Throughout the novel, an unnamed parent tells their child about the history of the Protectorate and the Day of Sacrifice. The parent lost a baby to the sacrifice years ago, and they say that an evil Witch demands an infant every year. Long ago, the Witch forced dragons to make the volcano explode, and killed a wizard. She poisoned the forest and the Bog that was once life-giving. According to the parent, it’s pointless to stand up to the Witch.
In the novel’s present, young Antain, an Elder-in-Training, is participating in his first Day of Sacrifice. It’s horrific: the mother refuses to hand over her baby, and Antain’s uncle Gherland doesn’t even stay in the woods to make sure that the Witch gets the baby. The narrator says that this is because, as far as Gherland is concerned, there is no Witch. The Witch is a convenient narrative to keep the population sad and under control. But unbeknownst to Gherland, a kindly witch named Xan has been saving abandoned babies for the past 500 years. She doesn’t ask why their mothers abandon them, but she feels it’s her responsibility to rescue them. Xan takes them to the Free Cities after feeding them on starlight, which is why they’re called Star Children. This year, Xan discovers a baby with a crescent moon birthmark. She takes her time traveling with the baby, and after 10 days accidentally feeds the baby moonlight. This enmagicks the baby. Xan knows this means she can’t leave the baby with anyone else, so she names her Luna and decides to keep her. At home, the swamp monster Glerk isn’t impressed while the dragonling Fyrian is ecstatic. Fyrian is the size of a dove, but he believes that he’s a Simply Enormous Dragon and lives in the land of the giants.
Glerk and Xan watch closely as Luna grows, as they know that her magic is going to burst at some point. They know that there’s something they should remember and that sorrow is dangerous, but they don’t know why. When Luna turns five, her magic erupts. Luna has no idea what she’s doing and Xan can’t teach her. Xan also grows weaker and weaker. Finally, Xan places Luna in a cocoon and returns to the old castle to reconnect to her memories of Zosimos, her mentor. After nine days of study, she comes up with a spell to encase Luna’s magic in a small grain in her brain, turning her into a compliant student of magic. It will open when Luna turns 13, at which point Xan will die. When Luna wakes up, she feels different. She goes into trances whenever she sees or hears anything to do with magic, which disturbs Xan. However, Xan tells herself that Luna will be fine and that it’s too sad and dangerous to think about what might happen in the future.
Meanwhile in the Protectorate, Antain remains haunted by seeing the madwoman’s baby taken. He goes to visit Sister Ignatia, the head of the Sisters of the Star. She allows him to visit the madwoman, whose cell is filled with paper birds and maps that say, “she is here.” Antain is terrified, and the madwoman’s birds attack him. His face scars horribly and he resigns from the Council of Elders. Though he loves his job as a carpenter, Antain feels as though he lost something important. His prestige grows, but he continually walks past the Tower and thinks of the madwoman. One day, he speaks to Sister Ignatia. He thinks that she looks oddly content until Sister Ignatia asks him to help Ethyne, a former Sister, move out of the Tower—she broke tradition and decided to leave the Sisterhood. Ethyne is the only person who looks Antain in the face without flinching, and Antain feels hopeful.
Meanwhile, the madwoman can’t remember her own name, but she doesn’t see the point in remembering anyway. She knows that she’s mad, but also that her madness has allowed her to discover magic and conjure paper out of the bends in the universe. She feels horrible for hurting Antain, but she watches him closely. Because of her magic, the madwoman now knows where the Witch lives. She draws maps leading into the forest and one day, Antain catches one.
As Luna grows, she experiences headaches and dizzy spells. Xan visits the Free Cities twice per year but she only takes Luna once per year, something that annoys Luna. Luna begins to realize that Fyrian never seems to get any older. One morning as Luna muses about Fyrian, she has a flash of a memory of a woman with black hair. Fyrian calls to her and she comes to, crying. She lies that she’s just thinking about how much she loves her family. Meanwhile, as Xan rescues another abandoned baby, Antain steps in her path. Antain is now married to Ethyne and is now filled with hope, so he suspects that he can reason with the Witch.
Later that year, Xan takes Luna to the Free Cities. Xan’s health is clearly deteriorating, but she insists that it’s not. On the trip, Luna sits with Star Children and tells them about her memories of a woman with black hair. Xan insists that they’re not true, but neither Xan nor Luna will talk about the truth. Luna begins to experiment with telling stories about a girl who can’t remember things and a grandmother who lies. One night, she and Fyrian fall asleep. When Luna wakes up with a burn on her thigh from Fyrian’s snout, she tells him to get out. Fyrian finds himself in a strange place and though he tells himself that his feet always stomp and his wings always blow leaves off of trees, he knows that this is odd. He finds a pair of tiny boots and then hears growling. He remembers Xan telling him to call for her if he needs help. Calling for Xan doesn’t help, so he calls for Luna. Fyrian finds himself back in Luna’s bed. In the morning, Luna discovers a pair of mysterious boots. They make her head hurt, so she throws them in her trunk and decides to ask Glerk about them. By the time she gets outside, she forgets about the boots.
Luna leaves Xan a note and goes up the hill to draw. A crow follows her and Luna remembers that somehow, she created the crow yesterday. Fyrian sneakily tags along and falls asleep while Luna goes into a trance and draws. She wakes up after noon and runs back home, feeling as though everything is strange. Xan is still in bed. Luna discovers that she drew a detailed map of how to get to the Protectorate. At the same time, Antain tricks the Council into letting him speak. He insists that since Ethyne is pregnant and their baby will be the next one sacrificed, he’s going to go into the forest and kill the Witch. Luna wakes up with a pounding headache and walks to a large stone. It lets her into a workshop where paper swirls around her. She’s able to read the word “magic,” and the papers tell her about how, 500 years ago, magicians enmagicked a child. That child and Zosimos tried to fight someone called the Sorrow Eater, but Zosimos died. The papers tell Luna that the child in question was Xan. In the Protectorate, Sister Ignatia assures Gherland that she’ll go into the forest to kill Antain herself—the Witch must kill him. She seems to taste Gherland’s sorrow with pleasure.
Xan wants to explain everything to Luna, but she knows she can’t. Knowing that there’s an abandoned baby waiting for her, Xan leaves Luna a note and turns herself into a swallow to fly to the Protectorate. Luna discovers the note later, and doesn’t finish reading it before tearing it up and heading into the forest with the crow, though a scrap of paper from the note crawls into her pocket. The madwoman watches Antain step into the forest and thinks that it’s too dangerous for Antain to go alone—the Witch lives in the Tower and will follow him. She turns the bars of her window into paper, and her paper birds carry her into the forest. Later that morning, Sister Ignatia assures Gherland that she’ll deal with Antain. They discuss that Antain’s journey is stirring up hope. In the swamp, Glerk and Fyrian discover the empty house and Xan’s note. It’s been repaired with Luna’s magic and it’s missing the word “magic.” Glerk decides that it’s time for Fyrian to grow up, so the two of them set out to help Xan and Luna. They sense that the volcano is rumbling ominously, like it’s about to erupt again.
The madwoman’s birds let her down in the swamp, where she discovers that her baby’s name is Luna. She turns to caring for the animals, wearing a pair of boots that she’s found. Luna is terrified, especially when a flock of paper birds descends around her—though she decides that they’re okay when they protect her from Sister Ignatia, who’s trying to find her Seven League Boots. Sister Ignatia goes to fetch them from the madwoman and congratulates herself on starting all the stories in the Protectorate about the Witch. At the same time, Antain, terrified, throws a rock at a sparrow (who is actually Xan) and breaks her wing. He carries Xan with him. Fyrian begins to grow at an alarming rate. Back in the Protectorate, Ethyne discovers that Sister Ignatia is gone and begins to plan a rebellion. Knowing that Sister Ignatia is the Witch who cultivates sorrow, she locks unsympathetic Sisters up and opens the library for everyone. Mothers who lost babies inexplicably begin to experience visions of their children growing older, and they feel hopeful for the first time.
Luna suddenly remembers being a child. She saw Xan create a scrying glass and went into a trance, and Xan insisted that she and Glerk couldn’t tell Fyrian the truth—he’d tell Luna that Xan is a witch, which would mean that Luna then wouldn’t be able to see Xan. Luna instinctively creates a scrying device and asks it to show her Xan. It shows her a sparrow. Xan tries to convince herself that Luna is safe at home as Antain starts to talk about the Day of Sacrifice and tells Xan that he’s going to kill the Witch. Xan is horrified to learn that the babies she saved were sacrifices. Antain hears someone coming up the hill. At the top of the hill, Luna, the madwoman, Xan, and Antain all converge with paper birds. Luna recognizes the madwoman as her mother and returns Xan to her human form, and Antain realizes that none of these women are the Witch he seeks. Xan apologizes for taking the abandoned babies and asks for forgiveness. As the three women stand, Sister Ignatia—the Sorrow Eater—appears. Antain is confused, but the madwoman explains that Sister Ignatia is the Witch and tells Luna that a piece of her is still human. Luna peers into Sister Ignatia’s heart, which is encased in a pearl, and sees that it’s full of sorrow. She unlocks it just as Fyrian and Glerk bound over the ridge. Fyrian now knows that Sister Ignatia is responsible for his mother’s death, so he threatens to kill her. Xan and Glerk talk him down and then, as the volcano rumbles, Fyrian says they need to get to the Protectorate to save everyone from the eruption.
In the Protectorate, Luna joins hands with Xan and the madwoman, and Xan coaches her through creating protective bubbles around everyone. In the aftermath, everyone in the Protectorate remains hopeful. They put the Elders in prison and Luna begins traveling to the Free Cities to tell the Star Children the truth about their parentage—and that their hearts can always hold more love. Luna discovers that her mother’s name is Adara. Xan and Sister Ignatia’s health deteriorates, and on the day that the first Star Children arrive in the Protectorate, Xan dies of happiness. Glerk takes her to the Bog with him. The parent explains to the child that the kind and generous Witch claimed the Protectorate, and that is the reason why they’re prosperous and healthy.
