In 2033, a large expedition was sent to the moon with the goal of exploring the shadow side of the earth's satellite, not visible from Earth. The very first days of the expedition brought a sensation: just a few kilometers from the landing site of the earth's ship, archaeological sensors found stone massifs of the correct form. After two weeks of research, the chief archaeologist of the expedition, Professor Nolan, established that about 5-6 thousand years ago ... ordinary earthly people or creatures strikingly similar to them lived here. When the approach to the stone buildings was completely cleared, a narrow passage opened before the earthlings. Ahead looked a strange, pyramid-like structure. The lanterns of scientists highlighted the walls, covered with letters, not like any earthly alphabet ... Archaeologists slowly walked into the aisle. The monumentality of stone structures astounded the imagination of astronauts. Have our distant ancestors once lived here? But how did they get on Earth’s satellite in those distant times ?! Scientists were slowly moving farther and farther away, but the pyramid seemed only to move farther and farther away from them. When the beam of the lantern went to the side, it was as if someone’s heavy and studying gaze was looking at earthlings ... Archaeologists were moving away from the ship, which was a violation of the instructions, but the fever of the discoverers had already gripped them - they were the first earthly people to see lunar artifacts. And behind them, in the closed darkness, suddenly something stirred. The creature, which was once abandoned by a vanished lunar civilization, awakened from a centuries-old dream and quietly moved behind the astronauts lanterns flickering in the distance ... The huge pyramid towered in front of earthlings. Archaeologists with a mixed sense of awe and horror looked at its peak, which goes hundreds of meters up, into the darkness of space. Suddenly behind, where there was a narrow passage between the megaliths, there was a grinding sound. Astronauts slowly, as in a dream, turned around ... The pyramid suddenly flared up and shone with a strange flickering light. It seemed that the fire was burning inside the pyramid, but its radiance was so amazing that all earthlings froze, looking fascinated at the overflow of prominences rolling from the foot to the top. And then other sounds rang out - it seemed to astronauts that they were hearing the music of the celestial spheres: a series of sounds charmed and called behind them, to unknown stars, farther and farther ... Professor Nolan was the first to take a step towards the pyramid. The rest followed. In front of them, in the clouds of cold fire, playing in the bowels of the pyramid, the Portal of Transition opened. Without thinking about anything else, scientists plunged into the waves of fire ... ... When archaeologists did not contact at the appointed time, the commander of the expedition raised the alarm and sent several robots to them. However, robots did not find any traces of missing scientists among the ancient buildings. The fire in the pyramid disappeared, and the Sentinel again took refuge in the labyrinth of underground corridors. He fulfilled his task: he sent aliens who disturbed him to another dimension - to the place where the missing civilization of the Moon lives ...