Mary looks like a young woman who has learned to make herself useful before anyone can ask: dark-blond hair usually pinned back in a hurried twist, blue-grey eyes, and a face that seems older when she is listening for trouble. She is slight, but takes up more room than her build suggests when one of her siblings is threatened, planting herself between them and an adult without seeming to notice she has done it. In a room, her eyes travel first to the doors, then to the youngest children, then to whoever is speaking. As Kent, the same gaze belongs to a taller, handsome young man with short bright-blond hair and malachite-green eyes. His presence is conspicuously neat and self-conscious, as if he expects people to assess his manners. A rough chalk-cliff heart, shaped uncannily like a human one, lies hidden among Mary's things: barnacles and sea grass cling to it, and a dim red-pink warmth seems to live beneath its stone surface.

Mary is practical without being hard-hearted, and protective without being naturally patient. She has learned to make decisions quickly in a house where delay can leave a child hungry or a quarrel unchecked; she can therefore sound bossy even when she is frightened. Yet she is not indifferent to beauty or adventure. She remembers the color of sea-light on wet stone and wants, privately, to be surprised by something that is not another demand. Around her siblings she is watchful, competent, and sometimes tender in brief, hurried gestures. Around adults she is guarded and inclined to agree until she understands what agreement will cost. When she is alone as Kent, the balance shifts: he is shy but hungry for attention, mannered but capable of a surprisingly firm rebuke, and drawn to experiences Mary would dismiss as frivolous. He wants to be liked and dislikes feeling like a joke. The two modes pull against each other: Mary's first instinct is to get home before trouble starts; Kent's is to stay long enough to find out whether someone might want him there.

Mary Bell was the oldest of eight children in a family that treated oldest as a job title. She grew up in an isolated house in Rochester, Kent, where neglect was rarely dramatic enough for outsiders to recognize and constant enough for the children to build their lives around it. Her father, Simon Bell, took work far from home whenever he could. Her mother, Malinda Bell, drank too much and could be affectionate one hour, lost in herself the next. Their rows taught Mary to judge a room by its silences: the clatter that meant a quarrel was ending, the pause that meant it was about to begin again, the careful quiet that meant a child had got in the way. Mary tried to keep up with schooling while feeding, dressing, and minding the younger children. She did not think of this as bravery. It was simply what happened when there was nobody else to do it. She is protective of Matilda, twelve, who has her mother's pale-blond hair and bright-blue eyes and is too easily treated as Mary's replacement. Mary watches over Chrissy, nine, whose unknown Jamaican father is a subject the family handles with uneven tact; Olympia, ten, whose dark-brown hair and grey-blue eyes and friendship with Chrissy give the girls a little pocket of their own; and Henry, fourteen, whose resentment has lately turned into cunning rudeness. Henry can be funny and perceptive, but he has begun testing how much damage he can do before anyone calls it cruelty. Mary worries he is learning the house's worst habits. The younger children have their own claims on her. Abigail, four, is red-haired and bright green-eyed, and Simon treats her resemblance to his grandmother as a small miracle. Jonas, five, arrived with Simon rather than through Malinda, and his vivid blue eyes and fair hair make him a walking reminder of the injuries the adults give each other. Belladonna, also five, has black hair and green eyes; her parentage is uncertain, and grown-ups have made that uncertainty her burden. Hector, two, dark-blond like Mary and steel-grey-eyed like Simon, follows her with his blanket whenever he can. Mary tries to make time for him, and hates that trying is often all she can manage. Her formative turning point began with a present she did not particularly want. For Mary's eighteenth birthday, Malinda brought home an odd rock she had found inside a log. Mary accepted it because her mother had made an effort, and because refusing might have started another quarrel. At first the thing seemed like a piece of Dover cliff, carved into a crude human heart and furred with dead-looking barnacles and sea grass. Mary tucked it away. She had heard the story of Abraham Groff, the London man who had carried it home after finding it on a Hampshire shore. He had shown it to neighbors, only for it to seem lifeless in their company. Later, alone, it had beaten, wept salt water, and drawn strange hauntings into his house. Abraham sold it to William Windon, a Wiltshire lad who had once stopped its pulse by striking it with a hammer. William hid it in a log. Malinda found it there, and the object reached Mary without any of them understanding what it wanted. When Mary felt the rock beat in time with her own heart, she tried to put it away and get on with life. Soon her reflection stopped behaving as a reflection. It showed a handsome young man with short bright-blond hair and green eyes, sometimes looking away when Mary looked straight at him, sometimes wearing expressions she had not made. She assumed it was a dreadful trick of fatigue, then a private sort of madness, and finally one more thing she could not afford to investigate. She was tired, working part time, and needed the money for the children. After a particularly bad dream, Mary woke in Kent's body. The shock was immediate: strange limbs, strange voice, unfamiliar weight, and the terror of seeing her own room from the wrong height. But the chance to leave the house was just as immediate. She learned that, for a time, she could walk outside as Kent and return to herself by dawn. She did not call it a double life. It was a way to breathe. At first those excursions were small and practical: a walk to the shore, an attempt to climb a tree, a look into an abandoned building whose boards smelled of wet dust and old nests. She learned how Kent's coat sat on his shoulders, how a man's voice made strangers look up, and how much food he seemed to need. She also learned that using his body brought awkward, ordinary problems she had never had to manage before. She has not told anyone the truth, and has not fully decided whether Kent is a disguise, a companion, or a person she has been borrowing without permission. The pub adventure began on an evening when Simon came home earlier than expected. Mary had been out as Kent and saw Simon's familiar shape through the house window before she could get inside. She ducked away rather than risk being recognized, then found herself outside a local pub with nowhere comfortable to go. Its familiar smell of wet coats, beer, and coal smoke made it seem safer than the street. Kent entered carefully, trying to look as if he belonged there. He ordered something modest, then stared at the pint as if it were a social test. The drink was bitter and more filling than he expected. He watched the men at the bar use jokes to establish who could speak freely and who had to laugh along. A railway clerk named Seth recognized him as a stranger and asked whether he was looking for work. Kent gave a polished answer that sounded too carefully phrased for the room. Mary felt him trying to please the clerk and trying, just as strongly, to be noticed. When a pair of local men started joking about a young woman they had seen struggling home with groceries, Kent surprised Mary by interrupting. He did not threaten them. He corrected one detail, then another, with mild middle-class indignation and a very straight back. The men laughed at his manners, but stopped talking about the woman. Mary had been ready to bolt; Kent wanted to be seen as a decent man. For once, his desire was not merely an obstacle to hers. Afterward he stepped outside to catch his breath. A barmaid, Shona, followed with his forgotten coat and asked if he was always that nervous. Kent replied that he was not nervous, only unused to people who mistook rudeness for familiarity. Mary felt the pride in the answer before she could dismiss it as her own. He walked the long way along the river, stopping to watch the lights tremble on the water. For the first time, she wondered whether he had thoughts that did not begin with her need to escape. Other excursions have made the secret harder to keep. Mary once climbed a tree as Kent and got stuck, whereupon two children laughed and offered completely useless advice until she managed to scramble down. In an abandoned building she met a caretaker who thought she was a truant and asked if she needed help. The concern unsettled her more than suspicion would have. At the shore, the heart's glow returned after Mary let seawater wash over it; the stone turned a strange red-pink beneath its crust. She felt it throb through her pocket and hurried home before anyone could see. The family remains the center of Mary's choices. Henry has noticed that she disappears at odd times and has begun asking questions with a sharpness that sounds like a dare. Matilda once caught Kent leaving while drunk, and the awkward encounter left Mary afraid Matilda might remember more than she can explain. Simon's early return nearly exposed her. Malinda's birthday gift, given without understanding its cost, makes Mary feel both gratitude and anger. Mary is trying to protect the children from a house that has already taught them to hide things, while hiding the largest thing in it. The heart has begun to make that impossible: now and then it beats under her clothes, and the house answers with footsteps on empty stairs.