A June afternoon in Krakow smelled of dust and an approaching storm. Everyone who could, took shelter under the awnings of cafés or hid from the sun in the shade of old tenement houses. However, Lena, Maks, Julia and Szymon had other plans. For them, it was the first day of summer holidays and no amount of heat could stop them from going out together. On a day-to-day basis, they were not very different from their peers, but they were united by their curiosity and love of unusual things. Lena, a talented illustrator, always had a sketchbook with her. Maks, a passionate photographer, never parted with his camera. Julia, a smiling sportswoman, was invaluable in situations requiring courage, and Szymon knew the history of every nook and cranny in the old town better than many museums. They met just by the Florian Gate, where tourists queued up for a selfie, and set off towards an alley that Szymon had mentioned more than once over coffee: - Apparently, once, during a fire in the city, an entrance to underground passages was discovered there. - They didn't believe in ghosts, but they liked it when Simon told them about forgotten tunnels and buried passages under the old town streets. The alley did indeed look inconspicuous. On the left, the stone wall was partly overgrown with ivy, on the right, an old iron door rusted so badly that it was difficult to see. Simon pulled a torch from his rucksack and Lena clustered with the others. The door gave way after a short tug, opening the entrance to the darkness with a groan. Even the first steps into the tunnels were like a journey to another world. Moisture, echoes and the smell of old brick. Maks took pictures, but the flash only momentarily illuminated the thick darkness. When they had walked a few metres, three branches of the corridor sprang up in front of them. Each led in a different direction - left, right and straight ahead. On the walls they could see fragments of old inscriptions and faded tiles. When Julia touched one of them, something clicked in the wall and a quiet metallic sound rang through the air. - Did you feel it? - whispered Lena, looking at her friends. Suddenly, a faint light and the echo of footsteps came from deep in the left corridor. Someone or something was approaching towards them. The friends exchanged quick glances, holding their breath. Each of them had to decide at that moment - turn back, hide, or head straight into the unknown?