At Birchwood, on the edge of the forest, stood houses with red roofs, while behind them the fields undulated and the old, resin-scented Badger Valley fell into darkness. The Silversmith - a narrow stream with sharp bends - ran like a thread of tin through the grasses, sloshing against the stones and silvering the night as the moon held its petal over it. Zosia was eleven years old, with freckles like poppy seeds and a thick notebook in which she drew animal tracks and pasted feathers. Her younger brother, Staś, carried a string forever untied and a stick knife in his pocket. Bzik, a shaggy mutt with ears like two poplar leaves, always listening for something, went with them everywhere. For a week, things had been happening in Brzeziny that everyone called in just one word: strange. It was as if the night had begun to make its own plans. In Mr Grzelak's orchard, the pear trees were losing their fruit not in any order, but in even triangles, as if someone was practising geometry with pears. On the line by the stables, individual socks were disappearing - always those with a coloured stripe. And Zosia's mum went through a whole drawer one morning because a bunch of keys with a red heart-shaped key ring had disappeared. "I must have buried it in my handbag" - she muttered, but the handbag was as it always is: heavy and full, and the keys weren't there. That night, as the wind was whipping through the tops of the pines, Zosia woke up because she heard tapping. Not the usual rainy kind, but rhythmic: three times quiet, twice loud, a halt - and then again. Lilac raised his head until his leafy ears were arranged in triangles. Zosia crept up to the window. The moon clung to the roof of the old mill at the curve of the Silversmith, making the waterwheel look like a serrated eye. Something vibrated by the pier. A shadow? Or was it just a twig? In the morning they went with Stas to the water. Zosia carried a magnifying glass and a notebook, Staś a stick, and Bzik himself and the fact that he can sniff every bush. The stream hummed like a kettle about to whistle. The smell of mint was in the air, as a whole sheet of wild mint grew on the bend and roughly dipped its leaves into the current. - 'Look,' whispered Staś and crouched by the bank. - 'That's not a dog. On the damp sand, next to a flat stone, were imprints: five fingers, clear, with a slight membrane between them. Zosia put her hand up for comparison - her fingers looked like birch twigs next to them next to the broad leaves of a burdock. - Not a fox, not a deer, not a badger - she muttered. - 'A blubber... that must have had a weakness for water. Lilac barked once and stopped, as if he was stupid. His nose wandered to a blackthorn bush. Zosia noticed a dishevelled thread on the thorns. Thin, blue, shiny. She pulled gently. A longer piece of thread slid out from the centre of the twig, smelling of... liquid soap? Mint? Staś put his nose to it; Bzik sneezed. - It's probably from some kind of net,' assessed Staś, squinting. - Or from a shopping bag. But what is it doing on a blackthorn so far from the road? Zosia wrote down: 'A blackthorn in the tracks. Blue thread on the thorn. The smell: mint. Night tapping." Then she added drawings: rippling water, a mill wheel, a small moon. In the afternoon, when the sun hugged the roof of their house for a while, Zosia announced: - We are going to the mill at dusk. We have to see what's tapping there. We'll tell grandma we'll be back before dark. We're taking a torch, a whistle, a string and a packet of dry apples for Bzik, because he'll pretend to starve again. - And we don't go into the water - Staś added, because mum kept telling him that. - Nor on thin planks. We promise. Grandma gave them a stern look, but said nothing. She just handed them an extra hat and a blanket: - As if you had to sit on a cold stone to keep our legs from getting sick. The sun was hiding behind the hill as slowly as if someone was pushing it with a stick. A clang of cranes came from the bushes, and a bat flitted overhead, nimble as a folded umbrella. The lilac took its steps softly, tail held low, but its eyes shone like two buttons. A whistle on a string swayed in Zosia's neck, a torch buzzed in Stasio's pocket. When they reached the bend of the Silversmith, the water seemed darker than usual. A stump stood in the middle of the current - like an island on which someone had left a narrow stick. The waterwheel stood still, although a quiet splash could be heard, as if someone was impatiently striking the sheet with their hand. A magpie cawed from the bushes and flew over their heads. It had something shiny in its beak, which fell and rolled down the planks of the platform with a clatter. An orange cap. - Well, at least we know who collects glitter," snorted Staś, but he soon became serious. - What about the keys? Bzik ruffled the fur on the back of his neck. Such a quiet murmur came from his throat that it sounded more like a question than a warning. A chill fell from the sky and the smell of silt and wet bark rose from the water. Zosia wrapped herself in a blanket, then picked up the cap and laid it on the edge of the pier. Lilac sniffed carefully along the edge. Suddenly, he stopped at one of the planks and looked at Zosia as intently as if he could speak. - What's there? - She whispered. She crouched down. The plank was grey from the damp, and just below it, at the edge, something thin and metal was lost in the grass. Zosia reached out with her hand, but Staś grabbed her wrist. - 'Don't touch it. It looks like wire. They turned up the torch and switched it on for a moment, shielding the light with their sleeve. A loop appeared in a narrow streak. A wire thin but strong, slightly hidden in the grasses and tied to a stake. Someone had obviously put on something that shouldn't have been put on. Zosia felt her heart start to beat colder, and Bzik curled his tail as if something reminded him of a bad memory. - It's not for humans - whispered Staś. - But for someone else it could be... - Muskrats come here, sometimes otters - Zosia swallowed her saliva. - If they step on it... At that moment, the tapping sounded again. Three times quietly, twice loudly, a halt. On the other side of the waterwheel, the reeds rustled. The plank under Stasio's foot squeaked. The boy took half a step back. The moon came out from behind a cloud, scattering silver dots on the waves. And then a head rose from the water: smooth, with whiskers like thin brushes, eyes glittering like wet beads. For a split second it seemed to be just a shadow and a reflection. But no - the creature dived, then emerged again, this time closer to the platform. In its mouth it held something that turned white and flashed red. - See? - Staś leaned over, insisting his hand on the post. - It's a pendant! A red heart! The keys! Zosia felt the blood hit her cheeks. - Mum... - a whisper broke out. Lilac tightened the leash so that Zosia had to tighten her fingers on the handle. The otter - because it had to be an otter - looked at them the way one looks at something unfamiliar: curiously, alertly, but without fear. She turned her head, tiny drops sprang from her whiskers and shimmered in the light. And then... A deafening splash came from the reeds, as if someone had lowered a heavy stick onto the water. The mill wheel twitched and moved lazily by a centimetre, then by two. Something else shimmered on the log in the middle of the current - a thin string, previously invisible, tightened and loosened slightly. The tapping came back again: three quietly, two loudly. The magpie squawked nervously, as if someone had entered her larder. And the wire loop, the one at the edge, suddenly trembled, as if alive, as if responding to all the commotion. - 'Zosia,' Staś said in a voice that Zosia had never heard from him; an older boy's voice. - She swam straight for the sloop. The otter dived for a second, swam even closer and moved its whiskers as if to sniff the air. A bunch of keys rang against the tea-coloured tooth of the wheel. Lilac scowled, then sprang up in an instant as if it were a spring, and tightened the leash so violently that Zosia took a step forward. The board groaned, hesitated under her weight. There was a low, quiet hooting from the forest side - so serious that the boys in Zosia's class would have instantly bent their knees. Somewhere up above, the wings of an owl cut through the air with a noiseless movement. Zosia grasped the post with one hand and blew the whistle upwards with the other. Her throat became as dry as if she had eaten a handful of chalk. A wire, thin as a hair, flashed in front of them and undulated as the water current brushed it from underneath. The otter emerged a third time, already so close that they could count all the drops on her fur. She continued to hold her mother's keys in her mouth, her red heart flashing like a small lantern. The loop twitched just in front of the vibrating whiskers, and Susan took in air to shout