While trying to explain the dimensionally transcendental nature of the TARDIS to his new companion Leela,the TARDIS lands them inside Storm Mine 4,a gigantic mining device scraping a desert area clean for rare minerals. On board there's a crew of nine humans and an enormous amount of servile robots-the non-verbal "Dums", the green colored "Vocs" and the silver "Super Vocs". All Voc robots supposed to control the Dums and each are individually numbered. All is not well on the ship however. A crew member named Chub has been murdered. Each of the crew,led by Commander Uvanov,each suspect the other of murder to gain a foothold in the profit the all seek from the mining. The doctor and Leela are soon caught in the middle of all this,both being thought of as suspects in the murder. But inconsistencies began to pop up. Leela finds herself confronted by one of the "Dum" robots who is somehow about to talk. And the doctor,having narrowly escaped. Soon another crew member, Zilda is murdered after having suspected the Commander was in fact the culprit.
The doctor and Leela meanwhile are freed from the Commander's captivity,due to his suspicion of them,by Poul who,as apparently was Zilda's also deceased brother,suffers from a "robophobia",resulting in Poul descending into madness. After earning the trust of the remaining crew when another of them,Toos,is threatened by a Voc but Poul is not,the doctor comes to realize a mad scientist Taren Capel is behind a robot revolution happening on the mining vehicle. Being raised amid robots Capel believed they were superior to humans and had programmed them to work against the miners. Though Poul was originally an undercover agent hired on to prosecute the conspiracy,he is soon too mentally dysfunctional to accomplish his mission. In the end the doctor is able to use a homemade explosive device to immobilize the robots,as well as Capel's own device to end his unhappy existence before departing with Leela in the TARDIS.
This story is an extremely well written who dunnit story. Not only is it an excellent vehicle for Louise Jamison's Leela,adapting extremely well to the strange environments she thrust upon herself by joining the doctor,but also to the character set up the story itself provides. The crew members of the Storm Mine 4 are all motivated purely by their own greed and pettiness in accusing each other of murder. And each of them never suspecting in any way their robot servants who have been reprogrammed. The story of money clouding judgement is broken largely by Poul who,through his own fears realize their mineral bounty is not as important as the other miners think it to be. The dysfunctional protagonist Taren Capal is a human being believing himself to be a "brother" to the Vocs and Dums. It's up to the doctor,the level headed outsider,and his clever new companion Leela to show everyone involved the right direction to take themselves in this compelling tale.