The Keeper of Traken appears in the TARDIS seeking the Doctor's help. A mysterious and evil force has invaded the Traken Grove, threatening the bioelectronic source which unifies the Traken Union.
When the Keeper is attacked by an unknown assailant, the Traken Consul hold Adric and the Doctor under sentence of death, as Kassia falls under the spell of the Melkur, a calcified statue of pure evil.
Kassia, now the willing servant of the Melkur, is to become the new Keeper of Traken when the old Keeper dies, thereby granting the Melkur access to the source of infinite power.
Kassia completes the transition to Keeper and now the only way that the Melkur can be destroyed is by destroying the source. But the Melkur is only a shell, the real power lies within.
