There is something in the house. I've known that since the day we came here. I think it comes from outside—from the woods that press against the fence a little closer each year. When we bought the house, Simon said the trees were a blessing that most people would envy. One step beyond the garden, through the little gate and there is a natural playground that changes with every season. Bluebells in springtime, cool walks beneath the canopy in summer, toadstools and crunchy leaves in autumn, muffled silence of snow in winter... what more  could children want?

"Children,' I said, "but, but..."

Simon waited, mouth in a mock gape, eyebrows raised. "Butbutbut what? Oh come on Lada, it's beautiful! What could be wrong? It's the perfect time, isn't it?"

Yes. What could be wrong? Why not? Why not raise a family in a house in the country?

But there is more to this story.

I liked the woods in the long ago, though I always stayed close to my parents. The fear started with the arrival of my Aunt, one summer when I was eight. People said there was something wrong with Aunt Raisa. She wasn't allowed to be alone for too long, and moved between the houses of her relatives like a sickness. She was to remain with us until the autumn, and I knew my mother was opposed to it because I'd heard her arguing with my father—flinging her long dark tipped fingers about and hissing at him. I remember his red mouth, framed by his neatly trimmed beard, pursing around his bared teeth.

"It is family," he said. "My turn. It is duty. Obligation."

They circled each other like dogs for three days, heads down, petty retaliations played out across the dinner table, but it was no good. Aunt Raisa came with two suitcases and set up in the end bedroom where she stayed for three days, coming out only at night.

"Such a small, dark child,' she said when we were first introduced. "Such a little babushka."

I didn't know what that meant then, but I knew it wasn't meant kindly. She sat looking at me, muttering in Russian as though casting a dark spell, but later, at bedtime, she came to my room and sat down, making barely a dent in the mattress. Instead of reading my chosen book, she told me a story of her own, about the land and the people we came from.

"In the forest outside Luchegorsk," she said, as though I knew the place well, instead of being born in Lancashire. "Deep in the trees, walks the Leshiy. He has long smooth hair and the eyes of an animal. He will lead you astray if you let him...."

The smell of trees came back to me. It seemed wedged beneath my fingernails and smudged into my skin. The dry, dusty roughness like the back of a rhinoceros.

"Once there was a girl," she said, "a girl like you, who was forbidden to enter the forest, but one day she did. Indeed, she could not help herself, for there comes a time when every girl must enter the dark forest..."

The corners of the room crept closer, stealing the warmth from my bed so that goose pimples washed over my skin.

"What happened to her, tetya?" I whispered. "Did the girl die?"

She shook her head. "Only in the way that every girl must die so that the woman can be born. A girl enters the forest and a woman comes out. Her face has changed, her eyes are secret. Her parents mourn."

"She could just stay out of the wood. I will."

"It would be best never to go into the trees alone," she whispered, looking less and less like herself as the minutes passed and more and more like one of the wooden dolls that sat on the shelf behind my head, "but something always takes us there. Often a man. Even as your Papa likes to take you—into the trees not knowing that you are afraid. If you should find yourself there one day—if you should get lost and darkness comes down then you must get off the path where Leshiy likes to walk. Cover yourself with leaves like a hedgehog and wait for the light..."

I wanted her to stop, but she went on, until my mother came, putting her head round the crack in the door. "It's time for Lada to sleep," she said, the smile hovering around her lips in a tipsy balancing act. Aunt Raisa nodded and stood up, walking sideways to the door, her eyes fixed on mine. "I will finish another time," she said quietly, "but remember. You must be a strong woman, for the ways of the world are hard."

She never did finish the story. It was my mother who read to me after that, every night until Aunt Raisa moved on.

That's where this story really begins.

Things changed after that. The woods, and me. Which of the two it really was, I cannot say, but our family walks were things to be dreaded. There was something there in the dim green, behind the trees, that moved as I did, round and round the trunks so I could never quite see it—and nor could anyone else. They called me timid because I would hide behind people—men especially, because they were taller, and wore longer coats when we were out walking.

"Please, please, Papa, Mama," I would say, pointing with a starfish finger. "There, there, look—in the branches..."

"I see nothing Lada," they would say, and Papa would whoosh me in the air and charge into the trees, mistaking my screams for excitement instead of terror. He didn't seem to see, as I did, the dark, woody figure that reached for us as he ran, round and round the trunks until he slipped and we fell into the leaves and the smell of rot and damp struck our faces.

I stayed away from the woods when I grew older. I went to school instead, far away from my family and afterwards took a job in a museum, working in the quiet and dark of the vaults, with the strange things they kept there, too many to display. Things were normal then, although sometimes I had trouble with the exhibits in the vault. Sometimes things moved or were broken. Sometimes I lost part of a day, or would cry and be unable to go in, remaining beneath my quilt, trembling though I didn't know why. It was a relief to meet Simon, who worked in the city. it was a relief to be married, to be able to hide behind a man once more and not have to speak.

But I have to hurry. This is not important in the telling of this story.

I became a strong woman in my own way, like Raisa had told me to; even in the house by the woods. A strong woman for my children.

I have two now. Three in fact, but the one upstairs is not my own. It looks like my newborn son, I admit that, but that is the trick of it. It happened gradually and I saw it coming. Things began to unravel like a skein of yarn that the cat has got hold of.

I told Simon, but he was busy and didn't believe me. Then I told him again: "Something has happened. The thing that lives in the woods has stolen the baby. The child upstairs is not Maxim."

Simon stood for a long time looking at me and then he ran upstairs, his footsteps thudding into and out of the children's rooms. I imagined him picking them up, exploring them with his long fingers—the eggshell bones of their faces, the line of their pearly teeth, the slippery strands of hair. He didn't believe me. When he came down he was breathless and looked at me as though he was afraid. He said I should see someone. He said he would arrange it and until then, perhaps my mother should come. He promised to call her on his way to the airport in the morning, and the moment he got back from Milan, we would talk. He didn't want to go at all, but his work is what brings the money. The money that pays for the house in the woods and all the nice things.

As soon as he'd gone and the light of morning stole through the windows, I knew I couldn't wait that long. This is the second day since. My mother has not come, and something is wrong in the house. I'm just not certain what it is. The fridge is clean. No rotten fruit or vegetables lie forgotten in the trays. The cushions are plumped, my needlework put safely away; no pins lie waiting on the floor for an unwary foot. The curtains are closed against the frost patterned windows and from the kitchen comes the smell of new bread. It may not be this room that is at fault. It may be that whatever is wrong lies upstairs where there are too many shadows at the turn of the stairs. I was going to go up earlier. I had something to do there, but the darkness put me off. Whatever it was couldn't have been so important, and the shadows were all wrong, because it was early morning when I was last there and it couldn't be much more than that now. The window on the landing didn't lose the light until later. Much later.

The phone keeps ringing. The house phone and the mobile, but I won't answer. I have to get the baby back before my mother comes. She may take the children away with her. She certainly won't understand that he's been replaced with whatever it is that lies in the cradle now, with eyes like beech nuts and its endless screech for milk. Milk I must not give, for I read that the only way to win back my child is to starve out the changeling.

Today it has stopped screeching, and the shadows are even darker at the turn of the stairs. Outside the frost still peppers the lawn, and the fence and gateposts are thick with rime. My footsteps show up dark green in their trail across the frozen grass. Last night I took the children outside, so that when the Leshiy came in to return my baby, he wouldn't find them there and take them instead. They were safer in the woods while he was busy inside. In the trees, where Aunt Raisa told me to go if I should ever be lost and have to stay. In the depths, in a leaf pile.

Waiting for morning, I came back and I watched and waited. I left the back door open and sat in the shadows to watch him pass, but he didn't come. I can't stand another night of that, so I shall go out again to the leaf pile and get in with the children. When it is morning again, Simon will be home and he will come and find us. Everything will be alright then. Everything.

But even that is not the end of the story.

It is in here with us. There is barely light to see yet, but I know. Out there amongst the trunks, something is walking; crying. I think I hear a man's voice, calling my name, but that is likely to be a trick. A trick of the Leshiy. I will be a strong woman and stay still.

It is silent here, but for the occasional drip or the snap of a branch. It is impossible to sleep, but that's just as well. I have bitten my lip without noticing so my mouth tastes of iron, and the children are cold; so cold. Their arms are frozen twigs and their lips, when I kiss them, like summer fruits taken from the freezer. The monster has found them in the safety of the leaves and taken them in its grasp. It's close now, sucking the warmth from my limbs, its black claws clutching my bare arms, squeezing. A strange whistle marks its ragged breathing and now, in the early light, clouds of its hot breath bloom before my face as it breathes. In, and out, and in...

Katherine Searle lives not far from London and works in a Secondary School. She is currently editing her second novel (under the pen name Polly Johnson) and working on a poetry collection. She loves history and myth and looking beneath stones to find what lies hidden beneath. Follow her on Twitter