Each volume of the series primarily follows a duplicate of a government agent as he lands on his prison planet and begins to both investigate the menace to the civilized worlds and find his position in his new society.
The organisms destroy their host when he or she leaves the Warden Diamond, making the planet system the ideal prison colony for the Confederacy, a massive space empire.
Through technological advances, the government duplicates the personality of the agent (who remains unnamed throughout the series) and implants "him" into four brain-dead host bodies.
The four hosts are then sent to four different planets in the Warden system and have no choice but to fulfill their assignment of locating and defeating each of the Four Lords, delaying the expected alien invasion and finding out vital information on the infiltrators.
From there, it is, in the first three books, purely an adventure story involving the copied agent trying to assassinate the local Lord, getting a girlfriend and learning a lot about himself in the process.