Staszek, Lena and Boris have always been fascinated by space, but never imagined that one day they would actually go there. It all started when they won a competition for the best rocket design and visited a European Space Agency base as a reward. As they tried on real spacesuits under the watchful eye of astronauts, the alarm suddenly blared and the lights flashed red. - Quick, to the emergency capsule! - shouted one of the engineers. No one had time to ask questions. Moments later, the children were already sitting tightly strapped into their seats, hearing a countdown in their headphones. 3... 2... 1... Launch! The rocket broke out of the Earth's atmosphere and rushed towards the Moon. After a long moment of silence, Lena unbuckled her belts and looked through the porthole. - Can you see it? - she whispered. On the surface of the Moon, in the infinite darkness, there was the faint outline of some building. When the capsule landed softly, the children quickly stepped outside. To their eyes appeared an old research station - whitewashed with dust, with large, dusty solar panels and a strange, giant clock above the entrance. - This must be the Moonwatch Station! - Boris remembered the legend of the first explorers of the hidden side of the Moon. The entrance was ajar. Inside, there was semi-darkness and the walls were illuminated by incomprehensible symbols. In one room stood three clocks, each indicating a different time - and all ticking backwards! Staszek came closest and then the clocks emitted a protracted mechanical beat. Suddenly the closed door at the end of the corridor creaked open.... A quiet knocking sound came from inside. Who or what had caused it? The friends exchanged communicative glances and moved slowly towards the door....