Four friends find a rusty key and a strange map under the Old Bridge in Nadwiśle, and what it opens is far more secretive than they expected.
Everyone in Nadwiśle knew the Old Bridge. It was wide, wooden, and always gave a tired creak when anyone crossed it. Julek, Kinga, Bartek, and Zosia went under it almost every day on their way home from school, never thinking twice about it.
Then, one stormy afternoon, when the wind had blown the clouds apart and the puddles shone on the road like spilled silver, Julek spotted something wedged between two damp boards. He crouched down and pulled it free.
It was a rusty key and a torn piece of yellow paper.
Bartek leaned over his shoulder. "That looks like a joke."
"A very strange joke," said Kinga, taking the paper carefully. It was part of a map, drawn on what looked like the corner of an old napkin. The lines were faded, but one thick mark pointed straight toward the shadowy space beneath the bridge.
"There’s nothing down there except mud and rotten planks," Zosia muttered. Still, she was already stepping closer.
Kinga turned on the torch on her phone. The beam cut through the dark under the bridge, catching broken stones, wet beams, and a shape none of them had noticed before: a narrow metal door half-hidden behind a pile of loose rock.
Julek swallowed and held up the key. It fitted the lock perfectly.
The click sounded too loud.
For a second, nothing moved. Then the door swung inward with a slow, shuddering groan, and a cold breath of air slipped out from the darkness inside.
From somewhere beyond the opening came a sound like footsteps.