The evening in Brzasek was the colour of a broken shell, and the wind carried salt and gossip. It was said that the old lighthouse in the shallows wakes up at 9.17pm, even though there has been no electricity there for years. Lena, a 16-year-old with pockets full of notes, measured the sound with an old flea-market radio. The low murmur came back every night, always just as even, as if someone was turning an invisible crank deep underwater. In her notebook, Lena scribbled lines and numbers and drew maps of the bay beside it. Today the murmur had a tail, three longer beats and one shorter one, repeated three times. Olek, a neighbour with a workshop garage and a drone in the boot, arrived with torches and silence. Together they ran the footage through his laptop, looking for a pattern until the numbers came up. 54°44'N, 18°21'E - the middle of a sandbank, right next to a lighthouse, enclosed by a chain and memories. Lena checked the tides: the night tide would expose the narrow isthmus for two hours, then the water would return sharply. - Do you fancy a walk? - She asked, pretending her voice wasn't trembling. - 'Only if we get back before high tide,' Olek replied, putting on his rucksack like a shield. They crossed the reed bed, where frogs clicked like stuttering metronomes, and descended onto the wet sand. The mist lay low, punctuated by a streak of moonlight, and the armour of crabs crackled underfoot. The lighthouse rose out of the darkness, tall and shabby, with a balcony that remembered other voices. The chain by the door bore fresh scratches, as if someone had already tried to enter and didn't want to leave a mark. - Guardians of nature? - Olek leaned over, studying the steel. - 'Or someone who knows the signal,' Lena whispered, removing a thin pick from a bundle of clips. Inside, it smelled of rust and flowers that someone had once dried on a string under the stairs. Spiral steps, from which the paint had fallen off, led to the laterna, where a dead Fresnel lens rested. A leather pouch lay on the windowsill, and inside it was a key with the R-742 mark stamped on it, and a small paper ticket. "Don't turn it on if you don't know what it will respond to" - read Olek, wrinkling his eyebrows. Lena felt the radio vibrate in her pocket; the murmur from downstairs suddenly merged with the silence, forming a tense, almost musical line. - 'R-seven-four-two... it repeats in the night,' she whispered, tightening her fingers on the key. They went lower, where the corridor was narrower and the walls cooler than the air. By a heavy door with a brass lock hung a sign: "Do not open at high tide". Lena put the key down and heard the lock respond with a single clatter, almost like a sigh. At the same instant, the lighthouse vibrated and the lens above them, without electricity or a human being, moved a hair's breadth. Three lights flashed from the sea, far beyond the mist, then one, and then three again. - 'Whoever it is, they know we're standing here,' said Olek with a voiceless mouth, as the metal began to soften under the key, and the door moved a centimetre.