The Doctor is chosen for a very special mission - to retrieve and assemble the Key to Time - by none other than the White Guardian. The Doctor is also provided with a new companion, the Time Lady Romana, and warned that there is also a Black Guardian who will do all in his power to obtain the Key to Time.
The Doctor is also provided with a Tracer which will locate each of the segments, and it leads the TARDIS to the planet of Ribos, where two con-men are attempting to sell the planet to the Graff Vynda-K. To make the planet look more valuable the leader, Garron, plants a lump of Jethryk as bait, whilst Unstoffe pretends to be a local talking of a Jethryk mine.
The Graff Vynda-K discovers that Garron and Unstoffe are con-men, and pursue them into the catacombs, where the natives fear to tread. Several fights break out, and the Graff Vynda-K's men are all destroyed. The Doctor disguises himself as one, and the Vynda-K tries to use him to deliver a bomb, but the Doctor switches it back, and the Vynda-K is destroyed.
Romana uses the tracer and the Jethryk transforms into the first segment of the Key to Time.
