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.