The Doctor, Nyssa and Tegan intend to visit Prague, but they find themselves underground in a lighted tunnel. They are several hundred kilometres below Prague, where the Svarozhich Project is taking place. The project has been plagued by cockroaches and slime mould. Dr Jane Flora was brought in to investigate the infestations.
Project security arrives to bring in the Doctor and his friends, but Flora greets them and takes them on a tour of the facility. Unknown to the project workers, several of their colleagues have been killed and replaced by something that has retained their memories, something that has made the roaches and slime.
As the Doctor and his friends help Flora in her investigations, they find the dead, desiccated bodies of Kern Shaw and several others. When Dr Walters confronts them, they realise he is an impostor. "Walters" is really Szlth'TOK, a member of an Earth species older than humans or Silurians. The project has killed several of his people, and for revenge he intends to kill everyone involved in the project, as well as all of those in Prague.
Szlth'TOK's plan is averted when project director Abloran, with the Doctor's help, negotiates a peace. The project will be abandoned, and Szlth'TOK's people will not kill the humans.
- Fifth Doctor
- Tegan Jovanka
- Kern Shaw
- Melva Hill
- Jane Flora
- David Wingate
- Jasmine Yannis
- Anwar Eisenberg
- Yankel Abubakar
- Fatima Peterson
- Tegan mentions Amsterdam and London; Nyssa has always wanted to see San Francisco.
- Nyssa and Tegan want the Doctor to teach them how to fly the TARDIS.
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