The plaster had long since fallen off the old school building in the city centre, and no one had repaired the rusty swings in its courtyard for years. However, Weronika, Kuba and Basia never cared about the appearance of the building - this is where their extraordinary adventure began. That day, just after their last history lesson, the three friends did not go to the bus stop as usual. Instead, they headed for the basement of the school. Veronika had heard the day before from the caretaker that there used to be unused tunnels under the gymnasium, which no one dared to rediscover. After a few minutes of wandering through dark corridors, among cobwebs and creaking pipes, Basia stopped at a half-open door behind the old cloakroom. The faint light of telephones illuminated the narrow staircase leading downwards. Without a word, they moved forward, not wondering what they would find at the end of this descent. There was a poignant chill at the bottom, and footsteps echoed off the dusty bricks. At the very end of the corridor, they spotted something that looked like a mirrored sheet stretched between the walls. Veronica approached first - the surface seemed to ripple and glow with a soft blue light. Cuba carefully touched it with his hand and his hand disappeared on the other side. - It's ... impossible - whispered Basia. - Is this... some kind of passage? At the same moment, a quiet groaning of the hinges came from behind them. The door they had come through began to close slowly. Veronica looked at her friends. Everything could depend on the decision of what they would do in a moment. A light pulsed on the other side of the portal, as if something or someone was waiting for them to pass on....