After The Doctor and his new companion Francis confronted the evil mummy pharaoh king Badru (DW: Curse Of The Mummy) they go to the planet Skaro long before the Daleks existed.
They saw what it was like when there were a lot of Thal people and the Daleks hadn't taken over everything. But on the planet Skaro they find out that there is an evil entity like a parasite that is influencing the Thals to be evil. It is possessing them.
The Doctor and Francis must find an ancient artefact called The Sacred Goblet Of Wisdom to return the Thals to their ordinary selves. But it was lost somewhere many centuries ago. It is hidden somewhere deep under the ground of planet Skaro. So The Doctor and Francis go deep underground.
They begin a quest to search for The Sacred Goblet Of Wisdom but then the evil entity that is possessing everyone, possesses the body of a Thal priest and tries to stop The Doctor and Francis from finding The Sacred Goblet Of Wisdom. So then they must find it quick or else the evil entity that is possessing the Thal priest could find them and murder them.
They find the Sacred Goblet but then the evil entity finds them and tries to take it from them. He does take it from them. They fight over it and try to get it back. They wrestle, punch and kick, and tackle each other but they cannot get it back. Francis takes it back. Then the evil entity tries to take it back. But he can't because the host he is possessing is elderly and slow so then he decides to possess Francis instead because he is youthful.
The evil entity now in Francis confronts The Doctor. The possessed Francis mortally wounds the Doctor. But the Doctor uses the powers of the goblet to heal himself. Then he uses its power to destroy the evil entity and return the Thals and Francis to normal. The Thal people thank the doctor and Francis. The Doctor says "You're very welcome." and then he and Francis leave Skaro in the TARDIS.