Landmarks No.24

The Ribos Operation.
For a series that had been running for 15 years, there had only been one genuine sequel (Monster of Peladon) and only some very sketchy story arcs - Ian and Barbara's quest to get home, the Monk's revenge, the Doctor's exile.
Finally, the production team decided to try a whole season of interconnected stories which could be enjoyed in their own right, but would reward the dedicated viewer with an on-going mission to collect six objects of immense power - with the balance of the cosmos at stake.
As an experiment, the Key to Time season  - which this story launched - was a brave one, but sadly the results were a little underwhelming - with a rather rushed and frustrating conclusion.
These days we are cursed with story arcs - not just in Doctor Who. Some are very well done and certainly make you want to tune in to each story to see how it develops. The best are those which build slowly, and don't appear every week.
Some people used to watch The X-Files purely for the alien conspiracy arc. Other felt these episodes tedious and annoying, and far preferred the stand-alone stories.
What story arcs do need to have is a conclusion. It is so annoying to get to the end of the series and you are still left none the wiser (worst case scenario being the various US shows which end on a cliffhanger - then don't get renewed).
This story is also significant for the introduction of Romana - a member of the Doctor's own race as a companion.
There is also the début of the Guardians, who represent the balance of light and dark, order and chaos, in the Universe.