Zosia spends July in Brzegowo, a small town by the sea. Every day she gets up early and wanders around the old pier. She is accompanied by Mirek, a cousin who knows all the creaking planks. Today Zosia carries her grandfather's compass, found in a dusty workshop. The needle goes crazy as they approach the end of the pier, where the seagulls circle. "Maybe there's something underneath," - whispers Mirek, squinting from the sun. Under a plank with an anchor engraved on it, they discover a rusty iron ring. A tug, a second, and the flap gives way, revealing a staircase into darkness. There is a smell of tar and salt from below, and water splashing quietly. "Are we going down?" - Zosia asks. "Just for a while. We have torches." The stones are slippery and the ceiling low; you can still hear the waves above you. On the wall someone has painted arrows, a date and a small boat. The compass needle suddenly stops whirring and points to an iron door. Behind the door something hums quietly, like an old engine, but more evenly. Next to the door hangs a brass lever with a sign: "Do not move at the drain". Mirek touches the inscription with his finger and glances at Zosia uncertainly. "How do we know when the tide is out?" - he asks in a half-hearted voice. It begins to drip from the ceiling, the drops forming a trickle along the steps. Behind the door a metallic knocking sounds three times, rhythmic and close. The compass trembles in Zosia's hand; the pointer twitches like the hand of a clock. "Are we opening?" - whispers Mirek as the lock itself makes a sudden click. Suddenly one torch goes out, leaving them in the uncertain twilight.