Five minutes after midnight, as sleepy darkness covered the town of Roztoki, a yellow message appeared on Adam's computer screen: "Signal received. Position: 51°35'N 21°01'E". The boy was stunned. For weeks he and his friends Maja, Janek and Lenka had been working on the shortwave radio, trying to catch unusual transmissions. However, this signal was different. Short, intermittent, as if someone was desperately trying to communicate something. "It's over there in the woods where the old observatory stands!" shouted Adam. The others looked at each other with a mixture of excitement and anxiety. Years ago there had been rumours that strange lights had been seen in the abandoned building, but none of them had ever dared to go there at night. Maja glanced at her watch. "If we're going to check it out, we need to move now. The next signal may never come." In a few minutes they had gathered torches, energy bars and a map. As they crept through the dark courtyards, all they could hear was their quickened breaths and the quiet rustling of the grass. Before entering the forest, they stopped, looking at each other anxiously. "What if it's a trap?" whispered Janek. Lenka, who always held her head high, raised her eyebrows. "What if it's someone who needs help? We won't check - we won't find out." The forest was completely different at night than during the day - the sounds they had hitherto associated with birds or the wind now sounded ominous. The beams of the torches barely pierced through the dense branches. After thirty minutes of walking, they reached an overgrown path leading straight to the observatory. The building loomed among the trees like a shadow from another world - boarded-up windows, dingy walls and a huge metal dome. Suddenly, the radio in Adam's backpack crackled, making a sound like a whisper: "You're close... help...". They froze, feeling a cold shiver run down their backs. Maja, who had the strongest nerves, moved first, the others following her. Just as they were about to go inside, they heard a crack and the door to the observatory swung open with a shrill creak. There was light coming from inside - golden, pulsating, completely incongruous with the abandoned place. Adam squeezed his breath and exchanged brief, communicative glances with his friends. Was it someone sending a signal? Or was something else waiting for them inside? At the same moment, a voice thundered from inside: "Please don't close the door...".