Twelve-year-old Kara Westfall and her brother, Taff, have been outcasts on the island of De’Noran since the night their mother was killed for being a witch. Kara, then 6, was accused of being a witch as well, but Fen’de Stone, the leader of their town, was thwarted in his efforts to prove it.
One day Kara uses her uncanny ability to calm animals to treat a neighbor’s horse for an infection. Although not paid what the man promised for her help, she is grateful for the seed he gives her. After school, she and Taff go to the store. She hopes the storekeeper will give her a few drops of medicine to ease Taff’s cough in exchange for her seed.
Grace, the daughter of Fen’de Stone, stops her. Where Kara is despised in the town, Grace is revered. Grace convinces the other store patrons that Kara stole her seed, and they force Kara to give it to Grace. That night, in an effort to make Taff feel better, Kara reads to him from the book they have been writing. In it, a sister is able to heal her sick brother with the help of a beast named the Jabenhook. The Jabenhook sits upon the brother’s chest and wraps him in its arms. When it flies away, the sickness goes with it, and the brother is never ill again.
Later that night, Kara goes to the fields to harvest the hushfruits her father has neglected to pick. A strange black bird, with eyes that change colors, appears. When Kara will not follow him into the Thickety, a dense forest on the outskirts of town, the bird steals her mother’s necklace from around Kara’s neck. This forces her to follow him or lose it.
The Thickety is home to plants and insects that kill humans. It is also the dwelling place of Sordyr, a demon. People are forbidden to enter it. Only one person — a boy named Simon Loder — has ever survived when he crossed its borders. Simon spent several nights in the Thickety when he was 8, but it took his mind. Now haunted and mostly mute, he is the constant companion of Grace Loder.
Even though it is forbidden, Kara chases the bird into the forest. He leads her to a grove and conveys with its eyes that she should dig at a specific spot. There, Kara discovers an old book and recognizes it as once belonging to her mother. Afraid it might be a grimoire, a witch’s book of spells, she hesitantly opens it, but finds the pages are blank. The bird returns her mother’s locket.
She looks up to find a man in an orange-hooded cloak watching her. Kara knows it is Sordyr, the forest demon. He offers her a giant black seed that he draws from his chest. If she accepts the gift, she will be his slave forever. She runs away, but not before he calls her name.
Kara cannot leave her mother’s grimoire alone, even though the pages are blank. When her only friend, an older boy named Lucas, steals it as a joke, Kara is furious. She insults him as she snatches it back. Lucas is a Clearer, a poor person who is forced to spend his days cutting back the poisonous plants the Thickety spreads overnight.
Suddenly, Lucas and Kara are attacked by a gra’dak, a vicious creature from the forest. Kara, horrified, silently begs it to stop, and the creature does.
In her bed that night, Kara knows that she somehow commanded the gra’dak. When she opens the grimoire, a beautifully rendered drawing of the creature now graces the first page. As she studies it, she realizes the drawing is almost 3-dimensional. She sees symbols within it and begins to read them almost without thought. When she pulls herself from her trance, she discovers she has called another gra’dak into being. She sends it out into the night knowing now that she is a witch exactly like her mother.
In the following days, Kara becomes obsessed with the grimoire. She cannot let the book from her sight, allowing thoughts of it to consume her. One night, when boys from the town come to throw dung and blood at their barn as a cruel joke, Kara calls a horde of rats to attack them.
Fen’de Stone brings his special guards — the graycloaks — to investigate the Westfall property. She manages to convince him that the boys are making up stories. When she goes to find the grimoire from its hiding place, she discovers it is missing. A note from Constance Lamb, a neighbor and old friend of Kara’s mother, explains that she has the book.
Constance tells Kara about how the grimoire took over her mother’s soul. She changed from a loving friend to a paranoid one. On that fateful night long ago, Constance guarded Kara’s mother while another friend, Abby, took the grimoire to destroy it. Kara’s mother threw Constance into a mirror, permanently scarring Constance’s face.
Kara’s mother then killed Abby and her husband. Still, the mother would not cast the last spell in the book because, if the last page is used, the witch becomes the property of the grimoire and whatever dark entity gives it its power. Kara promises to destroy the book, but finds she cannot.
Kara runs to a small grove of trees and casts several spells in order to alleviate the pain that being without the grimoire has caused her. Grace and her servant, Simon, find her. Kara is surprised to see that the Fen’de’s daughter can see things on the pages. Grace casts a spell to make it snow. Kara manages to get the book back. She tries to keep it with her at all times.
Grace comes to Kara’s house that night to beg to look at the grimoire again. She tells Kara that Sordyr sends her wonderful dreams at night. Kara calls up a horde of bugs that digs into Grace’s skin. The girl flees, but Kara knows she will be back for revenge. It comes on the night of a special town festival.
After being tricked by Grace, Kara and Taff wind up in an abandoned house with her and Simon. Simon tries to strangle Taff, but Kara manages to free him before her brother dies. She sends a swarm of stinging insects to attack Grace.
Taff begs her to stop. She is about to listen to his plea when Simon slams Taff’s head to the floor. Kara commands the insects to kill Simon, which they do. Before she can see if her brother is still alive, she is arrested. Grace gives the authorities testimony that Kara is a witch. Kara is thrown into a deep well to await a trial.
Inside the well, the black bird comes and shows her pictures of the past. Kara learns that her mother had taught her friend Abby about magic. It was Abby who killed her husband and threw Constance into the mirror. Abby then cast the last spell to try and revive her husband. Kara could not see what happened to the woman, but she heard Abby’s screams of terror and pain.
The bird then showed Kara’s mother after she gave birth to Taff. Kara’s mother knew she would be tried as a witch, so she begged her husband to condemn her. That way, their children might have the chance to have a normal life. If he defended her, they might kill him, too.
Kara is questioned by the Fen’de and tells him that the grimoire is in the Thickety. He claims they will follow her the next day to where she buried it. Lucas tries to free Kara but is imprisoned with her. They are left alone for three days.
Finally, Fen’de Stone unlocks the cell. Grace has been terrorizing the town with magic. The Fen’de begs Kara to stop his daughter from killing everyone. Kara goes home to see Taff. She is determined to get the grimoire from Grace so she can try and heal him. Her father admits that he has her mother’s grimoire — the simple notebook that he has been writing in for years, but every morning, the pages are empty.
Grace has Abby’s grimoire. Kara takes the book in to her brother and tries to heal him, but her words have no effect. She finds the book they made together and tells the story of the Jabenhook. The grimoire creates the creature, which then heals Taff.
Kara faces Grace in a horrific showdown of magic. Kara calls creatures to kill Grace but stops them before they finish the act. Each spell weakens Kara a bit more, while Grace seems to gain more power. Grace rains giant boulders down on Kara, and Kara calls the spiders from the Thickety to help her. They quickly spin webs that stop the rocks from hitting her.
Kara runs to the schoolhouse, only to find Grace waiting for her. Grace has called Simon back from the dead, and she orders him to kill Kara. Kara calls the Jabenhook to her, but instead of using it to heal her exhaustion, she sends it to Simon so that he can be free of suffering and allowed to die. Both girls only have one page left in their grimoires. Grace demands they dual in the streets.
As Kara begins to summon the last spell in order to save her family, her grimoire bursts into flames. Each page becomes a bird that flies away, forming a tunnel of light until the grimoire disappears. Before Kara’s last spell can kill Grace, she stops the creature and picks up Grace’s grimoire from the ground. Unrepentant, Grace calls a spell from a scrap of the last page of her grimoire. A great chasm opens in the book, and arms reach out to drag Grace down into it even as Kara tries to save her. The battle ends.
After that, life returns to some semblance of normalcy on the island. Kara’s friend Lucas decides to try his luck in the world, but promises to return one day. Kara’s father becomes a respected leader again in the town. He calls a special meeting of the all the residents. As he speaks, he calls for the eradication of magic from the island. Kara realizes that the man speaking is no longer her father at all. The spirit of Timoth Clen, the legendary man who first rid the island of magic, has possessed him.
As the people turn on Kara, planning to stone her to death, she and Taff flee. Even without her grimoire, she realizes that she can still do magic. She calls a horse to her. She and Taff manage to escape into the Thickety.