The Master (Roger Delgado) has stolen some top secret information giving details of one of the most destructive weapons in the known universe, and it is up to the Doctor (Jon Pertwee) to get that information back. The Doctor's quest takes him to the planet Uxarieus, where he becomes caught up in a dispute between the human settlers and the Interplanetary Mining Corporation. Meanwhile, the Master remains at large, and with murderous robots and rampant lizards to contend with, the Doctor's search for the all-important doomsday machine become evermore difficult.