One of my all-time favourite Dr Who productions, when I saw it the first time round, as child, I found it pretty scary stuff. The homicide plot-line is indebted in great to Agatha Christie; and her murder mystery thrillers. There also references to Isaac Asimov's Three Laws of Robotics. What made this particular teleplay so cerebral was the way in which a very clever script, is enriched with great sets/costumes and good acting that has made this set of four episodes a real classic. You even get new psychological clinical terms such as Robophobia or 'Grimwade's Syndrome'. This was second outing for Tom Baker and his new companion and rumour has it that they were not on the best terms to put mildly.
I do not want to give spoilers, so there the briefest of plot overviews:
On a planet far far away, a colossal sand-miner factory vehicle called Storm Mine 4, is slowly mining the surface of a vast, desolate desert in search of precious minerals. The sand-miner is crewed by nine humans and plethora of different humanoid robots - the black ones are 'Dums' that cannot speak, the pale green are 'Vocs', and a silver is a 'Super Voc' which controls all the 'Dums' and 'Vocs'. The robots conduct a routine scan of the area and locate a large sandstorm (Dune reference), which the humans decide to chase, as the storm will stir up heavier minerals to the surface. A human meteorologist, called Chub goes to gather an instrument package to place into his weather dirigible to study the storm. However, he is found dead,on another part of the vehicle a familiar blue box materializes - so begins another Dr Who adventure.
It is interesting to note that sand-miner design and function were heavily influence by Frank Herbert's Dune, . Another point interest aspect, well I found interesting anyway, is that at one point in the narrative the sand-miner's engines are about to run out of overheat, threatening the factory vehicle plus crew with imminent destruction. They find the human engineer responsible for controlling the power-plant is dead, strangely enough he is called Borg!
For me this narrative had good twists and turns, and is a real must see on DVD. This particular story has to be somewhere in my top ten Dr Who adventures.