Big Finish Doctor Who/Recap/026 Primeval

Just why did Nyssa collapse at the end of "Four to Doomsday"? The Doctor isn't sure, either, and it has him worried--so he takes he back to her home planet, Traken.

Traken being unavailable in the present (the Master accidentally destroyed it in "Logopolis"), the Doctor opts for Traken several thousand years before Nyssa's time; before even the Keepers. There the foremost healer of the day, Shayla, diagnoses Nyssa with acute exposure... to evil.

The Doctor barely manages to stifle a snicker at this diagnosis, but Shayla is being perfectly serious. She goes off to inform the High Council about the young woman dying of evil poisoning and the off-worlder responsible for it. The Doctor is really not laughing now.

Tropes

 * Outgrown Such Silly Superstitions: Traken.
 * Religion of Evil: Kwundaar-ism.
 * Xanatos Gambit: The Doctor's plan to regain control of the Source.