Reluctantly cancelling his well-earned holiday, the Doctor sets off in the TRADIS to trace and re-assemble the six segments of the Key to Time on which the stability of the entire Universe depends. Assisted by the argumentative Romanadvoratrelundar and K9, he lands on the planet Ribos for the first segment and finds himself entangled in the machinations of two sinister strangers, Garron and the Graff Vynda Ka. Who are they? Risking his life within the monster-infested catacombs of Ribos, the Doctor has to use all his wit and ingenuity to find out...