Humans landed and walked on the moon 6 times between 1968 and 1972. They didn’t return there until 2044 when Virgin Galaxy landed a ship with over 50 settlers who built a permanent base there. Over the next decade Virgin Galaxy raised the first space hotel – Lunar Delight – and from 2054 the moon became a meaningful and attractive holiday location.
During this period the number of semi-permanent settlers on the Moon rose to well over 5000. Until 2056 everything seemed fine. Over 10,000 inhabitants of Earth spent a minimum of one week traveling to and then exploring sites of interest on the moon.
Then in late 2056, a group of tourists disappeared without trace whilst out on a half day excursion to the Sea of Tranquility. Investigations by the local police and then the Space Patrol found no trace of the settlers or their lunar carrier. Tours were suspended for several months but eventually re-opened. In July of 2057 a distress call to Earth from the main base on the moon, Armstrong Centre, led to a battalion of space cadets heading to the moon at breakneck speed. The distress call described large sections of the Armstrong Centre disappearing into the surface of the moon.
An advance guard of the space cadets got there in under 4 hours and found no trace of the Armstrong Centre. It took over a year for Earth’s military forces to uncover an underground alien stronghold that had been built on the Moon, sometime between the late 20th Century and the time when humankind had returned to the Moon, by the ultimately named ‘Sequestrans’.
After a fairly short battle, the Seuqestrans on the moon were all destroyed. There were not many of them. As a species, they appeared to have the capability to adsorb matter into their bodies. Individually they could do this on a small scale but when they worked collectively they could adsorb large structures. This was discovered during the early skirmishes between them and the space cadets.
To this day Earth is not sure where the Sequestrans actually came from. There was no sign of any spaceship that they may have been used to land on the Moon and no records, that could be identified, of their history.