Doctor Who

The season introduces William Hartnell as the First Doctor, an alien who travels through time and space in his TARDIS, which appears to be a British police box on the outside. Carole Ann Ford is also introduced as the Doctor's granddaughter Susan Foreman, who acts as his companion alongside her schoolteachers Ian Chesterton and Barbara Wright, portrayed by William Russell and Jacqueline Hill, respectively. Throughout the season, the Doctor and his companions travel throughout history and into the future. Historical stories were intended to educate viewers about significant events in history, such as the Aztec civilisation and the French Revolution; futuristic episodes took a more subtle approach to educating viewers, such as the theme of pacifism with the Daleks.

Milestones this season included not one but two encore appearances by the Daleks; the departure of the original three companions, Susan Foreman, Barbara Wright and Ian Chesterton; and the first appearance of someone else from the Doctor and Susan's home planet, and the first Time Lord enemy to appear.

At twelve episodes, The Daleks' Master Plan was the longest story in the show's history until superseded by the fourteen-episode The Trial of a Time Lord in 1986. The season also had the greatest number of different companions (seven) and producers (three). This was the longest season in the history of the franchise to date, at 45 episodes. It also featured the first appearance of actor Nicholas Courtney, although not in his most famous role as the Brigadier. The Daleks' Master Plan also saw the return of Peter Butterworth as the Monk, who became the second Doctor Who enemy to make a return (after the Daleks themselves) to the show. This was also the season of The War Machines, to date the only televised story in which the taboo of never referring to the Doctor directly by the name "Doctor Who" is broken, with the exception of Missy's teasing in World Enough and Time.

This season was a season of major changes for Doctor Who, including some that have influenced the series and franchise ever since. Most notably, season 4 saw William Hartnell's First Doctor hand off the baton to Patrick Troughton's Second Doctor in the very first regeneration. When Hartnell announced his plans to leave the series, the show's writers came up with the idea of having the Doctor change his appearance and personality. In 1966, while it was not uncommon for actors to change in ongoing TV and film roles, this was possibly the first time a change in appearance was written into the very nature of the character.

No complete stories from the season were known to have survived until the early-1990s when archivists recovered a complete version of The Tomb of the Cybermen. A second complete story, The Enemy of the World was recovered in 2013, along with a nearly complete recovery of the story that followed it, The Web of Fear. The season is also notable for having Patrick Troughton play a double-role as both the Doctor and Salamander in The Enemy of the World.