Why Every Classic Companion Left Doctor Who

When Maureen O'Brien left at the end of The Myth Makers, she was replaced by handmaiden Katarina, played by Adrienne Hill. Where Doctor Who had previously had companions from contemporary Earth or the far future, Katarina was unique because she hailed from ancient Greece. 

Nobody realised the problem with this until Hill had already been cast and written into the following story; The Daleks' Masterplan. If Katarina had never even seen a light bulb before, how on Earth was she going to cope with the sorts of advanced technology that Doctor Who encounters week by week?

Katarina was quickly written out of The Daleks' Masterplan, so quickly, in fact that her death scene was the first footage Adrienne Hill filmed after being cast!