In this novel, renegade Purple pretenders ally themselves with the Medusae as a means to regain their empire. But the Medusae, who are totally unlike humans in all ways, turn on the Purples, seeking to destroy all humans and move to the Solar System, as their own world, far older than Earth, is finally spiraling back into Barnard's Star. One of the Purples, John Ulnar, supports the Legion from the start, and he is the fourth great warrior. His enemy is the Purple pretender Eric Ulnar, who sought the Medusae out in the first place, seeking to become the next Emperor of The Sun.