The Doctor is set a quest by the White Guardian, a powerful being who monitors and controls time. He is to collect and assemble the Key to Time, the disguised segments of which are scattered through time and space. Once assembled, the Key will be used by the Guardian to restore the balance of the Cosmos. The Doctor has little choice in the matter and is given a new Time Lord assistant, Romanadvoratrelundar, or Romana for short, and a locator to help him find the segments and convert them into their true form.

The first segment is traced to the city of Shur on the planet Ribos where a con man 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. Unstoffe, disguised as a guard, tells the Graff an elaborate tale of hidden maps and caves full of the blue mineral, which he claims the locals call scringe-stone.

The Doctor realises that the lump of jethryk is the first segment, but before he can steal it, Unstoffe spirits it away again. (It was he who placed it in the case in the first place.)

The Graff realises the deception and sets about hunting the con men down, while they, together with the Doctor, Romana and K-9, head for the catacombs under the city.

The Graff captures the travellers and the con men, takes back his gold and confiscates the jethryk. However, before he can kill his captives, the Riban guards blow up the entrance to the catacombs. The death of his officer and friend Sholakh in the ensuing rock fall unhinges the Graff's mind. He storms into the dust to be killed by a thermite bomb with which he intended to destroy the catacombs entirely. The Doctor meanwhile reaches safety with the jethryk.

Unstoffe and Garron leave with the Graff's ship of plundered loot, and the Doctor, Romana and K-9 leave with the first segment of the Key.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}