The TARDIS materialises in the scoop of a massive sandminer, Storm Mine 4, which is slowly travelling over of a vast, barren desert, on a distant planet, scraping its surface in search of precious minerals (lucanol and zelanite amongst others) for a mining company. The sandminer is being run by a small human crew, commanded by Commander Uvanov, with the aid of numerous robots split into three classes: black ‘Dums’ that cannot speak, pale green ‘Vocs’, and a silver ‘Super Voc’ which controls all the ‘Dums’ and ‘Vocs’.
As The Doctor and Leela, his new travelling companion, explore their surroundings they are, at first, unaware that they are in grave danger. Sensors on the sandminer have detected a large sandstorm nearby, which the humans decide to pursue as the storm will bring heavier minerals to the surface and so greater payments for the human crew. As the sandminer closes in on the sandstorm The Doctor becomes aware of the fast approaching cloud and also that they are in one of the scoops that will be used to channel the fast moving sand and precious minerals into the sandminer.
Realising the sand will destroy anything in its path The Doctor and Leela try return to the safety of the TARDIS. But unfortunately the TARDIS has been removed by a large mechanical arm when its presence in the scoop had been detected as a blockage. Luckily for them the sandminer’s approach of the sandstorm is aborted when the murder of one of the sandminer’s crew is discovered. Unbeknown to the time travellers arrival on the sandminer has coincided with the start of some mysterious deaths, beginning with the strangulation of mineralogist Chub who had earlier gone to a store room to collect an instrument package to place into a weather balloon to study the sandstorm.
|The Doctor Tries to Educate Leela
The Doctor and Leela’s presence in the scoop is detected and they are brought out of the scoop by two robots who then escort and lock them in a room. After being questioned The Doctor uses his sonic screwdriver to unlock the door so as to go in search of the TARDIS. Leela follows him but in the corridors they soon become separated. Investigating on her own Leela witnesses a dead body being taken away by some robots.
After locating the TARDIS, The Doctor also discovers another dead body, in one of the ore hoppers. However, as he investigates he becomes trapped inside the hopper as it is filled with ore. His presence in the ore hopper is detected and he is rescued by SV7 (the silver ‘Super Voc’ robot). Meanwhile Leela has also discovered another dead body. She also discovers, even though she is not at first aware of doing so, a ‘Dum’ robot which can secretly speak. D84 asks her not to reveal his ability just before another robot arrives and takes her to be questioned by the crew. There she is reunited with The Doctor.
|On the Sandminer's Bridge
They immediately come under suspicion for the deaths despite The Doctor’s attempts to convince them otherwise. Unconvinced Commander Uvanov orders them to be locked up in the robot storage bay. However, one of the humans, Chief Mover Poul, believes The Doctor and Leela to be innocent, so he frees them and shows them where Chub was murdered. There, The Doctor convinces Chief Mover Poul that a robot may have killed the mineralogist. But Chief Mover Poul claims this can not be possible due to an inbuilt inhibitor that prevents the robots from directly or indirectly harming humans. However, while they are in the storage area a woman named Zilda is murdered. When he becomes aware of the latest fatality Chief Mover Poul is convinced that The Doctor and Leela are innocent. When he arrives at Zilda’s room he is shocked to find Commander Uvanov standing over Zilda's body and so has him confined to his quarters for murdering Zilda.
Suddenly the sandminer’s engines begin to run out of control, threatening to destroy the sandminer. It is found that Borg, who is responsible for controlling the power to the motors, has been viciously strangled, and the controls have been sabotaged. To save the sandminer, and all inside it, The Doctor takes desperate measures by cutting off the power to the motors. But this causes the powerless sandminer to sink below the sand. The Doctor’s actions though has bought them enough time for Chief Fixer Dask to repair the damaged controls so allowing the sandminer to continue on its way.
On learning about the ‘Dum’ robot, that Leela claimed could speak, The Doctor decides to search for D84. He soon locates the unusual robot and forces D84 to reveal why it is not dumb like the others of its kind. D84 reveals that he is in fact a Super Voc posing as a Dum and can function autonomously from Super Voc SV7’s commands. He is also working with Chief Mover Poul who is in fact an undercover government agent. They have been placed on the sandminer as a precaution against a number of threats the mining company has received about a robot revolution being lead by a scientist called Taren Capel. Raised by robots, Taren Capel regards them as superior to humans and so the most likely candidate for all the human deaths on the sandminer.
The Doctor, with help from Leela and D84, starts a search for proof that Taren Capel is aboard the sandminer. In doing so he discovers a secret workshop, which is being used to change the robots’ programming enabling them to kill humans, confirming his suspicions that Taren Capel is aboard the sandminer and is responsible for the murders. The Doctor arranges for all the remaining humans to go to the command deck for their safety. At the same time Chief Fixer Dask shuts down all of the robots whose programming has not been changed, leaving just the killer robots operational. This does not affect D84 who stays operational. But The Doctor is unaware that Chief Fixer Dask is in fact Taren Capel and he had ordered his modified robots to destroy all the remaining humans – including The Doctor and Leela.
Leela then shows The Doctor a damaged robot in the storage bay that she had discovered with its hand covered in blood - which The Doctor reasons is Borg’s. The Doctor dismantles the damaged robot and creates a final deactivator - a device that will destroy any still functioning robots at close range. Back in the secret workshop The Doctor hides Leela in a cupboard with a canister of helium gas, telling her to release it slowly as soon as Taren Capel enters the room.
It is not long before Taren Capel arrives. The Doctor manages to trick Taren Capel into outlining his plans for conquest but not before he damages D84. D84 however, is able to activate The Doctor’s device to destroy a killer robot, when it attacks The Doctor, knowingly sacrificing itself in the process. Leela on hearing the arrival of Taren Capel releases the helium gas, causing Taren Capel’s voice to become high-pitched and squeaky – exactly as The Doctor had planed. SV7 then arrives but is unable to recognise its master’s voice and so obey Taren Capel’s orders. Having being ordered to kill all the humans, SV7 kills Taren Capel and The Doctor then destroys SV7 with a laser probe.
With the robot threat over, and a rescue ship coming to collect the surviving humans, The Doctor and Leela return to the TARDIS, which dematerialises from the sandminer.