The Key to Time is the umbrella title that links all six serials of Season 16 of Doctor Who. The arc was originally conceived of by producer Graham Williams, who had proposed it as part of his application for the producer's job in 1976. The name refers to the powerful artifact whose segments the Doctor, Romana and K-9 are searching for during the season.
A figure presenting himself as the White Guardian commissions the Doctor, Romana and K-9 to find all six segments of the Key to Time, a cosmic artifact resembling a perfect cube that maintains the equilibrium of the universe. 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, the Guardian claims that the forces balancing the universe are so upset that the segments of the Key must be found and assembled, to stop the universe so that he can restore the balance.
The first segment is disguised as a lump of Jethryk on the planet Ribos. The second is actually the planet Callufrax, but that planet has been shrunk by the space-hopping pirate planet Zanak. The third is the Great Seal of Diplos, having been stolen by a criminal of that planet. The fourth is part of a statue on the planet Tara. The fifth has been consumed by the monster squid Kroll, causing it to grow to gigantic proportions. The final segment is in fact a female humanoid — Princess Astra.
In the final episode, the purported White Guardian attempts to take the Key from the Doctor. However, the Doctor sees through the figure's charade, reveals him as the Black Guardian in disguise, and once again scatters the segments of the Key across time and space.