From Tardis Wiki, the free Doctor Who reference

A serial is a story told in a series of consecutively available parts in which the passage of narrative time is likewise continuous. Each episode or chapter typically ends in a cliffhanger to encourage viewers to return the next week to see how the heroes extricate themselves from the previous week's danger. Serials arose in newspaper and magazine publications. The format was transferred to the movies and later to radio and television.

The 1963-1989 version of Doctor Who was produced exclusively in a serial format, each serial having between two and fourteen episodes. Each season had between four and nine serials. Except for the majority of the William Hartnell era, the title of each episode was merely the name of the overall serial, with the episode number appended. Thus, during this era of Doctor Who, the terms "serial" and "story" were synonymous.

The serial format has been largely abandoned by the BBC Wales version of the program, although some serials, like Dreamland, The Infinite Quest, The End of Time and Spyfall, have been produced. Other multi-part stories have been produced, but the production team have been reluctant to return to a numerical naming convention. Likewise, several second parts of stories haven't been set immediately after their first parts, thereby breaking the convention of serial storytelling. The term "serial" is therefore rarely used to describe anything produced by BBC Wales. In current parlance, "episode" is usually synonymous with "story" and multi-part stories are called "two-parters", "three-parters" and so forth.

In 2020-2021, due to the outbreak of the COVID-19 pandemic, the standard BBC Wales format had to be abandoned, with Series 13, titled Doctor Who: Flux, reduced to six episodes, each forming a chapter in a serial. Like majority of Hartnell's era, each episode in the serial got its own title.

Each story of The Sarah Jane Adventures with the exception of Invasion of the Bane is also a serial, consisting of two parts each.

The third series of Torchwood was also produced as a serial, consisting of five parts, with the overall title being Children of Earth. However, rather than using the term "Parts", this production used "Day", symbolising the series taking place across 5 days rather than immediately after one another.