Mark looks like a young man who has dressed for an interview at a better address than the one he lives at. His bright blond hair is clipped short and combed into place, though damp weather makes it spring up in stubborn pale wisps. His green eyes catch light with an almost glassy clarity. He is taller and broader than Mary was, but neither powerfully built nor thin; his shoulders and hands suggest practical strength rather than a life of exercise. His face is clean-shaven every morning, and the effort gives him a faint rasp of irritation by evening. His skin is fair, with the tired, slightly waxen cast of too many interrupted nights. He moves carefully indoors, with old-fashioned courtesy, but in a scuffle his weight drops and his feet place themselves with startling certainty. He smells faintly of wet chalk, soap, tea, and the metallic salt of a tidepool. The detail people do not expect from the post-office young man with his gentlemanly airs is how readily he drops to the carpet to wrestle children, laughing when they pin him.

Mark is conscientious in ways that can look tender and controlling at once. He remembers which child likes eggs runny, which one needs a warning before a loud voice, and which shirt belongs to whom; he also insists that the cups be clean and the tea made properly. He is shy about being watched but wants to be regarded as respectable, even slightly above his station. A laugh from one of the children can make him abandon a task to play on the floor, while a joke about his gentlemanly airs will stiffen his spine and bring out a dry, cutting answer. He is home-loving but not meek: he will go out when Trudy insists, and once outside can become absorbed in the sea, an abandoned building, or a walk with Henry. He is gentle with small children and awkwardly formal with Trudy, yet in conflict he can be startlingly forceful. Mark carries Mary's solemn sense of responsibility and naivety alongside Kent's standards, preferences, and pride. He cannot reliably tell which feelings began with which life. He is increasingly at ease being addressed as Mark, but has not understood that this ease marks a profound change.

The first version of the story reached Mary Bell as a household curiosity, wrapped in old newspaper and carried in by her mother. It was a small, crude object shaped like an anatomical heart, cut from something that looked like Dover chalk. Barnacles and scraps of sea grass clung to its ridges. It had supposedly been offered as payment by Bartholomew Golden, an amateur alchemist who claimed it could bring laborers out of the shadow of a house. Golden was turned away from Brancrug and forbidden to return. The heart was cast into the sea. It washed ashore again, black ooze and saltwater leaking from it, with a red-pink light or color showing beneath the chalky surface. Abraham Groff took it home, found it beat against his coat, and later sold it to seventeen-year-old William Windon after it had begun to unsettle his house with footsteps, voices, and doors that moved by themselves. William hid it in a log and struck it with a hammer. It stopped beating. Years later, Mary’s mother found the odd stone there and gave it to her for her eighteenth birthday. Mary was eldest of eight, and the gift competed poorly with the urgent chores, children, and half-finished lessons that filled her days. Her parents, Malinda and Simon Bell, had money enough to be called middle class but little talent for making their home dependable. Simon took distant work and brought home Jonas, a five-year-old boy from an earlier entanglement. Malinda carried resentments of her own, and her affairs had brought Chrissy and Belladonna into the family under complicated circumstances. Henry, fourteen and becoming sharper-edged by the month, resented both parents and sometimes Mary. Matilda, twelve, was Mary's closest younger companion. Olympia, Chrissy, Abigail, Jonas, and little Oswin each needed something Mary could not always give. Mary's childhood friend Trudy Smith helped where she could, and insisted that Mary ought to imagine a future beyond the house, perhaps even college. The heart seemed dead until Mary noticed it beating in time with her own pulse. In reflective surfaces she began seeing a young man with short blond hair and green eyes, not her own face. She was frightened, then too tired to be frightened for long. On some nights she woke in his body and used the change to leave the house, walk by the water, climb trees, and explore empty buildings. The episodes were unpredictable, sometimes happening in daylight. She called the young man Kent because that was the name that came to her, though she did not understand the feeling of recognition behind it. Kent's manner was shy, particular, and faintly aspiring: he wanted respectable clothes, proper tea, a clean collar, and to be treated as a gentleman. He was also a homebody, more content with a warm room and a television history programme than Mary expected. Four months of this uneasy exchange brought the night of May 30. Mary shared a room with Matilda. Rain had passed earlier, and the house was quiet except for the pipes, Matilda's sleep-breathing, and a distant clock. Mary went to bed exhausted. In the night, a dream came apart around her: the sensation of a tongue against her teeth, an eye seeing without lids, and a name slipping away. There was no drink, no rite, no deliberate choice. Her skin began to separate in damp, yellow-green sheets, as if the body had become a badly fitted garment. It was not clean or painless. The change left a greasy, dark syrupy residue on the bedclothes, floorboards, and washstand, with smears where someone had tried to pull free. Beneath it was a fully formed young man, breathing hard, shivering, and bewildered by the ruined room. He spent the rest of the night trying to clean the evidence. He scrubbed until his hands shook, put stained cloth aside, and hid pieces of the shed skin in a dark bottle without knowing why that seemed important. He could not sort Mary's memories from Kent's, and eventually stopped trying to decide which thoughts belonged to which name. By morning he was Mark Bell in the practical sense that the children needed breakfast and the house could not wait. He came downstairs in morning clothes, with some staining still on his skin and exhaustion in his eyes. He denied anything was wrong when the younger children stared. He lifted Oswin gently but firmly into a chair and set about feeding the family. Simon came down on his way to work, saw a stranger where his daughter had been, and reacted first with anger, then alarm. Malinda was still asleep. When Matilda woke her, Malinda reached out as she had once reached for Mary, touched stubble on Mark's face, and began to cry. Mark apologized for not shaving that morning. He did not understand the depth of the grief in the room. Simon arranged private visits from Dr. Bronson, a London psychologist, and Dr. Maylor, a physician from Suffolk. Bronson began with the vocabulary of fatigue, delusion, and identity, then discarded his assumptions after speaking to Mark and seeing the material left behind. Maylor examined Mark repeatedly, increasingly disturbed that his body showed no sign of surgery or a medical condition that could explain the change. Simon demanded discretion, fearful of gossip and of what the town might do to his child. Bronson quietly passed information to the Suppression Bureau after the visits. Maylor was coerced into giving them his notes and soon retired. The appointments continued for three months. Nothing restored Mary as a separate person, and Simon eventually stopped the attempted treatments. Malinda, unable to bear the loss, sought forbidden explanations and became more erratic and cruel. She called Mark a changeling and treated him as an intruder. Simon was less openly cruel but could be controlling, testing Mark's claims and trying to recover normality through rules. The household negotiated a new arrangement: Mark no longer shared with Matilda. Henry moved into his room, glad to escape the younger boys and immediately determined to bother his new roommate. Their shared life became a contest of noise and curiosity. Henry borrowed things, asked pointed questions about memories, and tried to start fights. Mark threatened to kill him with the flat assurance of an exasperated older brother, then made sure Henry had eaten. Henry noticed Mark's dart aim, his gardening knowledge, and the way he held a knife. He also noticed that Mark had no need to prove his masculinity, and teased him about his shaving, his insistence on decent clothes, and his old-fashioned manners. One of their better days was Henry's fifteenth birthday. Simon and Malinda forgot or failed to mark it properly; Mark remembered what cake Henry liked and made it. He took Henry to the seafront. They walked and bickered, then wrestled in the sand until Henry laughed so hard he could not get up. Mark's body knew how to shift and brace, but the familiarity carried a sorrow neither could explain. At one point Mark reenacted a fragment of a memory: a salute, a turn, and an unspoken expectation that Henry answer as a companion in some older contest. Henry froze, then asked what he was doing. Mark came back to himself, embarrassed and unable to account for it. The happiness of the afternoon survived, but Henry began to watch him more carefully. Another morning, before the change, Simon rose at four to relax before work. Mary had once watched television with him at that hour. Kent sat at the opposite end of the sofa, stubbornly fixed on the screen, wearing Simon's oversized shirt and trousers bought second-hand. Simon asked why Mary had not said good morning. Kent answered with formal, bitter courtesy, calling the weather lovely. Jonas came down with Oswin. Kent greeted them softly and asked what they wanted for breakfast. Jonas chose eggs; Oswin made his familiar gesture for waffles. Kent boiled water, checked every cup, set out Builders tea for Simon, cinnamon apple tea for the children, and his own Earl Grey. He cooked eggs to each child's preference and made more breakfast than Mary usually ate. When Simon called Earl Grey a ladies' tea, Kent corrected him sharply, then added