[2] The story takes place in an era in which 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.

One of the Legion's most important tasks is to guard the current Keeper, a beautiful young woman named Aladoree Anthar.

Through the machinations of his uncle, a powerful politician with a hidden agenda, John Ulnar is assigned to Aladoree's guard force at a secret fort on Mars.

When she is kidnapped by a huge alien spaceship, John and the three other survivors of the guard force follow her kidnappers to a planet of Barnard's Star.

John Ulnar's uncle and his nephew have allied with the Medusae as a means to regain their empire, and have kidnapped Aladoree to ensure that AKKA is not used against them.

The Medusae, however, turn on the Purples, seeking to destroy all humans and move to the Solar System, as their own world, far older than Earth, is spiraling into Barnard's Star.

Brian W. Aldiss and David Wingrove, noting that Williamson had produced much better work, dismissed The Legion of Space as a Gosh-wow!

[4] Thrilling Wonder Stories found the novel to be "space opera with a vengeance -- with passably good characterization imbedded in an interplanetary chamber of horrors that .