The season consisted of four serials and fourteen episodes. Unlike season 23, there was no overall story arc, and the episode allotment - two four-episode stories, two three-episode stories - would be retained for the remainder of the original series. Notably, the season featured the introduction of new companion Ace, who would be a prominent figure in the spinoff fiction that kept the Doctor Who name alive following its cancellation in 1989 and was the final official onscreen companion until Rose Tyler in the revived series.
Unlike the previous regenerations of the Doctor, the Sixth Doctor's happened at the beginning of the season's episode. Colin Baker, who played the Sixth Doctor, would not come back to shoot a few episodes leading up to the regeneration as he felt insulted at being replaced after a short time as the Doctor and was displeased to make fans think he was still the Doctor for another year when he was only offered one story before his dismissal. His Doctor's life was instead wrapped up through an audio series entitled The Sixth Doctor: The Last Adventure in 2015.
Contrary to common belief, season 10 kicked off in the last week of December 1972 — not in 1973, as would be expected. Season 10 actually began nine years after season 1 started. In fact, The Three Doctors began nine years to the week after The Daleks first aired.
For the purposes of this list, "Series 4" is considered to be the production series 4, which ran all the way from Time Crash to The End of Time.
The years seen in this section may seem decidedly "off". Remember, however, that this list only gives the first year in which an episode from a series was broadcast. David Tennant, unusual amongst other Doctors, began and ended on special episodes, not regular ones. Thus, his series actually begin in 2005, 2006 and 2007 — not 2006, 2007 and 2008 as is commonly thought.