"The Wishsong of Shannara" is the third novel in a series that features members of the Ohmsford family completing quests to save the Four Lands. In this novel, Brin Ohmsford and her brother, Jair, were born with Elven magic they call the wishsong. Brin is able to alter nature with her wishsong, while Jair can only create illusions. Their father has forbidden them from using the magic, but sometimes they still perform magic in secret.

Brin and Jair think the magic is merely a toy, until the day the Druid Allanon arrives. He asks Brin to help him destroy the Ildatch, a book of dark magic that overpowers any who try to use it. Mord Wraiths, also known as dark walkers, guard the Ildatch and they are searching for Allanon. Gnomes and other creatures also protect the dark magic, because they are under its spell.

Brin, Allanon, and family friend Rone Leah make their way toward Graymark castle and the Maelmord, with many side adventures along the way to battle monsters and fight the dark magic. Brin struggles not to use her wishsong, once she realizes it can be used for harm. She worries that she will lose herself and lose her ability to control the magic. More than once Brin is forced to use the wishsong to do harm to her enemies, and she is horrified by the sense of glee she feels afterward. Allanon adds magic to Rone's sword, the Sword of Leah, and Rone becomes obsessed with the magic it contains. When Allanon is dying after being attacked by a Jachyra, Brin promises him that she will destroy the Ildatch. Brin and Rone meet an old man named Cogline, who is familiar with the land around the Maelmord. He reluctantly agrees to help Brin reach her destination, along with his adopted granddaughter, Kimber, and their giant cat, Whisper.

Brin's brother Jair is left behind when Brin and Rone leave on their quest, because he is young and he must explain Brin's absence to their parents. But then Jair encounters a Gnome and Mord Wraiths searching his house, and he realizes he must leave for his own safety. He takes his father's Elfstones and travels toward Rone's homeland, Leah. He hopes to find help for Brin and also plans to send word to his parents, but he is taken prisoner by the same Gnome he had seen previously. The Gnome, named Slanter, eventually becomes Jair's friend and protector. Others join Jair's group, including a Weapons Master, Garet Jax; a Dwarf, Elb Foraker; a Prince of Elves, Edain Elessedil; and a Borderman, Helt. Jair is visited in a vision by the King of the Silver River. The King offers Jair magic to help Brin, in exchange for Jair's Elfstones. Jair reluctantly agrees. The King tells Jair to travel to Heaven's Well, near Graymark castle and the Maelmord. The King gives Jair a pouch of Silver Dust so Jair can remove the poisons from the Silver River. The King also gives Jair three things to assist him in helping Brin: first, a vision crystal, through which Jair can see an image of his sister; second, extra strength to anyone who helps Jair during his quest; and third, the power to use the wishsong one time only to create reality instead of illusion.

Brin finally reaches the Maelmord, tricking her companions so that she can finish her quest alone. The Maelmord is a living, breathing forest, and Brin realizes that she must join with it rather than fight it. As she uses her wishsong, Brin feels her own self slipping away. When she reaches the Ildatch at last, her former self is all but gone and she is in danger of joining with the book's dark magic forever.

Jair throws the Silver Dust and the vision crystal into Heaven's Well, just in time to see an image of Brin clutching the Ildatch and looking transformed by its evil. He uses his one chance to create reality with his wishsong, traveling to join Brin and remind her of her humanity. Brin destroys the Ildatch, thereby destroying the dark magic, the Mord Wraiths, and the Maelmord.