[1] The film is loosely based on the novel Baron Munchausen's Narrative of his Marvellous Travels and Campaigns in Russia (1785) by Rudolf Erich Raspe.
[2] In the year 1787, the astrologer Sirius seeks proof of the existence of the Selenites, a fabled race of Moon dwellers said to possess the talisman of eternal life.
The adventurers fall into an underground lake and are attacked by swimming monsters, but are rescued by flying creatures and soon meet the Selenites, who are humanoid but have three legs and crescent-shaped heads that are detached from their bodies.
The Green Means launch an attack from their nearby spacecraft, sending their flying saucers to invade the Selenite kingdom.
The Earthlings join the battle, driving back the Green Means, stopping their leader from stealing the talisman, and rescuing the Selenite king and queen.