Jason sees a raft full of musicians called the Giddy Nine floating down the river. For reasons unknown, the Giddy Nine are intentionally sailing their raft toward a waterfall to destroy themselves. Jason tries to use a bow and arrow to shoot a rescue rope toward the raft, but instead of hitting the raft, his arrow hits one of the musicians in the shoulder. Jason's rescue attempt is unsuccessful, and the raft full of people plummets over the waterfall.
In shock, Jason runs into the forest and spends the night there. He doesn't know how to find food or shelter. The next morning, Jason comes upon a building called the Repository of Knowledge. Inside the building is an old man known as the loremaster. The loremaster informs Jason that he is in a country called Lyrian, and Earth itself is simply known as the "Beyond." An evil king named Maldor, the last living wizard, rules Lyrian. Since Jason wants to return to Earth, the loremaster allows him to search the Repository for information, but he warns Jason not to unlock the door to the library's upper level.
Heedless of the loremaster's warning, Jason unlocks the door that leads to the forbidden level. There he finds The Book of Salzared, which is bound in human skin. He learns that Maldor may only be defeated if someone speaks one magical Key Word in his presence. The book shows Jason only the first syllable of the six-syllable Key Word. When the loremaster discovers that Jason has read the Book of Salzared, he tells Jason to search for Galloran, the Blind King, who will help him defeat Maldor.
Because the evil king is already aware of Jason and will now try to kill him, Jason must find the other syllables. At Galloran's castle, Jason meets another Beyonder. Rachel is about Jason's age, and Galloran suggests that they work together to defeat Maldor. Galloran bestows knighthood upon Jason, making him Lord Jason. He sends Rachel and Jason on their quest to find the Key Word's syllables.
First, Jason meets Jugard, a man who tells him the fourth syllable and directs him to the town of Trensicourt to find Nicholas, the next adviser. On their way to meet Nicholas, Jason and Rachel encounter the still-living, disembodied head of Ferrin, a magical person known as a displacer. Ferrin's supernatural nature allows him to live even when parts of his body are cut off. Jason and Rachel help Ferrin find his separated body parts and reattach his head. Ferrin joins the children on their quest.
In Trensicourt, Nicholas says that the Key Word's third syllable is written within the town's royal lorevault, and an additional syllable can be found on an island in the center of Whitelake. To gain access to the lorevault, Jason challenges Trensicourt's chancellor to a battle of riddles. Jason wins, enters the lorevault and learns the Key Word's third syllable.
While still in Trensicourt, Jason receives an invitation from Duke Conrad, a servant of Maldor, asking Jason to join a party called the Eternal Feast. Maldor's enemies can enjoy a life of constant pleasure at the Eternal Feast if they will only stop trying to defeat him. Jason ignores the invitation and travels with Rachel and Ferrin to Whitelake.
Whitelake's surface is a peculiar form of quicksand, so Rachel has to run lightly across the lake to reach the island where she meets a displacer named Malar. Only Malar's head and one of his arms remain on the island, as a punishment for rebelling against Maldor. Malar tells Rachel the fifth syllable of the Key Word, and says the sixth syllable can be found in the Sunken Lands. Malar also tells Rachel that their friend Ferrin is a spy for Maldor. When Rachel returns from the island and confronts Ferrin, he admits to being a spy and leaves Jason and Rachel to continue their journey alone.
While traveling to the Sunken Lands, Jason and Rachel meet Jasher, a member of the Amar Kabal, another magical race. Jasher says he has been sent by Galloran to protect them on their quest. In the Sunken Lands, Jasher points out a giant tree where the Pythoness, the keeper of the sixth syllable, lives. Inside the tree, Jason discovers that the Pythoness has died, but her daughter Corrine tells him the sixth syllable.
Now Jason, Rachel and Jasher need only the second syllable to complete the Key Word. This syllable is tattooed on Kimp, a man serving Duke Conrad at the Eternal Feast. To find Kimp, Jason accepts Maldor's invitation to the Eternal Feast at Harthenham Castle, and after a few days, he spys the syllable in Kimp's shoulder tattoo.
When Jason asks to leave the Eternal Feast, Duke Conrad forbids him from departing and challenges him to a duel to the death. Jason chooses billiard balls as the duel weapon, counting on his baseball pitching skills to help him win. Jason knocks Duke Conrad unconscious with a ball but does not kill him. Jason leaves Harthenham Castle but is followed by Kimp and Duke Conrad's boarhounds, which attack him. Jason escapes with minor wounds, but his friend Jasher is killed in the fight. More horsemen begin to pursue Jason, and he is separated from Rachel just after he communicates the final syllable to her.
Jason goes to the Felrook stronghold and speaks the Key Word, arimfexendrapuse, in Maldor's presence, but the word has no effect. Maldor reveals that the Key Word is just an elaborate ruse, designed to keep heroes questing after a useless word instead of focusing on defeating Maldor directly. Saying that Jason is a worthy opponent, Maldor offers to let him become one of his advisers. When Jason refuses, he is put into the Felrook dungeons. While imprisoned, Jason is bitten by a venomous snake, locked inside a small iron container for long periods of time and given only a little food and water. Eventually, Ferrin rescues Jason, smuggles him out of Felrook and leads him to a cave known to be a pathway to the Beyond.
Inside the cave is a pool that Jason must jump into to reach Earth. Jason feels guilty for leaving Rachel behind in Lyrian and for leaving the citizens of Lyrian defenseless against Maldor. When he tries to run away from Ferrin, Ferrin ties Jason up and pushes him into the pool to send him home to safety. Jason catches hold of Ferrin's hand, which comes off, and he falls through the water, ending up in a cornfield on Earth. After putting Ferrin's hand in a bag to carry with him, Jason finds a nearby farmhouse and knocks on the door. He is intent on finding a way back to Lyrian to help Rachel. When a woman answers the door, he asks to use her telephone.