The Guardian of Time gives the Doctor a difficult task,and a new assistant. Together they must search for the six segments of the Key to Time, to prevent the Universe from being plunged into eternal chaos. On their mission, the Doctor and Romana travel to the planet Ribos where the first segment of the key is protected by the monstrous Shrivenzale.

The Graff takes the Doctor, Romana and Garron prisoner and then sets about trying to track down Unstoffe and the jethryk.