Lunar-7 base had been closed for years, or at least that's what all the official reports said. Meanwhile, right there, among the frosted domes, Tola, Michal and Ali slowly glided the lunar rover. For the first time in their lives, they felt that they were really part of something big - the Young Explorers Mission, the first science programme for teenagers on the Moon. Tola, the youngest of the three, stared intently at the scanner screen. She was from Gdansk, loved puzzles and codes, and had dreamt of setting foot on another planet since she was a child. Michal, the group leader, had a head full of ideas, but it was his composure that made the others feel safe. Ali, a technology genius, was the son of an engineer and never parted with his hand-built tablet. The sun was fading behind the moon's horizon, casting blue-silver shadows. The Lunar Valley of Silence, where the old base lay, seemed peaceful - all too much so. As soon as they entered the main corridor, the base's security systems automatically activated. The door closed behind them with a heavy bang. - 'Relax, it's just automation,' chuckled Ali, tapping on the tablet screen. - 'I'll disarm it in a minute. However, at that moment Tola suddenly froze. A flashing red LED flashed on one of the walls. Underneath it, someone had painted an unfamiliar symbol by hand, quickly - it resembled a cross between a spiral and the letter omega. - 'Someone was here before us,' she whispered. Suddenly something else answered them in the echo of the empty corridor - a quiet hum, and then a distorted voice came from the base's speakers. The words were spoken in Polish, but were interspersed with foreign, unearthly sounds: "Do not open ... door... they are coming back here... Time... is shrinking..." The three young teenagers looked after each other in horror and fascination. Before them was a mystery bigger than they could have imagined. Then, from the deep shadows of one of the rooms, there was the sound of footsteps. Someone - or something - moved towards them.