The story takes place in an era when humans have colonized the Solar System but dare not go farther, as the first extra-solar expedition to Barnard's Star failed and the survivors came back as babbling, grotesque, diseased madmen.
They spoke of a gigantic planet, populated by ferocious animals and the single city left of the evil "Medusae".
While attending a Great Books course, Williamson learned that Henryk Sienkiewicz had created one of his works by taking the Three Musketeers of Alexandre Dumas and pairing them with John Falstaff of William Shakespeare.
However, since the character displays few other signs of such a background, and since he bears an English first name going back to the Norman Conquest, Williamson seems to have rather implied a mixture of ethnicities and cultures during the centuries of spaceward expansion.
The Legion itself is the military and police force of the Solar System after the overthrow of an empire called the Purple Hall that once ruled all humans.
(This rationale for an invasion of Earth - the invaders coming from an older, dying planet, and having an existential need to find a new home - dates back to H.G.
Their vast spaceships also have very effective plasma weapons, very similar to those the Romulans had in a Star Trek episode called Balance of Terror.
Williamson then wrote The Cometeers, in which, twenty years after The Legion of Space, the same characters battle an alien race, this one of different origin.
The Cometeers cannot be destroyed by AKKA, as they are incorporeal from the Universe's point of view and exist for the most part in an alternate reality.
The ruling Cometeers feed on their slaves and literally absorb their souls, leaving disgusting, dying hulks in their wake.
Giles broke into a secret chamber guarded by complex locks and force fields that the incorporeal Cometeers could not penetrate.
Another novel, One Against the Legion, tells of a Purple pretender who sets up a robotic base on a world over seventy light years from Earth, and tries to conquer the Solar System using stolen matter transporter technology.
The story also features Jay Kalam, lobbying to allow the New Cometeers to leave the Solar System in peace, as many people were demanding that AKKA be used to obliterate the departing swarm of planets once and for all.