During the later part of the war, the government issued a pamphlet on how to recognize changelings. Violet read it (a green tinge of the features; propensity to cruelty) and laughed. The real signs had been far more pervasive, far less clear. Sometimes she thought she had only realized she wasn’t human when she was fourteen. Sometimes she thought she had always known.

The external, everyday things were always easy. She liked French, hated mathematics, and complained about her governess. She sailed toy boats with Thomas, bridled when he was patronizing, and once threw her oatmeal at him. She cried when a picnic was rained out, when she fell and scraped her knee, and when her governess disciplined her.

Other things were harder. None were inexplicable.

She did well at her piano lessons, but all music was only a string of notes to her. She supposed this was what Papa meant when he talked about his old tutor who was tone-deaf.

There were nights she climbed out her window into the garden because she could not bear to be inside another moment, and she could never go back in till she had danced herself breathless. Mama shook her head and said that Aunt Maisie, too, had been a tomboy.

She didn’t cry when her kitten or Grandmama died. She poked the kitten and she stood respectfully at the funeral, but both times she was curious, then bored. Thomas had once read her a poem that said hopeless grief was passionless.

She knew she was different. She knew everyone else felt the same.

Then the summer she was fourteen, they stayed with Papa’s family in the countryside. It was the last summer before the first rumours of the war began; a summer of sunshine and slight, warm breezes, croquet and boating and tea on the lawn. Thomas was back from his first year at Oxford, and he spent more time with her than he had in years. They went horseback-riding and translated Latin together; he told her stories about life at Balliol, and she showed him how much her drawing had improved.

But one bright summer evening, everyone was busy and Violet took her sketchbook to the river alone. She settled in her favourite spot, on a rock half-hidden by drooping willow branches, and began to sketch the leaves. At first there was no sound except the trickle of the river and the scratch of her pencil on paper, but after a while she realized that the river-noise had a rhythm and a tune and meaning, as no song ever had.

Beyond the willow-branches, the river was silver with the sunset light. In the middle, her bare feet just brushing the surface, stood a tall woman with pale hair and pale eyes. She wore a white dress with lace at the neck and wrists, as one might wear to a tea-party; but streaming out from her shoulders were great, half-transparent butterfly wings that shimmered blue and cream and pink and deep, royal purple as they drifted open and shut.

Violet stared, reduced to a racing heart and dizzy head and not a scrap of thought.

The woman smiled at her and said, “My child.”

Her heart still beat fast, but the fear was gone as she watched the woman step across the water to her. When the woman’s toes touched the pebbles on the shore, Violet said, “You’re my mother.”

“Yes,” said the woman. Even standing on the drab shore, light clung to her hands and the folds of her skirt. “And you have been in exile, but I shall show you how to come home.”

She cupped Violet’s chin with cool fingers and tilted her head up; Violet closed her eyes and stood, sketchbook falling to the ground. She supposed this moment should be difficult, that she should be thinking of her family and home, but it was not hard at all. Not at all.

Cold fingers brushed her back, and her shoulders loosened. She knew that her wings were blossoming; she could feel their colours in her throat. When she opened her eyes, the world was different: shadows were longer but filled with hidden glimmers, and the house was hazed with mist but she could see leaves on a tree half a mile away.

“Come across the water,” said her faery mother.

Violet returned to the house as the grandfather clock in the parlour chimed seven; in the human world, she had not been gone over a quarter of an hour. Her wings were hidden and her hair, which had flown free and tangled in Faery, was neatly braided. She felt as if she had been opened up and re-made, then sewn back together and wrapped in her normal clothes.

Thomas leaned out of the study and smiled across an invisible infinity. “I say, Violet—I’m reading a bit of Virgil; would you like to help?”

It had been easy to leave, and it was easy to follow him into the study and laugh at how many declensions he had forgotten. That evening felt almost real, just as all the evenings before had felt almost false.

For the first few years, she only passed information, while the reports of faery incursions began to grow. Then—when they went to London for Violet’s introduction into society—three things happened. The faeries turned the Prime Minister’s fingers into twigs and his eyes into acorns. Papa died. And Thomas discovered what she was.

There was a curfew after the attack on the Prime Minister, but it made no difference to Violet’s family. They were all staying at home anyway, listening to Papa’s breath rattle and guessing how much longer he would last. Violet had wondered a few times if she would need to hurt her foster-family, but in the end it was a purely human sickness that killed him. All she had to do was stand by and give Mama damp cloths to wipe his forehead.

There was a song called “Swans at Sunset” that to her was just a string of notes with a sentimental name; but Papa loved it, and she played it every evening, pounding the keys so the sound would carry to the sick-room. As she worked through the measures, she remembered Papa’s throaty laugh, and teaching him to play patty-cakes, and handling his rock collection as he told her about where he had found the pieces.

The man she remembered didn’t seem to have much to do with the withered body upstairs, and neither of them had anything to do with her. She spent hours listening to the clock and hoping he would die before the next chime so the waiting would be over. But she still played “Swans at Sunset”; maybe she had picked up the habit of love, if not the substance.

When Mama finally came downstairs and told her it was over, Violet said, “Oh,” and went to pay her respects, bubbling inside with happiness because she could spend the rest of the evening undisturbed. Then she curled up in her room and finished The Moonstone.

Thomas discovered her the day after the funeral. They had gone back to the country to bury Papa in the family plot, and when the sun touched the horizon she slipped down to the river to make her report. This time they let her visit Faery, and she came back with her wings still unfurled. She stretched, enjoying the feel of mortal sunlight on gossamer membranes—and heard the click of a pistol.

She turned, and saw Thomas holding the gun steady, his lips pressed together.

“Where is my sister?” he whispered, biting off each word.

The last remaining bits of the old Violet, who had babbled proudly to everyone about her older brother and always put on her best dress when he came home, shredded and blew away on the evening breeze.

“I don’t know,” she said.

“When?”

“We were switched as babies,” she said. “It wasn’t your fault you didn’t notice.”

The change in Thomas was a little like the change in Papa as he withered on the sickbed. Suddenly there was a new Thomas, wide-eyed and desperate, who had nothing to do with the brother who had slapped her on the back and taught her Latin. And as with Papa, she knew he was gone and felt no regret, for she had changed equally. He had loved her, and now hated her. She had been his sister, and now was not. There was no one left to be sorry.

“Are you going to kill me?” she asked. She was almost certain she could confuse him with a glamour and escape.

Thomas drew a shaky breath. “No.” He lowered the gun. “I’m finding her. No matter what it takes, I swear I’ll get her back. Then maybe I’ll come for you.”

Violet nodded and turned back to the river. She knew she didn’t love him because it didn’t hurt to leave.

“Did you always know?” he demanded.

She knew he was asking if the sister he loved had ever existed. He was a human and he wanted to know what was in her heart.

“Yes,” she said, because she was a faery and had no heart. Intentions mattered nothing; and her nature was that she had always been a traitor.

Afterwards, people often asked her why she had worked for the faeries even though she had been raised by humans. When she told them how it felt to stand in Faery after the grimy dream of the human world, and that she could not stay there until her task was done, they took that as reason enough.

But for the faeries there was no such thing as reason. There was only theirs and mine, us and them. She knew at last why she had never cared for her family: they were not hers. She knew why she would work for the faeries: she was theirs.

To the extent that she had been tainted by humans, and therefore needed a reason, she thought that she worked for them because they gave her an answer.

Miss Stanton’s School for Young Ladies in Yorkshire was cold and damp, its paint peeling on the walls. The girls stood in rows for inspection every morning, their hair parted down the middle and pulled into painfully tight braids. By the end of the day, Violet’s gums hurt and her shoulder blades ached with the need to let her wings free.

But the school sat on the edge of the moors that the faeries were raising to life, and they needed someone to make sure no one who guessed the truth came away again. So a handful of leaves and a mouthful of glamour became a letter from the school’s patron that made the headmistress, Miss Stanton, not only hire Violet but keep her when she was in trouble.

“Miss Thornton, I cannot permit you to give your students such things to read.”

Violet kept her voice submissive. “If I am to teach them French, ma’am, I must give them something.”

“We have several French Bibles.” Miss Stanton drew her thin eyebrows together. “I think they should provide you with sufficient material.”

Violet strongly suspected that Miss Stanton had never heard of Candide before yesterday, and she was torn between wanting to laugh and wanting to turn Miss Stanton’s knobby fingers into twigs.

“I understand,” she said.

“Indeed?” Miss Stanton let out a little huff. “I realize, Miss Thornton, that you find it amusing to treat your students in this fashion. But I have a duty to safeguard the souls of those in this school—including yours.”

And I must guard my people, who have no souls, thought Violet, but she did not say it aloud; making any more trouble could endanger her mission. So she looked at the table and nodded, thinking that when she was done here, she would drive Miss Stanton mad to run naked over the moors.

That night, Agnes Thompson was missing at curfew. In twenty minutes, Miss Stanton turned the whole school upside down; then she started search parties. Violet tried to make them wait for morning, but Miss Stanton would have none of it. So she walked into the damp spring night with one of the porters, and when he turned his back she took a leaf from her pocket and blew it onto the wind, thinking, They are coming.

The wind shifted, and she knew that the moor, already more than half-alive, had heard her. Violet smiled and hummed a scrap of faery song. Mist began to rise out of the ground.

The porter stumped along, lantern held high, bleating, “Miss Thompson! Miss Thompson!” Then he paused, staring at the base of a bush. “What’s this, then?”

Violet peered around his shoulder and saw a cluster of imp-eggs, glowing blue in the darkness. She thought to the wind, Now.

Jewel-bright butterflies bubbled out of the ground, glowing ruby and amber and lapis lazuli, and they rushed up through the porter as if he were mist. He collapsed with a soft, choked noise, his chest shredded and bleeding where they had touched him. The butterflies corkscrewed up into the sky, then descended in a rush to twirl about Violet, who laughed at the crazy scraps of colour.

“Go find the others,” she said, and they streamed away into the darkness. The mist had thickened into fog. Violet tilted her head and let her wings unfurl. Every now and then she heard shouts in the distance, as the butterflies found the intruders one by one. At this rate nearly all the school staff would be dead by morning; perhaps the girls could be lead away to serve the Faery Queen. They were all young enough.

“There you are.” Violet spun around to see Miss Stanton emerging from the fog. “We must get back to the school at once; something’s not right here. I’ve sent the others back already.”

The air trembled and told Violet that four had made it back alive. That was more than she would have believed Miss Stanton capable of saving, and Violet looked at her with a measure of respect.

Miss Stanton stared back, beady eyes gone wide, and Violet realized she was watching her wings open and close.

Violet almost laughed. “You see why I’m not worried about my soul.”

Colour caught at the edge of her vision, and she turned to see the butterflies spinning lazily towards her.

“They’ll kill you.” Miss Stanton’s voice was high and reedy.

Violet ignored her and held out a hand, waiting for them to cluster on her palm—

“You fool,” said Miss Stanton, and stepped in front of her.

The butterflies sank into her and then gushed up from the back of her head. Violet did not feel them as they settled on her hands and her hair, did not listen to their laughter in her mind. She was staring at the ugly woman crumpled on the ground, her mind repeating a single concussed thought: She died for me.

Miss Stanton had not loved her, had not needed her, had known she was not human. Had still died for her.

Violet dropped to her knees in the grass. She had thought she understood humans. When they talked of love and altruism, they meant protecting mine. When they talked of bravery and moral choices, they meant destroying yours.

Despite what humans thought, faeries did know sacrifice; every day of the war they laid down their lives for their Queen and their kin. But not for their enemies. Not for strangers. They would never die for someone who had betrayed them, simply because she needed help.

For the first time in her life, Violet wanted to know why.

And for that the faeries had no answer.

There was no point to dying for someone who had tried to hurt you, and no point at all to dying for someone who had never been in danger. Violet knew it as surely as she felt her own heartbeat, and she could feel the butterflies laughing at the blood dribbling out between Miss Stanton’s wrinkled lips. But she knew, also, that something in that death had been needful and right.

Maybe it didn’t matter who was us and who was them, whether she was human or faery, and maybe it didn’t matter whether she loved anyone or not. Maybe there was something still she had to do.

She took a train to London, walked into the War Office, and said, “I am a changeling. I want to defect.”

“Nasty little fight, but we killed the buggers.” Major Harris’s voice echoed slightly in the tunnel. Then he saw Violet. He stiffened, mouth working uncomfortably, but didn’t apologize for his language.

The soldiers were all like that: they could not treat her as a man, did not want to treat her as a woman. Violet only smiled and unfurled her wings, laughing inside as he turned away uncomfortably.

“Right this way, miss,” said Colonel Weston. He was afraid of her, like the rest of them; Violet could taste his nightmares sometimes. But he still pretended she was a lady, and so Violet had tracked down his wife and laid protections on her. She appreciated anyone who, like her, pretended to be kind.

Violet followed the Colonel down the tunnel, trying not to gag. They had gassed the mound with sulphur to weaken its enchantments, then thrown jam-tin grenades full of iron filings to destroy them, and enough iron and sulphur still hung in the air to make her vision swim.

“We’ll have to hurry. We think they might have called for reinforcements.” He gave her a sidelong glance.

“I can’t tell if there are any nearby,” said Violet. “The fumes are still too strong. They’d likely come through Faery, anyway.”

Through a doorway she glimpsed the great white anchor stone. It was split clean across, and her wings ached in sympathetic pain: there would be no more easy passage to Faery through this mound. But come twilight, the faeries would be able to use any stream or forked branch to cross into the mortal world.

Colonel Weston shrugged. “Well, there’s not much point to holding the mound anyway. We’re just lucky they didn’t kill the prisoners this time.”

They were deep into the mound now, and the air had become clear again. Then Colonel Weston stepped through a doorway into the round prison room and raised his lanterns, and Violet could see the shadowed lumps of the prisoners twitching. He looked at her, and this time there was no fear in his eyes, only hope and desperate expectation. He wept for the prisoners as she could not, and he looked to her for hope; and that was another reason why she liked him.

She knew that humans needed signs, so Violet laid her hand against the wall. This deep in the mound, there were still some scraps of power; at a touch from her mind, great glowing white flowers bloomed across the domed ceiling, filling the room with light. Under the faery lights she went to each of the prisoners in turn. They had been changed inside the faery mound, and being still inside it, could be changed back: twigs to fingers, acorns to eyes, thistles to tongues, goat’s head to human. Each one healed under her hands, and maybe this was what mattered. Maybe it was.

At the height of the war, Violet was with Colonel Weston in Devon. All of Cornwall had fallen, as had Lancashire and Yorkshire, and great swathes of Wales and Scotland. Will o’ the wisps floated up the Thames to London, hobs and brownies roamed the streets at night, and the new King had gone into hiding. Everyone was terrified of possible treachery, and even the small towns were papered over with propaganda posters urging people not to submit.

The parade of pictures and slogans was endless. A square-jawed young soldier grasped a rifle, while beside him a young woman held aloft a flag: “BRITONS NEVER WILL BE FAERY SLAVES.” A green-faced, slant-eyed faery leered at screaming little girls: “THE FACE OF THE ENEMY.” The smoking ruins of a cottage, with bodies lying across the doorstep: “The village of Wattingham surrendered, and the faeries SLAUGHTERED every man, woman, and child. MEN OF BRITAIN, NEVER AGAIN!” A neatly-groomed housewife smiled over a bonfire: “Every flower is faery food. BURN YOUR GARDEN!” Two little girls knelt at their father’s knee: “Daddy, what are YOU doing to save us from the faeries?”

And everywhere, with a hundred different illustrations: “ONE TRAITOR CAN DOOM A CITY. REPORT SUSPICIOUS BEHAVIOUR AT ONCE.”

Even so, every day they heard of another town or village that had accepted faery rule. Violet could not be sorry that the mining had stopped in South Wales, or that the factories in Manchester no longer belched poison into the air; but the same people who smashed the machinery and broke iron gates were the ones who delivered children to the faery mounds and cut throats at the cromlechs.

The news from abroad was even worse. The Erlkönig rode freely across the Sudetenland; in Norway, King Haakon tried without success to stamp out the álfablót, while in Sweden the älvdanser met every night in Stockholm; witte frauen roamed the streets of Vienna. In France, the dames blanches sent matagots across the countryside and raised the Tarasque to attack Paris. The Hapsburg emperor and all his family were driven mad or cursed with donkey’s heads, and the Pope had gone into hiding.

They no longer got any news from Ireland at all: after the Irish had cast off British rule, they had broken into a civil war over whether or not to ally with the Sidhe. No one knew which side was winning; sometimes after dinner, the soldiers liked to discuss strategies for invading Ireland, but privately Colonel Weston admitted to Violet that the generals were drawing up plans for when Ireland invaded them.

But then the tide of the war began to shift. The Germans sent over some of the new flamethrowers, and though they were clumsy, the fire was elemental enough that faery magic could do little against it. Then they got the new Vickers guns, which could fire round after round of alternating silver-and-iron bullets, and better grenades. For the first time in over a year, the army went on the offensive. The official name for the policy was “sectional cleansing,” but most people called it “scorched earth”: working outwards from London, they killed every faery they could find, torched every moor and forest the faeries had awakened, and surrounded, gassed, and blasted every mound.

On the day the Yorkshire Dales burned, Violet finally collapsed. She crouched outside the whole day, rocking and keening as the ash fell on her hair and the moorland’s agony ripped through her mind. They had to hold her down and give her a double dose of laudanum before she would quiet. Since there was no way she could be discharged, her commanding officer promptly sent her south to join Colonel Weston’s unit in Devon, where they were trying to hold the Cornish border until the main campaign arrived.

Violet crouched in the ditch, Colonel Weston slumped beside her. A night raid on a faery mound had gone disastrously wrong and they were separated from the rest of the unit, the Colonel badly wounded by elf-shot. In the distance, she could hear the crack of guns and scattered booms from the men who still had grenades; the cold air pulsed with the silent faery-horns. Answering song bubbled up in Violet’s throat, and she clenched her teeth to keep it back. She doubted any of the humans abroad tonight would see morning, but she owed it to the Colonel to try.

Cautiously, she stood and cupped her hands towards the sky, then leaned back, her wings blooming. The air cradled her, caressed her fingertips, and in its eddies she could feel the men’s lives winking out, one by one, like vanishing fireflies.

“Are their deaths not beautiful?”

Violet opened her eyes and saw her faery mother at the edge of the clearing. She wore again the white tea dress, her pale hair floating free on the wind, her wings glistening.

“You know what you are.” Her voice thrummed with power. “Why do you resist?” Moonlight caught and clotted in her hair, and a wave of song crashed through Violet’s mind. She fell to her knees. The whole night had been a trap to make her use her powers, opening herself so she could be turned back to them against her will.

“Come back, my child, thread of my gossamer.” Her mother knelt before her and cupped her chin. “Come back across the water to your kin, and drink the sunlight on the fields of Faery,” she whispered, and Violet’s wings ached with longing.

Behind her, Colonel Weston made a wet, choked noise. Violet clenched her teeth. “No.”

“He is dying,” said her mother. “Unless you heal him.”

If she used her powers once, even just to heal, she knew that the last dams of her mind would break. Violet wondered if her mother had planned this part too.

“Either way he is betrayed. One way he lives.”

“I have orders. So does he.”

Her mother’s voice was thick with disgust. “How could you betray us for these gasping things of smog and dust?”

Violet thought of Miss Stanton and Thomas, of the army chaplain’s long sermons and the ragged, pointless songs of the soldiers. She could guess what any one of them would say, but they were all human replies, and here in the moonlight she could not pretend they were hers.

Instead she lifted her eyes and said, “Because their deaths are beautiful.”

Her mother’s fingers dug into her chin. “Do you think they’ll ever love you?”

“Do you think I’m human enough to care?”

There was a rustle at the edge of the clearing: a soldier stumbled through the trees. Her mother turned, and Violet flung herself to the side; as briars sprouted from the soldier’s eyes, her hands found Colonel Weston’s revolver and she brought it up.

Her mother went still. “You are not of them.”

Violet thought of the men she had cursed for the faeries, and the men dying tonight; the woods destroyed by factory pollution, and the fields screaming as they burnt in the war. “I know,” she said, and pulled the trigger.

“You will never come home,” her faery mother snarled as she died, and Violet was not sure that she cared.

After the war ended, nobody was sure what to do with Violet. Her mother had died of a fever, and none of her more distant relatives would take her, but the army would not let her go free.

Eventually Colonel Weston offered to take care of her, and since he had commanded her during the war, he was allowed to adopt her as his ward. He took Violet back to his country estate; after a while, Mrs. Weston stopped looking at her with fear, and even sat with her in the evenings to sew.

Violet embroidered roses on a pillow, sketched the parish church, and practiced playing songs that still made no sense to her. She took care that nobody saw her dancing in the woods, and when the longing for Faery was so bad that she could only curl up in her bed and shiver, she said she had a headache.

She still couldn’t weep for Mama or Papa, but she could remember them both with the hallucinatory clarity of faery memory, and she thought that if she could not be a daughter, at least she was a faithful monument. One evening she finally played “Swans at Sunset” for the first time since Papa died. It was still just a string of notes, and she wondered why she had waited so long.

Then one morning, as she sat practicing at the piano in the parlour, the maid came to her and said, “There’s a man here to see you, miss. He says—”

“Show him in,” said Violet, because she could feel him, she could tell.

A moment later Thomas stepped into the room. She did not turn around but continued playing “Swans at Sunset.”

“I heard about you sometimes, during the war,” she said.

His voice was lower than she remembered. “Sometimes I heard about you. Mostly from the faeries.”

He’d never joined the army, but had gone straight from nobody to legend: the half-mad son of a peer who charged into faery mounds alone and came out again alive. The man who’d sworn to walk into the Faery Court itself to find his sister.

“Did you find her?” she asked.

“Yes. She didn’t remember being human.”

“I didn’t remember being faery.” Her fingers moved smoothly over the keys.

“She made her choice. I made mine. What are you doing now?”

“Colonel Weston has been kind enough to adopt me as his ward.”

Thomas sighed, then stepped to the side of the piano, where she could see him. There was a scar across his cheek and lines around his eyes.

“I’ve just settled the estate,” he said. “Father left me the house in town, and Uncle Harold left me the old house in the country.” His fingers drummed against the wood of the piano. “If you want... you could come stay with me.”

“You know I’m no family of yours.”

“I think you’re the closest I have left.”

She stopped playing. Thomas watched her steadily, waiting for her answer.

It would not be true to say she had ever missed him, but she was now fairly sure that she had, all this time, been waiting for him.

“And what are you planning to do?” she asked.

He shrugged. “The war’s over, but they still need men with experience of Faery. Here, or... there’s talk of posts in the Orient. I might be gone sometimes.”

She could never exactly care for him, any more than he could ever make her kin. But she thought that she would like to try.

“We could study Chinese together,” she said.