Sarah Mitchell discovered the first thing was missing on a Tuesday. It was small—a tube of lip balm that she kept on the bathroom counter, the kind with the mint scent that she had bought at the drugstore on her way home from the hospital three weeks ago when she had brought the twins home. She reached for it one morning and found nothing where it should have been, and she thought nothing of it until she found the replacement: the same brand, the same scent, sitting on the counter in a pink cap instead of the blue one she had always used.

Claire had bought it. Sarah knew this with the same certainty with which she knew that the sky was grey and that newborns slept too much and that her husband Mark was gone every day from seven until six and came home tired and silent and fell asleep on the couch before the television was turned off.

Claire was fifty-four, Sarah's mother-in-law, a woman who had widowed two years ago when her husband—Sarah's father-in-law, a man Sarah had loved and missed more than she had expected to miss anyone—had died of a heart attack that had arrived without warning and left without permission. After the funeral, Claire had called Sarah and said: "I'm moving in. You need help."

And she had helped. Oh, how she had helped. Claire made breakfast every morning at six—oatmeal for Sarah, scrambled eggs for the twins, juice that was not from concentrate. Claire did the laundry every afternoon—sorting by colour, using the gentle cycle, folding everything with a precision that made Sarah's own folded clothes look like an insult to fabric. Claire soothed the twins at night when they cried, rocking them in the nursery chair with a rhythm that was almost hypnotic, humming a lullaby that Sarah recognised as a song her own mother used to sing.

"Everyone needs help with twins," Claire said, on the afternoon she arrived with two suitcases and a box of baby supplies that included things Sarah had not known existed: nipple thimbles, bottle warmers, cloth pads that clipped to underwear. "You're doing great, sweetheart. But you shouldn't have to do it alone."

The neighbourhood loved Claire. The women at the parenting group asked her advice. The delivery drivers smiled when they saw her on the porch with the twins. The woman who lived next door— a retired teacher named Patricia—brought over a casserole on Claire's first week and said: "You are a saint. Sarah is a lucky woman."

Sarah smiled and said thank you and went back to the nursery and sat in the chair and held one twin in each arm and wondered when her life had become a series of thank yous that belonged to someone else.

The second thing that was missing was a sock. Not just any sock—Maggie's sock, the one with the little blue stars that Claire had bought at the baby store on Main Street, the one that had ended up on Maggie's left foot at 3:00 p.m. and had vanished by 6:00 p.m., replaced by a sock that was the same colour but different in texture, rougher, cheaper, the kind of sock that makes a baby's feet sweat.

Sarah found it at bedtime, when she was taking Maggie's feet out of her pajamas and felt the rough fabric and thought: that's not the right sock. She checked the laundry basket. She checked the closet. She checked the drawer where Claire kept the twins' clothes, and there they were: twelve socks, all the same, all cheap, all arranged in pairs with military precision.

"Where did these come from?" she asked Claire, holding up one of the socks.

Claire was in the kitchen, chopping vegetables for dinner. She set down the knife and wiped her hands on a towel and smiled the smile that had become the most uncomfortable thing in Sarah's life: a smile that was warm and genuine and completely devoid of awareness.

"Oh, those? I bought some extras. The ones you had were falling apart. You know how babies grow—your feet don't stand still."

"But these are different."

"They're socks, sweetheart. They keep feet warm. That's all a sock does."

Sarah put the sock back in the drawer. She went to bed. She lay beside Mark, who was snoring, and she stared at the ceiling and she listened to the house settle around her and she thought about how much she wanted to scream and how quiet she would be if she did.

The third thing was the voice.

It happened on a Friday evening, when Sarah was on the phone with her mother, talking about nothing in particular—the weather, the twins' sleep schedule, the fact that Claire had started cooking dinner every night and that the food was really good and that Sarah felt guilty because she hadn't cooked anything in three weeks.

"Your mother sounds wonderful," her mother said. "You're so lucky to have her."

Sarah opened her mouth to say something—something that might have been a complaint or might have been a cry for help or might have been the truth, which was that she was not lucky at all, that she was trapped in a house that was being slowly, quietly, lovingly taken over by a woman who smiled too much and helped too much and loved too much in a way that felt less like love and more like occupation.

But before she could speak, she heard it. From the other room. From the nursery. Claire was humming the same lullaby, and she was speaking to the twins in a voice that Sarah recognised instantly.

It was her voice.

Not similar. Not reminiscent. Hers. The same pitch, the same cadence, the same slight upward inflection at the end of each sentence that made statements sound like questions. Sarah froze, the phone pressed to her ear, her mother's voice fading into the background like a radio tuned away.

"Claire?" she said, when the call was over and she was standing in the doorway of the nursery and Claire was sitting in the rocking chair holding Emma, who had finally fallen asleep.

"Hello, sweetheart," Claire said, still using Sarah's voice. "She's asleep. She was fussy all evening. I think she's teething."

"Who taught you to talk like that?"

Claire looked at her, and for a moment—just a moment, the kind of moment that exists between one heartbeat and the next—her face was something Sarah had never seen before. It was not a mask. It was not a performance. It was a face that had been waiting behind the smile, the helpfulness, the casseroles and the folded laundry and the perfectly arranged socks, and it was a face that was hungry.

"Nobody taught me," Claire said. "I've always sounded like this."

Sarah looked at her—really looked at her, the way you look at someone when something inside you has shifted and you can no longer trust your eyes—and she saw it: the hunger. Not sexual hunger, not material hunger. The hunger of a woman who had lost her husband and whose children had grown and left and called only on birthdays and holidays and the hunger of a woman who had realised that the only way to fill the space inside her was to become someone else's mother, someone else's wife, someone else's everything.

That night, Sarah did something she had not done since the twins were born: she stayed awake. She lay beside Mark, who was snoring, and she listened to the house. At 1:00 a.m., she heard footsteps in the hallway. At 1:15, she heard the nursery door open and close. At 1:30, she heard Claire humming.

She got out of bed and walked to the nursery doorway and stood in the dark and watched.

Claire was sitting in the rocking chair, holding one of the twins—Lily, the smaller of the two, the one who cried more and slept less—and she was singing the lullaby in Sarah's voice, and she was looking at the baby with an expression that was not love and was not hunger and was not any of the things Sarah had assumed it was. It was something more complicated: it was the expression of a woman who was practising. Practising what? Practising being Sarah. Practising being a mother. Practising being someone who belonged.

Sarah went back to bed. She lay down beside Mark and she closed her eyes and she let the tears come, quiet and silent and endless, the way tears come when you are too tired to cry and too afraid not to.

The next morning, she called a private investigator.

His name was David Ross, and he was fifty, thin, and wore a suit that had been fashionable in the nineties and had not caught up with the present. He sat at Sarah's kitchen table and drank her coffee and looked at her with eyes that had seen too many wives and not enough husbands.

"I need you to tell me everything," he said.

And she did. She told him about the lip balm and the socks and the voice. She told him about the humming at night and the way Claire looked at the twins and the way she looked at Sarah, which was the way a wolf looks at a sheep: not with malice but with the patient, methodical hunger of someone who knows that the sheep will eventually run out of places to hide.

Ross took notes. He did not interrupt. When she finished, he closed his notebook and looked at her for a long time.

"I'll need a week," he said.

A week later, he called her at the laundry mat, where she worked from three until eleven and folded other people's clothes and tried not to think about the fact that her own clothes were mostly in a drawer and that she had not bought anything new in three months and that the last thing she had bought for herself was a tube of lip balm that had been replaced by a pink-capped version without her knowledge.

"I found something," Ross said. "You're not going to like it."

"What is it?"

"Claire Montgomery had three children. All of them are adults. All of them live in different states. And none of them have spoken to her in over two years."

"Why?"

Ross paused. "That's the interesting part. I don't know. But I found something else. A voice recording. On an old answering machine she kept in her house. It's her ex-husband. Before you. He recorded it the month before he left her."

Sarah closed her eyes. "Play it."

There was a click, and then a man's voice, tired and resigned and carrying the particular kind of sadness that comes from loving someone who loves you in a way that is impossible to live with: "Sarah—well, I don't know if you'll ever hear this, but I need to say it. Claire will become anyone you need her to be. She will be the perfect wife, the perfect mother, the perfect friend. And that is exactly why I can't stay. Because there will be no Claire left. Only a reflection. And I can't live with a mirror."

The call ended. Sarah stood in the laundry mat, holding the phone, surrounded by the hum of dryers and the smell of hot cotton, and she understood everything.

That evening, she went to the nursery at midnight and found Claire in the rocking chair, holding both twins, humming in Sarah's voice, looking at them with the hungry, loving, terrifying eyes of a woman who had decided that she would be whatever it took to belong.

Sarah stood in the doorway and watched her for a long time. Then she walked into the room, picked up Lily from Claire's arms, and said: "It's my turn."

Claire looked at her. She did not argue. She did not protest. She simply stood up, smoothed her skirt, and walked out of the nursery, closing the door softly behind her.

Sarah sat in the rocking chair and held her daughter and sang the lullaby in her own voice, which was slightly off-key and slightly shaky and entirely, unmistakably hers.