I.

The seventh night of the star's light, Edmund Windsor woke to a world that had ended while he slept and did not yet know it.

The rose-nebula hung in the sky above Windsor Manor, taking up a fifth of the heavens, bright enough to cast shadows on the manicured lawns. It had been burning for seven nights and seven days, a second sun the color of crushed roses and cold fire. The servants called it the Dead Star. Edmund called it nothing at all, because naming it felt like admitting that something had happened, and he was not yet ready to admit anything.

He walked the corridors of the manor alone. Mrs. Pemberton's door was ajar; he could see her bed made, the morning tea still full on her side table, cold now, a thin skin formed across the surface. The stables were empty. The gamekeeper's cottage was dark. Every adult door in the house was open, and behind every door was silence.

He went to the library and opened the atlas. He spread the maps of Britain across the great oak table and traced his finger along the Thames, from Windsor upstream to London, downstream to the sea. Somewhere out there, across that thin blue line of water, was America. Somewhere in that America was a family name he had never heard spoken at Windsor Manor—Croft. His father had mentioned it once, in a context that had not seemed important at the time.

II.

The first week was the strangest. Children appeared from every direction, drawn to Windsor Manor the way moths are drawn to the rose-nebula itself—compelled by something they could not name. They came from London, from Berkshire, from villages whose names Edmund could not pronounce. Some were orphaned. Some were simply lost, wandering the English countryside in pajamas and bare feet, their eyes wide with a terror they did not yet understand.

Arthur Fairfax arrived on the third day. He was a small boy with thick spectacles and hair that refused to lie flat, and he carried a satchel full of books his astronomer father had been reading when the Dead Star came. Arthur spoke in a voice that was too measured for a thirteen-year-old, which was probably because it was. His parents had always spoken to him as if he were a colleague at Cambridge, which, in a sense, he was.

"It's not a star," Arthur said, spreading his father's notes across the library table. "It's a supernova. A helium flash in a star eight light-years away. The radiation alone should have killed us all, but—" He stopped, looked up at Edmund, and his composure cracked for the fraction of a second. "But it hasn't. It's killing something else. Something specific. Something that only affects—"

"People over thirteen?" Edmund said.

Arthur nodded. He was already thinking about the implications. Edmund could see it in his eyes, behind the thick glass of his spectacles: the great machine of his mind turning, calculating, building models.

By the fifth day, five districts of London had sent delegations to Windsor Manor. They gathered in the abandoned air-raid shelter beneath the Thames embankment—a place Edmund had discovered by accident, pressing through ivy that had swallowed an iron door whole. The shelter was damp and smelled of old concrete and old fear, but it was dry, and it was defensible, and it was a beginning.

"We need rules," said Clara Hastings. She was an orphan who had been brought to Windsor Manor when she was six, and she had spent seven years learning the subtle mathematics of survival in a house full of adults who loved her but did not understand her. She stood at the head of the crude wooden table Arthur had dragged into place and looked at the five delegates with an expression that was older than any child had a right to be.

"We need a constitution," Arthur said.

"We need food," said a boy from Southwark.

"We need to know if it's going to keep killing people," said another.

Clara looked at each of them in turn. "Both of those things are true," she said. "But if we don't have rules first, we'll kill each other before the star has a chance to finish us."

III.

The ice fortress was built in the North Sea, three weeks later, by children who had never seen the ocean until the week before.

It was not a fortress in any conventional sense. It was a collection of abandoned fishing vessels, rusted iron plates salvaged from a decommissioned shipyard, and enough timber to build a small cathedral. The children worked on it in the freezing北海 water, their hands bleeding, their teeth chattering, their faces turned upward toward the rose-nebula which still burned in the daytime sky like a wound that would not close.

Edmund stood on the shore and watched them work, wrapped in his father's greatcoat, which was far too large for him but warm. He felt the weight of the coat on his shoulders and understood, for the first time, what it meant to carry something that belonged to a dead man.

The first American ship appeared on the horizon at dawn. It was a steamer, painted white, flying a flag Edmund did not recognize. As it drew closer, he could see the children on deck—dozens of them, perhaps fifty, dressed in coats and hats that cost more than Windsor Manor had cost his grandfather. At the center of the deck stood a girl who looked to be about fourteen, with dark hair and a posture that could only be described as imperial.

Isabella Croft.

She came ashore alone, wearing a riding habit and carrying a leather portfolio. She walked up to Edmund, stopped at attention, and said in a voice that was perfectly modulated and entirely thirteen years old: "I represent the interests of the Croft family and the citizens of New York. I have come to propose an exchange."

Edmund looked at her carefully. He was thirteen years old, but he had spent his entire life learning to read people—the subtle hierarchies of Windsor Manor, the unspoken rules of servants and visitors and seasonal guests. He saw in Isabella Croft a mind that worked like a chessboard.

"An exchange of what?" he asked.

"Everything," she said. "My people will leave England. All of us. We will go to America, to the land my grandfather helped build with steel and railroads and money. And your people will come to America too, but on different terms. You will bring your culture, your traditions, your understanding of this island. We will bring our wealth, our technology, our energy. We will exchange civilizations, Mr. Windsor. And in doing so, we will save both of them."

Edmund looked past her, across the Thames, toward the rose-nebula burning in the morning light. He thought of Arthur's notes, of Clara's constitution, of the ice fortress rising from the北海 like a child's dream of a castle.

"How many times will it take?" Edmund asked.

Isabella blinked. "How many times will what take?"

"How many exchanges?" Edmund said. "How many times do we have to trade places before we figure out who we are?"

Isabella Croft looked at him for a long moment. Then she smiled, and the smile was both thirteen years old and a hundred. "Seven," she said. "I think it will take seven exchanges."

And they stood there on the Thames shore, two children under a rose-colored sky, and began the work of ending the world.

IV.

Years later, Isabella would sit on the veranda of the new Windsor Manor—which was not in England at all but in a valley in Montana, built by British children who had crossed an ocean and never looked back—and she would tell her granddaughter about the meridian.

"The meridian," she would say, pointing toward the eastern horizon where the first stars appeared in the twilight, "was not a line on a map. It was a line in the sky. That rose star. It was beautiful, wasn't it?"

"Yes, Grandma," the girl would say, and she would look east, and she would wonder if the star was still there, still burning, still watching.

And Isabella would think: they never figured it out, did they? The seven exchanges. The ice fortress. The constitution written in the dark beneath the Thames. The children who became something else and never knew when the change had happened.

She would look at her granddaughter's face—thirteen years old, wide-eyed, full of a future that none of them had planned for—and she would feel something that was not quite happiness and not quite grief, but something that lived in the narrow space between them, like a meridian line drawn across the heart of the world.

The star was still there. She could see it sometimes, if she looked carefully enough, in the pale blue dawn, a faint pink smudge against the light. Burning. Watching. Waiting for the next exchange.