Timothy was thirteen years old and loved to explore. However, even he never imagined that life in Astromast, the largest human colony on the Moon, would prove so exciting. He and his younger sister, Lena, attended the International Space School, whose halls were full of tall screens and models of planets rotating in the air. Each day here was different from the previous one, but none was quite like the one that had just begun. This morning the gravity was a little lower than usual. So the children were slipping slightly in the corridors, bouncing off the walls, but the maths teacher quickly called everyone to order. After lessons, Tymek and Lena met in the greenhouse, where tomatoes and basil were growing and the glass roof showed the crater Copernicus covered in moondust. - 'Did you hear about the signals Dad caught? - Tim glanced at his sister, who was watering the purple radishes. - Yes, he said, there are some strange pulses outside the dome! Maybe it's another robot scout? Shortly afterwards, the siblings headed for the B-zone, where the few inhabitants strolled between hangars and laboratories. There was a sweet smell of synthetic cocoa in the air, but Tim was more interested in the sound: a low buzz that could only be heard between the two oldest domes. - This must be it! - whispered Tymek. - Maybe we'll find something that none of the adults have discovered yet! Following the sound, they found themselves in a room marked 'Technical area - No admittance'. Lena pressed her ear to the door and opened her eyes wide. - Something's moving in there, Tymek.... The door unexpectedly opened with a quiet hiss, revealing a narrow corridor lit by blue lamps. At the end of the corridor, a control panel blinked red, and next to it... something moved in the shadows. Tymek stopped and grabbed Lena's hand. Together they took a step forward. - Are you ready? - He asked in a whisper. As they approached the panel, suddenly all the lights went out and an unfamiliar voice filled the room: - Identification required. Enter access code. The siblings looked at each other with wide-open eyes. What would they do once they got past that boundary? And who or what was that voice behind the door...?