The Doctor meets the White Guardian

While busy planning a holiday the TARDIS goes dark and a mysterious voice summons The Doctor from outside. There he discovers The White Guardian, a powerful being who monitors and controls time. The White Guardian recruits The Doctor and sets him on a quest to collect and assemble The Key to Time, the disguised segments of which are scattered throughout time and space. Once assembled, the Key will be used by The White Guardian to restore the balance of the Cosmos. The White Guardian however, warns The Doctor about The Black Guardian who also seeks these segments, but for an evil purpose.

The Doctor has little choice but to accept The White Guardian’s request and is given a tracer to help him locate the disguised segments and then convert them into their true form. The White Guardian also informs The Doctor that a new recruit from the Time Lord Academy on Gallifrey will assist him on his quest. This is Romanadvoratrelundar, or Romana for short. As The Doctor is soon to find out Romana may be inexperienced but this female Time Lord believes herself to be just as capable as The Doctor, who would rather seek The Key to Time on his own.

When The Doctor returns to the TARDIS he discovers that Romana is already in his time machine waiting for him. After introductions The Doctor places the tracer into the TARDIS console and they discover that the first segment is located on the planet Ribos.

The White Guardian

On arriving there they discover Ribos is an icy planet with late-medieval-type inhabitants who are unaware of alien cultures. But in the city of Shur a conman named Garron and his assistant, Unstoffe, are engaged in a scam to sell the entire planet to the Graff Vynda-K, deposed ruler of Levithia, who wants to use Ribos as a base from which to plan the winning back of his throne.

The Graff is particularly interested in Ribos for it is apparently a rich source of Jethryk, a rare and very valuable blue mineral vital for space warp drive. His interest is further piqued when he sees a large lump of the mineral in a display case of jewels in a reliquary. Arriving there The Doctor, after using the tracer, realises that the lump of Jethryk is the first segment, but he is unable to steal it. Unstoffe meanwhile, disguised as a guard, tells the Graff about a nearby lost mine with caves full of the blue mineral, which he claims the locals call scringe-stone.

Romana

Convinced that he is being told the truth the Graff provides a large sum of gold, as a deposit for the planet, under the agreement that is to be kept safely in the room with the crown jewels, watched by Ribos guards by day and a Shrivenzale beast by night.

The Graff however, is being tricked by the conmen and so later Unstoffe distracts the shrivenzale, recovers the piece of Jethrik (which he had earlier placed in the case), and takes Graff’s gold from a safe.

The Graff, on discovering the trick that has been played on him, and that he has lost all his gold, sets about hunting the conmen down along with their "accomplices" The Doctor and Romana. The Graff’s search leads him to a labyrinthine of catacombs under the city where The Doctor, Romana and K9 have already entered in their own hunt for the lump of Jethryk.

Garron Negotiates with the Shrieve Captain

The Graff captures the The Doctor, Romana and K9 and the conmen, takes back his gold and confiscates the Jethryk. However, before he can kill his captives, the Ribos guards blow up the entrance to the catacombs causing the ceiling to collapse on the Graff’s men.

With the gold and the piece of Jethrik, the Graff gives his last surviving guard an explosive to kill himself with. The guard, actually The Doctor in disguise, swaps the explosive for the Jethrik. Unaware of the swap The Graff storms off into the maze yelling like a madman - the death of his loyal officer and friend Sholakh in the rock fall having unhinged the Graff's mind – and is killed by the thermite bomb.

After leaving the Catacombs, Unstoffe and Garron depart with the Graff’s ship of plundered loot, leaving The Doctor, Romana and K9 to return to the TARDIS with the Jethrik.

Back inside the TARDIS, and at The Doctor’s invitation, Romana touches the tracer to the lump of Jethrik, which is transformed into the first segment of The Key to Time.