The TARDIS brings the Doctor and Sarah to the medieval planet Peladon, some half a century after the Time Lord's first visit. Now under the benign influence of the Galactic Federation, the Pel nobles enjoy new riches and new technology. But the Federation is at war with Galaxy Five, and Peladon has taken on a new tactical importance; it is rich in the rare mineral trisilicate, which is vital to the war effort. The Pel miners, however, are in revolt.

For too long they have been overworked and taken for granted by the Federation and the Pel royalty. They believe that Federation machinery has awoken the spirit of the legendary Aggedor, who is now slaughtering aliens and unbelievers. As open rebellion erupts, the Federation sends in a hard-line military taskforce - Commander Azaxyr and his Ice Warriors. The Doctor and Sarah must not only act as peace-keepers between the warring factions but also discover the truth behind the 'spirit'. They soon learn that the Galactic Federation has been infiltrated by traitors and spies, determined to destroy the alliance from within..