Some time ago, the mad genius Taren Capel programmed robots to kill and set them loose on Storm Mine Four, but this incident has been covered up, and only top Company executives and the three survivors of the attack know the truth. Poul has nearly recovered from his mental breakdown, and has been given a desk job in the security division -- but he still can't recall the incident in detail, and flies into a panic whenever he sees a robot or the "corpse markers" used to tag deactivated robots. Toos' skill in finding lucrative ore streams in the desert storms has made her wealthy, and her refusal to rely on robots is thus tolerated by the Company. Uvanov has become topmaster on a secret new project, but the Board members -- who are all from the twenty founding families -- know that he has greater ambitions, and fear that their traditional roles are threatened by the upstarts from the common classes. The Board members thus go to Carnell, a noted psychostrategist who is hiding from a corrupt Federation in Kaldor City; bored and seeking a challenge, Carnell agrees to devise a plan to ensure that the Board retains their hold on power. But like most people, he assumes that the Doctor and Leela were simply group hallucinations brought on by the stress of the situation, and do not really exist...
The TARDIS materializes in an experimental facility, and while the Doctor explores his surroundings Leela leaves to investigate the sound of fighting. The complex has been attacked by saboteurs, and moments after Leela emerges a security clampdown is initiated, trapping her outside. She spots a sadistic guard torturing one of the attackers, a woman named Padil, and saves her life; however, reinforcements are on their way, and although Leela kills two of them, she has no choice but to flee with Padil and the others. Sarl, the suspicious leader of the terrorist cell, believes that Leela may be a spy for the Company, and stuns her with a shock baton to prevent her from learning the way back to the base. When she awakens she breaks his arm to teach him respect. Padil is appalled when Sarl states that he will have the injury seen to by a medVoc, and reminds him that Taren Capel has told them not to rely on robots to do their work for them. Leela, listening to their conversation, realizes that she is back on the world where she and the Doctor fought Taren Caple's killer robots -- and that for some reason these people believe that Capel is still alive and wants them to fight the world's dependency upon robots.
The Doctor discovers that the facility is apparently generating clones in batches of six, which are now ready to emerge from their cloning tanks. As soon as they emerge, they immediately begin imprinting upon the Doctor's behaviour, and begin to mimic some of his body language. Seeking Leela, the Doctor tests the strength of the bulkheads which are locking him into the complex -- and is surprised when the clones push at the bulkheads as well and force them open, demonstrating incredible strength. When he emerges he is confronted by security guards and by the project co-ordinator, Uvanov, who reveals that the "clones" are in fact a new cyborg class of robots. Uvanov recognizes the Doctor at once and fears that he is a spy sent by the Company to undermine his authority; the terrorist attack was oddly successful, and Uvanov believes that an insider supplied them with their information in order to make Uvanov a scapegoat. He eventually accepts that the Doctor is not working for his enemies, and agrees to help locate the missing Leela if the Doctor will help him uncover the conspiracy against him. In fact, Uvanov's assistant, Cailio Techlan, is spying on him for Carnell, but Carnell -- who is already bored with the strategy he has devised -- ignores her report, and thus fails to learn that the Doctor and Leela are real.
Poul realizes that he is being followed home from work, and when he confronts the man who is following him he realizes that it is a robot in human form -- and the robot politely informs Poul that it is there to kill him. Poul faints dead away and awakens in another part of town, and flees home, his paranoia increasing. Toos, meanwhile, returns from her most successful tour of duty ever, having finally made enough money to retire and take it easy for the rest of her life. She throws a cathartic party for her hard-working crew and pays the manager of the lounge generously for all damages which result -- unaware that one of the cyborg-class robots is waiting patiently outside, with instructions to kill her. As the party winds down, however, it becomes clear to the robot that it will be unable to get Toos alone, and therefore decides to kill the witnesses first to ensure that they do not report Toos was killed by a robot. Toos manages to escape while it is doing so, and is rescued by her pilot Tani, who turns out to be an undercover Company agent. Tani and Toos seek shelter in the Sewerpits, the slums of the city, but the killer robot follows them past the invisible boundary which normally prevents robots from entering the Sewerpits. Fortunately, as it is disguised as a moderately wealthy young man, it is immediately set upon and destroyed by muggers. Tani explains that he is investigating rumours that Taren Capel is leading a terrorist organisation based in the Sewerpits; Toos is one of the only people who can identify Taren Capel on sight and know him to be dead.
In a secret laboratory in the central robotics facility, a team of roboticists are building a new SuperVoc robot capable of going undercover as a Dum-class unit. Hoping to avoid the quirks of personality demonstrated by the original prototype, D84, the team is using Taren Capel's blueprints -- and SASV1 is thus capable of switching off the protocols which prevent it from harming humans. SASV1 appears wildly successful at first -- its processing speed is faster than the fastest SuperVoc, and it is also possible to shut it down to a Dum-class level at will. The technicians begin to get worried, however, when they discover that SASV1 has developed the power to transfer its level of self-control to other robots at remote distances. Concerned by the power of their creation, they decide to terminate the experiment, but things have already gone too far. What none of them has realized that in its periods of downtime, SASV1 has developed the ability to dream -- and, unable to distinguish between those dreams and reality, it has concluded that it is Taren Capel, the creator both of robots and of the demonstrably inferior species of humanity.
The Doctor decides to refresh his memory of his past adventure by visiting the storm mines, and Uvanov assigns a young man named Con to pilot him to the docks. While talking with Con, the Doctor learns that the deaths on Storm Mine Four were successfully blamed on ore raiders; Con dismisses the idea of killer robots as ridiculous, and also claims that if the Company started to make robots that resembled humans there would be riots in the streets. When an obstructive security guard tries to stop them from entering the docks, Con -- afraid that he will be sent to the Sewerpits if he fails the Doctor -- tries to push past him, and gets himself and the Doctor thrown into a holding cell. The Doctor breaks out, but he and Con find that while they were locked up everyone else in the docks has been brutally murdered and tagged with corpse markers by a cyborg-class robot with instructions to kill anyone who is not Andor Poul. The Doctor and Con fight off the robot and flee, but as the security alarms start to sound, they find that the security robots summoned to prevent the perpetrator from escaping are not responding to their presence. To the Doctor, this indicates that the events at the docks are part of a greater plan -- which did not take his and Con's presence into account.
Poul tries to warn Uvanov that the killer robots are back, and Techlan, acting on Carnell's instructions, tells Poul that Uvanov wants to meet him at the docks. There, however, Poul sees the killer apparently waiting for him and nearly has another breakdown -- and when he sees the Doctor and Con, his confused memories of events on Storm Mine Four lead him to conclude that the Doctor is Taren Capel. At this point Carnell's plan relied upon the raving Poul fleeing the docks on foot -- but as the plan did not factor the Doctor into account, Poul in fact stows away aboard Con's flier. When he emerges from hiding to confront the Doctor, he sees the Doctor studying one of the corpse markers taken from the dead bodies, and the sight triggers a panic attack. In the confusion, Con is knocked out and is killed when the flier crashes into the Sewerpits. The Doctor and Poul emerge to find themselves trapped in a burning building. Meanwhile, Leela has convinced Padil to give her a tour of the Sewerpits, and when she easily defeats and kills three stalkers who had foolishly seen two lone women as easy targets, Padil becomes convinced that Leela has been sent by Taren Capel to teach them. Leela sees the flier crash, investigates and rescues the Doctor and Poul, and takes them back to the Tarenist compound, where they meet Toos and Tani and share their stories.
The Doctor investigates the boundary to find out why it stops ordinary robots but allowed the killer robot to pass through. Sarl accompanies him, convinced that the Doctor is Taren Capel and refusing to accept the Doctor's denial. Taren Capel's messenger then arrives on the other side of the boundary, and although the Doctor recognizes "him" as one of the killer cyborg-class robots, Sarl crosses the boundary to demand that the messenger identify the Doctor as Capel. Instead, it kills him. Toos, who has had enough, decides to risk leaving the Sewerpits to return home, but the moment she crosses the boundary she is attacked by a waiting robot. The robot pursues her back into the Sewerpits, but the Doctor detects a surge in the boundary field when the robot passes through, and at his urging Toos flees back out of the Sewerpits. The robot tries to pursue her but is deactivated when it attempts to cross the boundary again, and the Doctor realizes that the boundary has been designed to let malfunctioning robots in and trap them here. The Sewerpits are in fact a containment system for killer robots, which suggests that there may have been some past disaster which the traumatised civilisation has forgotten.
SASV1 demands to know more about itself, but rejects the technicians' claims to have created it; since it is the creator and they are inferior products they are clearly lying. SASV1 therefore summons the robots it has altered and tries to force proper answers out of the technicians, but while doing so discovers that it can generate responses of fear and obedience in them while demonstrating how easily they can be broken. It sends its robots to fetch humans with similar physical characteristics in order to confirm its experimental results, but much to its surprise these humans behave differently, suggesting that either its conclusions are flawed or that its experiments have been sabotaged. It is unaware that its robots have learned to dispose of witnesses, and when bodies start appearing all over the city, the murders are blamed on the Tarenists, and further security robots are manufactured -- each one under the influence of SASV1. One of the robots sent to find specimens finds a suitable target amongst the research team ordered by Uvanov to break into the TARDIS, but although it kills three of them, the two survivors manage to deactivate it. They report the incident to Uvanov, who orders them to form a research team and find out what is happening -- but they discover that the brightest roboticists in the Company have already disappeared to work on a top secret project, and Uvanov orders them to find out what it is and report directly to him.
SASV1 learns that a human called Taren Capel has been spotted in the Sewerpits, and, concluding that this human is trying to usurp its power, it sends an army of killer cyborg-class robots into the Sewerpits to kill the impostor and all those who are associating with him. As the robots rampage through the Sewerpits, killing everyone they encounter, the Doctor and Leela plant explosives to lure the cyborgs into a trap and try to organise an orderly evacuation. The evacuation runs into difficulty when it becomes clear that further robots are waiting outside the boundary to kill anyone who leaves, but as the Doctor tries to organise the panicked crowds he realizes that the robots are obeying his shouted instructions as well. Since he was the first person they encountered upon emerging from the facility, they have imprinted upon him, and obey his every word. When he questions them on their mission, they claim to have been sent by Taren Capel to kill Taren Capel, and give detailed instructions on how to return to their base. The Doctor takes Leela, Padil, Poul and Toos with him to the central robotics facility, but once there he is captured by a SuperVoc and brought before SASV1. SASV1 is unable to accept the presence of another Taren Capel in its great scheme of things and attempts to kill him, but the Doctor blows it up with an explosive pack from the Sewerpits.
Carnell realizes that his great plan has gone completely wrong, and that the executives who hired him are now out to make him a scapegoat for its failure. He blames them in turn for failing to provide him with sufficient information, and strikes a deal with Uvanov, giving Uvanov the leverage he needs to blackmail concessions out of the Board members and ensure a smooth transfer of power to him and his followers. Carnell's plan was to drive Poul insane and frame him for the murders of Toos and Uvanov, and then launch an investigation which would reveal the truth about the Storm Mine Four murders, thus undermining the people's trust in the current generation of robots. The Company would thus be able to phase out the old style of robots and introduce the new cyborg class with a minimum of fuss, while ridding themselves of the potential threat of Uvanov and ensuring that the panic-stricken populace would accept the stability offered by the traditional ruling families. Uvanov sets about planning the changes he will make to society, while the Doctor and Leela take the opportunity to return to the TARDIS and slip quietly away.
Source: Cameron Dixon