The sixteenth season of British science fiction television series Doctor Who, known collectively as The Key to Time, began on 2 September 1978 with The Ribos Operation, and ended with The Armageddon Factor. The arc was originally conceived by producer Graham Williams, who had proposed it as part of his application for the producer's job in 1976.^[1] The name refers to the powerful artefact, the segments of which are what the Fourth Doctor and his companions, Romana and K9, search for during the season. Anthony Read continued in his role as script editor, from the previous season. Douglas Adams also became script editor alongside Read.

A figure calling himself the White Guardian commissions the Doctor and K9, assisted by a new companion, the Time Lady Romana, to find the six segments of the Key to Time, a cosmic artefact resembling a perfect cube that maintains the equilibrium of the universe. Since it is too powerful for any single being to possess, it has been split into six different segments and scattered across space and time, disguised by the raw elemental power within them into any shape or size. However, since the forces balancing the universe are so upset, the White Guardian needs to recover the segments of the Key to stop the universe so that he can restore the balance. The White Guardian also warns the Doctor of the Black Guardian, who also wishes to obtain the Key to Time for his own purposes. In the final episode, the Black Guardian, disguised as the White Guardian, attempts to take the Key from the Doctor. However, the Doctor sees through the figure's charade and orders the segments of the Key to Time to once again become scattered across all of time and space, bar the sixth, which he reinstates as Princess Astra. Afterward, the Doctor decides to install a device called a randomiser into the TARDIS' navigation system for a period of time to make his following voyages unpredictable to evade the Black Guardian.

Douglas Adams took over as script editor from Anthony Read for The Armageddon Factor. Season 16 consists of one long story arc encompassing six separate, linked stories.

The Key to Time was broadcast from 2 September 1978 to 24 February 1979.