Caves of Androzani.
It was good then, it's good now.
I just listened to Mission to Magnus a 6/Peri lost story by the same writer as Caves featuring Sil and the Ice Warriors.
The two combined made me wonder if any companion was more perved over by the villains than Peri.
God bless her, may watch the Planet of Fire next - for reasons that have nothing to do with bikini clad American companions.