"Poison" is the name of a sixteen year old girl living in a small village in the Black Marshes. When her baby sister Azalea is stolen by phaeries, she goes out into the world to look for her. Helped by a wraith-catcher called Bram, she finds the passing-place to the Realm of Phaerie. She faces the terrifying Bone Witch and helps a girl called Peppercorn escape from the house. All three, guided by Peppercorn's phaerie cat, venture into the Realm of Phaerie. They are sent to steal the Lady of Spiders' knife in return for Azalea, but the Phaerie Lord goes back on his word when they return. They escape to the Hierophant's Realm, where Poison learns that they are all fictional characters in a story written by the Hierophant. When the Hierophant is murdered, Poison uncovers the Phaerie Lord's plan to gain the power of the Hierophant. Poison foils this plan, and is chosen as the new Hierophant herself. She resolves to create a hero to liberate the Realm of Men from the phaeries.

When Poison's sister Azalea is stolen by phaeries on Soulswatch Eve, and a changeling left in her place, Poison decides she will go to the Phaerie Realm to get Azalea back. An old man called Fleet gives her advice, money, and supplies. Poison hitches a lift out of the swamps with a wraith-catcher called Bram.

Bram takes Poison to Shieldtown. Here he helps her to locate a man called Lamprey, who makes her gamble her life on a riddle. She answers correctly and he tells her that there is a passing-place to the Realm of Phaerie in the Bone Witch's house.

Bram takes Poison to the Bone Witch's house. She explores, but the night comes quicker than expected and the Bone Witch emerges. Poison is forced to run from her, and follows a strange cat up some stairs to a girl called Peppercorn's room. Peppercorn has lived here as long as she can remember, looking after the house in exchange for not being eaten. She helps Poison to hide, but when the Bone Witch has gone back downstairs she tells Poison that she cannot stay here. Poison goes to the cellar to hide, but the Bone Witch finds her and puts her in a cage in the kitchen.

To Poison's surprise, Bram appears and releases her from the cage. He says that the cat came out of the house to fetch him. Poison comes up with a plan, and with Bram's help they tip the witch's ingredients into the cauldron, then pull out some of the poisoned bones from the mixture. The Bone Witch's dogs eat these and die. They then skin the dogs and fool the Bone Witch into following them. She topples over the balcony, falls into her own cauldron, and dies.

Poison, Bram, Peppercorn and the cat, Andersen, leave the house into the Realm of Phaerie. They make it to the Phaerie Lord's palace, but Lord Aelthar will only let Azalea go if Poison steals a dagger for him from the Lady of Spiders. They all travel to the Realm of Spiders, where Poison manages to outsmart a giant spider and climbs into the palace. She steals the dagger and leaves.

Back in the Phaerie Lord's palace, Aelthar's secretary, Scriddle, takes the dagger and locks them in a room. With Andersen's help they escape, and then sneak to Aelthar's rooms. Aelthar is angry about something the Hierophant is doing, and is planning to visit him. Poison decides that if this Hierophant is Aelthar's enemy, then perhaps he will help them. They follow Aelthar into the Hierophant's realm.

Here they see Fleet, who explains that he is an Antiquarian, one of the Hierophant's workers who record stories. He shows Poison the Great Library where all the stories are kept. He then takes her to see the Hierophant, who tells her that she is just a fictional character in the story he is writing. At first Poison cannot cope with this, but Bram persuades her not to give up.

She goes to see the Hierophant again but he has been murdered, stabbed with the Lady of Spiders' dagger. Poison accuses Aelthar, and Aelthar is then murdered by the Lady of Spiders. Poison finds out that it was actually Scriddle who planned the murder. Scriddle almost kills her, but his accomplice Lady Pariasa stops him and heals Poison with her magic. This is because Poison has already been chosen as the new Hierophant, meaning that the world will dissolve if she dies.

Poison takes the position of Hierophant. She finds out that her sister Azalea was returned to the human realm long ago, as time passes differently in the Realm of Phaerie. Azalea lived happily with her father and stepmother until she turned twenty, then she set off into the world to look for Poison. Poison is sad about this but has found a substitute sister in Peppercorn. Poison decides to create a hero to liberate the Realm of Men from the phaeries.