Simon always felt uncomfortable passing the old observatory on the hill, especially after dark. However, it was here, one night in June, that he stopped with Olga and Leos. They were carrying backpacks full of biscuits, notebooks and torches. It was supposed to be a simple holiday adventure - exploring forgotten places at night, taking photos and making up thriller stories to laugh about their own fright later. The observatory, despite the years, was still impressive. The stone walls remembered the days when the farthest reaches of the cosmos could be seen through a huge telescope. Now everything was covered in cobwebs and dust, the air had a peculiar metallic aftertaste of mystery. When they entered the main hall, Leoś immediately noticed something unusual. In the corner stood a large, round slab resembling a mirror, but its surface seemed to ripple and shine, as if a piece of the night sky had been enclosed there. "Is this some kind of art installation?" - Olga asked, approaching cautiously. Simon felt a shudder; he touched the slab with his fingertip - and suddenly his reflection disappeared and his hand was pierced by an icy wave. For a moment, he saw something - a lit-up city he did not know, and silhouettes of people dressed in strange clothes. Leo pulled out his mobile phone and took a picture, which in a split second turned into a chaotic mosaic of colours. "It can't be a mirror. It's some kind of ... something," he muttered. When he tried to touch the surface of the tile, he felt a subtle tug, as if something on the other side was waiting for him to take that step more. Olga was intrigued by the number engraved in small letters on the plate's frame: 511. This was no coincidence. Old, scattered notes could be seen under the ceiling. Simon climbed onto a nearby table and took down one of them. The words were in English: "Only the brave see both sides. Only the chosen walk the bridge." Suddenly the air thickened, the devices in the backpacks began to switch on by themselves, and a faint, then increasingly intense blue glow ejaculated from the plate. The shadows around them began to move, and from the centre of the slab's surface emerged a shape, completely human but faceless.... The friends froze, feeling that making a decision - to escape or to cross the threshold - would never again be as easy as they thought. This was the moment when the mysterious world was reaching out to them, and they had to decide whether to grasp it.