Summary: originally designed as a children’s show, the Classic Doctor Who format at this time was half hour episodes making up whole stories of 2, 4, 6 (and occasionally, gulp, 12) episodes, though the stories themselves only got individual names towards the end of the First Doctor’s run – most story titles were decided on later by fans, TV historians, DVD releasers etc.
The Doctor and his companions generally alternated between historical and SF stories, with very few set in contemporary times. Unlike these days, the historicals were usually “pure” in that the stories involved no alien or science fictional element, other than the fact that the main characters are time travellers and have perspectives from other times.
Many episodes and whole stories from these years were destroyed by the BBC (well technically all of them were, but many were saved or recovered) which means sadly that some of the best stories are no longer available to view. Hardcore fans can revisit them through audio recordings, or the good old Target Book novelisations, which are no longer in print but have a thriving existence in the second hand book market (and, more recently, have been made available from the BBC as audio books).
At this stage in Classic Who history, the TARDIS cannot be controlled by the Doctor, which means that any time he leaves a particular place and time, they are in the hands of fate. This gives extra tension to the companion journey, as in many cases they have no way of knowing if and when they can ever return home. Several companion leaving stories thus comprise either of accidentally landing within a year or two of their point of origin and jumping off the bus while they have the chance, or picking the first random planet that seems to have something to offer.