Circular Time (audio anthology)

 was the ninety-first monthly Doctor Who audio release produced by Big Finish Productions. Released in 2007, it was the company's first attempt at an audio anthology in the main monthly range. It featured the Fifth Doctor and Nyssa. Its critical and popular success paved the way for the release's then-unique format to be repeated annually.

The release contained four separate short stories, including:
 * Spring by Mike Maddox from a story by Paul Cornell
 * Summer by Mike Maddox from an idea by Paul Cornell
 * Autumn by Paul Cornell
 * Winter by Paul Cornell

The theme of the compilation, as suggested by the title, was the passage of time — specifically the time of the friendship between Nyssa and the Fifth Doctor. A story was taken from each of the major periods of their friendship's life — spring, summer, autumn and winter — to indicate how it matured. Each story was also of a different genre — adventure with aliens on a distant planet, celebrity historical, character piece, and high-concept adventure. Despite the presence of a cohesive theme, however, the stories shared no narrative continuity at all.

Time Lords

 * The Council of the Great Mother is a Time Lord organisation.
 * Time Lords can regenerate into non-humanoid incarnations.

Continuity

 * The idea Time Lords can assume non-humanoid incarnations was explored in TV: Mawdryn Undead, where the titular Mawdryn suffered from never-ending regenerations until his eventual death, which became increasingly mutated and inhuman as a result of a failed experiment to become immortal.