Aboard the Haven, a vast city in space, two rusty service drones named Servo and Diode are fleeing for their lives. They’re not in the best of condition and Diode’s hydraulics are starting to seize up, but Servo knows what their hunters will do to his friend and refuses to leave him behind. Diode pleads with Servo to get away while the others are busy with him and tells him to escape through a nearby access hatch. Moments later, a huge bulkhead bursts open and the Cannibalists approach the incapacitated Diode. Their leader, Titus, looks the grimy robot over with distaste, but is confident there are still some parts worth salvaging. Ripping him apart, Titus pulls out Diode’s vital parts and hands them out to whichever of his gang most needs them. Diode begs to be deactivated and allowed to die, but Titus ignores him and rips out his ’eyes’, which he then attaches to himself. Diode’s memory chips are then removed and eventually his batteries are drained off and used to re-charge the others. Once the job is complete, the gang cheer at their success and resume the search for Diode’s friend Servo…

The TARDIS materialises in a cold, dark metal corridor. The Doctor and Lucie notice the smell of hydrogen sulphide in the air which reminds Lucie of old car batteries. She guesses they must have landed inside an old factory. The Doctor finds some vacuum tubes and induction coils, but they’re all rusted solid. He opens a nearby solar screen which reveals that they’re hanging in space. He concludes that they must be inside a star city, a place that would normally be home to 100,000 souls, but the TARDIS scanners hadn‘t detected a single source of life. Apart from themselves, the city is completely dead.

Unbeknownst to the Doctor and Lucie, their conversation is being monitored by three Assembler robots, Domitian, Probus and Macrinus. They realise the two intruders must have only just arrived and they’re the first organics ever to have visited the station. The robots have never seen living beings before and they chuckle excitedly at the thought that their legend of humans returning one day has finally come true. Their protocol states that all new arrivals must undergo hazard evaluation, but the two visitors have already left the corridor and have entered the lower transit. The Assemblers are horrified as this is the territory of the Cannibalists! It looks like the visitors won’t be living beings for very much longer…

Lucie is getting a bit tired of wandering down endless cold, dirty tunnels. The Doctor can sense something wrong here, but then they hear the sound of a metal door opening and closing nearby. When Lucie points out that there’s not supposed to be anyone else here, he reminds her he was talking only about organic life. Just then an angry metallic voice calls out to them, then it summons support from others. A group of hostile looking robots appear in the corridor and charge towards them…

The Assembler robots are watching events from their control room. They begin to panic as the Cannibalist robots move in on the Doctor and Lucie, but there’s nothing they can do as protocol states they’re not permitted to leave the Spire of Assembly. However, Domitian thinks there’s another way they can help the visitors…

The Cannibalists don’t look at all friendly and Lucie likens them to a cross between a biker gang and a cutlery drawer. The Doctor suggests a swift retreat back to the TARDIS, but suddenly a huge metal bulkhead slides down and separates him from his companion. Trapped in the corridor with the Cannibalists and with his sonic screwdriver unable to open the bulkhead, the Doctor tells Lucie on the other side to keep running. She goes to find help as the Doctor turns to face the advancing robots.

The other Assemblers turn to rebuke Domitian. His idea to lower the bulkhead was a disaster as he’s trapped one of the organics in with the Cannibalists.

Titus and the Cannibalists examine the Doctor with curiosity. They wonder if he’s a menial-grade maintenance drone, but notice that he’s covered in some sort of strange fabric. They scan him and discover he’s not a robot at all, which disgusts them. They’ve never seen anything like him before and decide the best thing to do is find out more about him - by dissection. The robots activate their appendages and prepare to slice the Doctor open to see what makes him tick. The Doctor quickly activates his sonic screwdriver and the Cannibalists all retreat in pain. Titus orders his fellow gang-members Crusher and Ripper to attack and they grab hold of the Doctor and push him against the service vents. Suddenly a vent opens up and the Doctor falls through into the access duct, his scream fading away as he disappears into the darkness. Titus calls down after him, warning him that they’ll be coming after him. They turn their attention to the bulkhead and quickly break it down…

Lucie soon finds herself lost in the maze of dark corridors. As she stops to find her bearings, a door slides across, trapping her inside a lift. It starts to ascend rapidly and an automated voice tells her she’s in the Spire of Assembly. The lift comes to a halt and the door opens to reveal three robots who introduce themselves as Assemblers Domitian, Probus and Macrinus and explain that their role is to construct, maintain and repair the Haven according to their protocol. Lucie thinks they look more like vacuum cleaners than robots. The Assemblers take great offence and say that cleaning is a job for menial-grade sanitation drones. Lucie tells them she has a friend who’s in trouble, but they reassure her he’s still alive and that he managed to avoid the Cannibalists.

The Doctor’s fall down the access duct was broken by a pile of scrap metal at the bottom. He hears a noise nearby and calls out to the robot who’s trying to hide in the shadows. Nervously, the droid emerges from cover and introduces himself as Servo. It was he who saw the Doctor had been trapped by the Cannibalists and opened the access duct. He explains that the Cannibalists kill other robots and make use of any functioning parts for their own meta-bionic drive systems. He’s now the only servo-grade maintenance droid left and he’s in a bad way as his wheel treads have became misaligned and he can no longer stand up. The Doctor offers to repair him and before long Servo’s motor systems are functioning at maximum efficiency again.

The Assemblers are overjoyed when they discover Lucie is human and declare that their legends were true after all. Lucie points out that she’s only a visitor and has nothing to do with this place. Domitian produces an archive recording of a human scientist but because Lucie doesn’t look anything like the middle aged man, he believes it proves Lucie can’t be human. For example, she’s a lot smaller than the man in the recording, she doesn’t have any facial hair-growth and she has two unorthodox protrusions emerging from her body. Lucie has to explain that humans come in two different genders. The other Assemblers are convinced by her words, but Domitian says that if she really is human, she‘ll be able to give them new protocols to save the Haven. He explains that things started to go wrong here when the maintenance drones develop malfunctions. It began with errors in their guidance systems, but then their positronic brains became corrupted beyond repair and somehow they found a way to override their control programmes.

Servo tells the Doctor about the legend that says humans will one day return to the Haven and make it their home. The robots have been preparing the city ever since, but no one ever came. The Doctor looks around the dingy corridor and points out that they’ve let things slide a bit. In fact, the whole place is a wreck and he suspects it’s been like this for a long time. Servo explains that the maintenance drones broke their programming and became the Cannibalists. They’re now rogue robots with free will, intent only on prolonging their own survival by using other robots as spare parts. Servo admits that he doesn’t follow the original protocols either, but he’s not like the others and he doesn’t want to hurt anyone. He’s embarrassed to admit it, but all he’s interested in is writing poetry!

Lucie tells the Assemblers that the Doctor will know what to do. Suddenly an alarm sounds and the three robots panic as the Cannibalists arrive outside the Spire of Assembly. Titus’s voice bellows at them over the communicator, revealing that his gang has come to collect the runaway drone who they believe is hiding here. The nervous Assemblers try to convince him that he’s mistaken, but Titus refuses to listen and says that if they don’t hand over Lucie, they’ll break in by force and take her themselves. He gives them 60 seconds to decide. Domitian is keen to do what they say, but the others argue that Lucie is human and the Cannibalists will kill her. While they’re arguing, Titus comes back to say he can’t be bothered waiting any longer and his gang are on their way up now…

Servo is expecting the Doctor to make fun of him, but in fact the Doctor is genuinely intrigued as he’s never met a robot that writes poetry before. He asks to hear some and Servo reads him an example. Although the poem is constructed from machine code (and sounds like a random combination of numbers and letters), the Doctor is able to speak the language and assures Servo that it’s very moving. However, he’s starting to get worried about what the Cannibalists are up to…

At the Spire of Assembly, the Cannibalists have broken into the elevator shaft. Domitian blames Lucie for what’s happening and demands that she gives them new instructions. She suggests they build a barricade and they start shifting some control panels towards the lift door. Once that’s done, Lucie asks if there’s another way out of here, but it seems they’re trapped as there are no other exits. In a state of panic, she asks if they can turn the power off. The Assemblers believe she’s asking them to press the ‘reset switch‘, but they tell her this is nothing more than a myth. Before they can explain further, the Cannibalists burst into the control room. Titus says he’s been meaning to visit here for some time as he’s heard they’ve been hoarding some decent technology. The Assemblers accuse them of contravening their protocols and order them to leave, but Titus refuses to be threatened by decrepit old rust-buckets like them. He picks up Lucie, believing her to be a new robot fresh off the production line, and literally drags her out of the control room, then the other Cannibalists surround the three Assemblers and force them to follow…

Servo explains that the Cannibalists spend most of their time in the lower transits and they only come to the upper levels to hunt when they need new parts. The Doctor wonders why they would choose to skulk in the basements when they could be roaming freely throughout the city. He begins to suspect that they’re actually hiding from something. Servo finally admits that there’s another kind of robot living on the Haven, one that even the Cannibalists are afraid of…

Lucie struggles against the grip of Titus, but she’s unable to prevent him from dragging her down a series of corridors towards a huge set of metal doors. The three Assemblers, who are being forced by the other Cannibalists to follow close behind, recognise the area and begin to tremble. They’ve arrived at the Generation Furnaces, the section of the Haven that powers the entire city. The ageing robots know that their parts are too antiquated and worn out to be of any use to the Cannibalists, so they’ve been brought here to be melted down and reduced to liquid metal…

The Doctor demands to know more about the other robot that lives on the Haven, but Servo says no one here ever speaks of her, because if they do, she’ll come for them. It’s said that she eats memories and no one has ever seen her without their hard-drives being de-fragmented beyond recognition. Suddenly Servo is overcome by fear. He tells the Doctor that by thinking about her, he’s somehow summoned her here. The Doctor watches in amazement as the mysterious robot appears. Servo warns him not to get too close, but the Doctor approaches it and demands to know what it is.

The robot confronting the Doctor introduces herself as Minerva and says she’s the voice of the station. She analyses the Doctor and discovers he’s organic, then asks him for his instructions. Realising she’s a cybernetic access point for the control system of the entire star city, he asks her if she knows where his friend Lucie is. Minerva reveals that Lucie is at the Generation Furnaces, accompanied by the Assemblers Domitian and Macrinus and the drone designated Titus. Minerva presents an image on the wall showing the Cannibalists threatening to throw Lucie to the flames unless she’ll give them details of her meta-bionic drive systems. The Doctor pleads with Minerva to stop them, but she asks for more specific instructions. He tells her to shut down the Furnaces and she agrees to his request.

The Cannibalists are disappointed when the flames in the Generation Furnaces go out and Titus assumes there must be a fault in the circuit breaker.

The Doctor asks how long the city can survive without the Furnaces and Minerva says the energy reserves will last only two hours. The Doctor asks her to show him to the area where Lucie is being kept, but before they leave he tells Minerva that his new friend Servo is afraid that she wants to eat his memories. She confirms that she is capable of performing cerebral erasure on him if that’s what he requests, but the Doctor gives her a strict order not to do anything to Servo’s mind under any circumstances. Minerva agrees and when Servo nervously comes out of hiding, the three of them head for the Furnaces.

Despite their best efforts, the Cannibalists are unable to get the Furnaces alight again. Titus wonders what caused the flames to go out as it‘s never happened before and Lucie claims that she was responsible. She reminds them of the legends about humans coming back to the station to take charge and says that if they don’t release her and the others, she’ll switch them all off right now. Titus isn’t convinced for a second and challenges her to prove it by deactivated them now. Domitian tries to back Lucie up, but Titus starts to attack him violently and Lucie is forced to admit that she was bluffing. Just at that moment, the Doctor arrives, accompanied by Servo, and demands that the prisoners be released. Encouraged by Titus, the Cannibalists laugh at the Doctor. He assures them he’s not making idle threats and gives Titus one last chance to co-operate, but when he refuses again, the Doctor calls out Minerva’s name.

Minerva appears and the robots back away in terror, convinced that she’s about to eat their brains. Titus persuades them to calm down and assures them they have nothing to fear. The Doctor points out that their attempts to get the Furnaces alight by setting the gas outflow to maximum is very dangerous. He calls on Minerva to light the gas and in seconds, the Cannibalists are completely enveloped in flames. As they race around in panic, trying to put each other out, Lucie and the remaining Assemblers use the distraction to join the Doctor, Servo and Minerva. They cross through a hatchway and prepare to close it behind them. By now Titus has realised Minerva is following the Doctor’s voice, so he synthesises the Doctor’s voice pattern and summons Minerva to join him. She accepts the command and heads back towards the Furnaces. The hatchway closes, sealing her in with the Cannibalists. The Doctor orders Servo to open the hatch again, but the robot is reluctant to do this as it would enable the Cannibalists to escape too.

The flames have been put out and most of the damage to the Cannibalists is minimal. Titus has lost his neural transmitter, so he cruelly tears out one of the synaptic relays from the brain of his follower, Ripper. He then orders Crusher to find a way to open the hatchway while he gets to work on Minerva. The other robots want to have her stripped down and cannibalised before she has a chance to wipe their brains, but Titus has a better idea. Minerva is the cybernetic control grid for the whole city, so Titus commands her to open the hatchway. She does so and the robots realise how useful it’s going to be to have her on their side.

The Doctor, Lucie, Servo and the two Assemblers stop once they’ve reached a safe distance. Macrinus reminds Lucie about the reset button, but Domitian dismisses it as a myth. Lucie explains to the Doctor that there’s supposed to be a switch somewhere in the Haven that will return everything to its proper setting. It would re-boot the whole city, erasing everyone’s memories and replacing them with the original control programmes from a back-up disc. It would be a chance to start up all over again with a clean slate. Domitian insists there’s no such button and believes it’s just a metaphor, but Macrinus disagrees and wants to find it. Lucie points out that it means he too would be deactivated, but Macrinus thinks it’d be worth it just to see the Cannibalists stopped.

Minerva has been monitoring the conversation of the Doctor’s party and transmits a projection to Titus. The Cannibalists realise the Assemblers plan to shut them all down, so they decide to stop them before they can find the button.

Macrinus suggests they head for the Diaconian Spire, a section of the Haven that’s never used. He reveals that they’ve been living here for 50 years and the job of the Assemblers was to construct, maintain and repair the city before the humans arrived. Lucie learns that Servo likes to write poetry and he becomes defensive, claiming it’s very personal to him and is just his a way of documenting his emotions. He gives her an example and she tactfully suggests he should try making it rhyme, but he insists that it does rhyme when spoken in the original machine code. The Doctor is intrigued to learn that when the Assemblers were first activated, there were no humans around and the Haven was already in a run-down state. He begins to have a theory, but then they arrive at the Diaconian Spire. Unfortunately they’re unable to open the door and the Doctor suspects someone is overriding their instructions.

Just then, Titus arrives with the other Cannibalists and Minerva. The Doctor’s group is trapped and Titus tells them it’s time for them to beg for mercy. Thanks to Minerva, he can now control every device in the city and he’s been listening in all the time as the Assemblers plotted against him. He mocks Macrinus for believing in the old myth of a reset switch, but says he’ll never find out for sure as he plans to deactivate all his enemies. Domitian tells the Doctor that he’s going to create a diversion, then he and Macrinus rush the Cannibalists and start fighting with them. While the robots are all occupied, the Doctor asks Minerva to open the door to the Diaconian Spire, then he, Lucie and Servo slip inside. As the door closes behind them, the two Assemblers are slowly ripped apart by the angry Cannibalists…

Lucie is frustrated at being unable to help their robotic friends, but the Doctor tells her it was their own choice. They explore the Spire, but it appears to be empty and it looks like Domitian and Macrinus sacrificed themselves for nothing. The Doctor asks Minerva to search for the reset switch, but as she starts to give him directions it becomes clear that she herself is the switch. She confirms that she’s a cybernetic access point for the Haven and is capable of full-system re-initialisation. This is why there have been rumours over these years that she eats robotic brains - she has the ability to erase the mind of every robot in the city. Lucie suggests they activate the reset immediately, but the Doctor tells her not to be so hasty. For one thing, it’ll mean the end of Servo as well. Servo says he realises that, but is prepared to have his mind wiped if it’ll make everything else better. The Doctor insists that his life is important and he has to be sure that using the reset would really make a difference here.

The Doctor asks Minerva how old the Haven is and is surprised to learn that it’s been here for 4,900 years. Servo insists she must be wrong, but the Doctor realises this isn’t the first time someone’s activated the reset button. Minerva confirms this and projects an image of Assembler Antonius, an earlier inhabitant of the city. In the recording, he reports that the drones are resorting to cannibalism and the situation is so desperate, they’ve been left with no choice but to restore the city back to its original state in the hope that next time things will be different. Then Minerva shows them further recordings which reveal that the reset button has been accessed 214 times before. Each generation keeps repeating the same pattern - the Assemblers are switched on, they build the drones, the drones turn into psychopaths and then the Assemblers hit the reset switch and the whole thing starts over again. Lucie wonders what causes the drones to go mad, but it could be any number of things - a bug in their coding, a duplication error or even a computer virus. The Doctor points out that Servo is another example of a robot who’s broken out of his programming, but while the others were driven to cannibalism, he was driven to write poetry. It makes him greater than any robot that ever lived before. He may be a freak and an aberration, but he’s also a miracle of evolution.

Outside the Spire, there’s nothing left of Domitian and Macrinus but nuts and bolts. The Cannibalists turn their attention to the locked door, but it’s taking too long. Frustrated, Titus grabs hold of Crusher, sets his power cells to short circuit, then stands him up against the bulkhead. Crusher’s entire system overloads and he self-destructs. The resulting explosion leaves a gaping hole in the door through which the Cannibalists can enter…

The Doctor explains that the robots have literally evolved over the generations through the accumulation of copying errors, but Servo has broken the pattern. There’s never been another robot before with both free will and a conscience. Servo doesn’t want to hear any more and decides it’s time to bring an end to all the suffering here. Like Titus before him, he synthesises the Doctor’s voice and prepares to order Minerva to activate the reset button. The Doctor quickly stops him and tells him his actions won’t stop the suffering, it’ll simply prolong it, replacing one group of killer robots with another. The reset button is the cause of the problem, rather than the solution, and the answer isn’t to turn back the clock, but for everyone to learn from their mistakes.

Suddenly, Titus and the remaining Cannibalists burst into the room and prepare to attack the Doctor’s group. Servo warns them that if they take one step further, he’ll order Minerva to activate the reset, but Titus doesn’t believe he’ll do it as it’d mean he’d be committing suicide too. The Doctor and Lucie try to persuade Servo to find another way out of their problem, but then Titus himself decides to give the order, arguing that there’s no point in living any longer now that there’s nothing left on the Haven to kill. Adopting the Doctor’s voice, Titus tells Minerva to activate full system re-initialisation. The Cannibalists start to laugh maniacally as Minerva starts the process. The Doctor orders her to stop, but it’s too late.

Minerva erases the hard-drives of all the robots and then re-installs the star city control programmes. It suddenly goes quiet and the Doctor and Lucie realise everything here is effectively dead, all except for Minerva. Somewhere in the Haven there’ll be a machine building a new batch of Assembler robots and they in turn will start building new maintenance drones. Minerva projects another recording, this time of the scientist Lucie was shown before. His name is Protocol and he explains that once the humans arrive, the drones will be surplus to requirements so he’s installed a computer virus into the city’s programming which is designed to cause the robots to self-destruct. However, over thousands of years things have evolved and instead of destroying them it actually gave them freedom. Just then, Servo miraculously comes back to life, apparently unharmed by the re-initialisation process. He remembers who he is and he even recognises the Doctor and Lucie. The Doctor asks Minerva why she didn’t erase Servo’s memory and she reminds him that when they first met, he gave her strict instructions not to do anything to harm his friend under any circumstances.