Arriving on the planet Uhala in the Andromeda galaxy the Doctor and Donna find themselves in a civilization that is mixed between organic and mechanical species. The Doctor takes Donna to an art gallery to see some examples of Andromedan art. Spotting an unusual artifact, something like a rusty truck wheel encrusted with diamante, the Doctor produces his sonic screwdriver. He seems to think this isn’t a work of art at all. As he goes in search of the gallery’s owner Donna is surrounded by three supermodel types as she looks at the strange artifact. Suddenly she finds herself being transmatted. Donna, the supermodels and the display case containing the artifact arrive in a darkened room aboard a spaceship. A little fat man enters the room, and has her taken to an unused bedroom by the supermodels. Judging from their astonishing strength, Donna assumes that these ‘women’ are humanoid robots

The Doctor races back through the city streets to get to the TARDIS before the transmat traces vanish. Arriving at the ship he is met by a boy and a large robot. As he tries to slip by them all three and the TARDIS are also transmatted, this time to a different ship. The boy leads him through the corridors of a rather battered craft. He introduces himself as Boonie and the large robot as Mother. In the control room of the ship there is a middle-aged woman, Kellique, and from their behaviour the Doctor realises that the boy is the commander of the ship. It becomes apparent that they are in orbit above Uhala, observing the other vessel, Dark Light, where Donna is being held.

The fat man, Garaman, interrogates Donna. She tells him that the Doctor will soon be on their trail but her obvious lack of understanding about the realities of the universe, his galaxy or space travel in general merely amuse him. Dark Light begins to leave the system. Garaman is curious about the Doctor but when Donna decides not to tell him anything the fat man orders one of the robots to break her finger. He i s stopped by a three legged lizard who informs him that torture rarely works. The lizard’s name is Mesanth.

The Doctor is placed in a cell. He ponders on the artifact, which is what Boonie seems to be after. His screwdriver readings have told him that it contains some very advanced resonance coils. An elderly woman brings him a tray of food. She tells him that he is aboard The Sword of Justice and that Boonie is not actually a boy, but somewhere in his forties. Her name is Li’ian. She says that Boonie needs to talk to the Doctor.

Donna tries to impress Garaman and Mesanth with some tales of her adventures with the Doctor and some adventures of his own that the Doctor has told her about. These include dealings with the Racnoss, Ood, Magentans and the Jant. They are decidedly unimpressed by this but more interested when she tells them that she finds the presence of various robots on the ship a bit creepy. When they tell her that they view ‘Mechanicals’ as an inferior species she agrees and starts to complain about her treatment at the hands of Robot Santas. Garaman warms to=2 0her and offers her a guided tour round the spaceship.

The control room of The Sword of Justice contains Boonie, Kellique and four robots of bizarre and assorted designs. There is a bustle about the place as they prepare to arrive in the Karris system. Boonie tells the Doctor that the cultist ship has just arrived, too. He fills the Doctor in on the back story of the Cult of Shining Darkness. They are the wealthy followers of a scientist, a woman called Khnu em Llodis. After her disappearance (and presumed death) two years earlier they have been quiet, but two months ago they restarted their activities. They are organic supremacists who refuse to believe that machine intelligences are sentient. Boonie is trying to track them to find out what they are doing and stop it, whatever it is. During this conversation Dark Light vanishes from the sensors but the Doctor crawls beneath a control panel and boosts the sensor capacity to overcome their shields and find them again. Boonie begins to wonder what sort of man can arrive suddenly and double the capacity of his ship’s sensors.

Donna’s guided tour, led by Mesanth, shows her how palatial Dark Light is. At the end of the tour Garaman announces that they have picked up their pursuers’ trail, which pleases him. He is not bothered about being followed and would actually prefer to know where their adversaries are. Donna transmats to the surface of Karris with Mesanth, one of the supermodel robots and a well-muscled human called Ogmunee. It is a desert planet, much to the liking of the lizard. Sandstorms have made it impossible to beam into the tunnels below ground where there is another segment of the device that they are looking for. They find a door in some rocks and descend some rough steps.

Li’ian takes the Doctor to the ship’s ‘library’ where he finds out about Khnu em Llodis. She was a great robotist who some say went mad. She claimed to have discovered a dark heart in their empire that would rise up and destroy everything the empire had built. Li’ian says that everyone assumed this to be a reference to machine intelligences. Her mysterious disappearance soon after this pronouncement led many to think that her ship had been destroyed to stop her speaking the truth. She had been working on something secret at the time of her death and Li’ian thinks that it is the segments of this device that the cultists are seeking. Khnu’s death panicked the cult into scattering the parts around the galaxy.

Mesanth tells Donna that his species is peaceful by nature and prefers to avoid conflict. They are making their way through the tunnels to the home of a primitive ape-like species – the Jaftee. Donna sees some carvings on the wall showing some of these Jaftee being eaten by a huge, tentacled monster. Mesanth says that the presence of the monster is the reason they have brought the robot. The Jaftee, on the other hand, should prove no problem as they were seeded two years earlier to believe that the cultists are their gods.

Boonie is not pleased to find that Li’ian has told the Doctor about the cultists, but is somewhat mollified when Kellique says that the Doctor’s modifications to the sensors have allowed them to pick up a trace of the third segment under the planet’s surface. A first part was apparently hidden in a forest on Chao. The Doctor suggests transmatting into the caves beneath the surface to look at this segment before the cultists retrieve it. When the20atmospheric problems that prevent this are explained to him the Doctor sets about boosting the transmat like he did the sensors.

In the tunnels the Monster attacks the female robot that is leading the way, shattering it against the rocks. Donna wants to help the robot but her companions say it is merely a machine. Donna risks being attacked to go to the dying machine’s side but there is nothing she can do for it. When the monster reappears Ogmunee produces a powerful gun and shoots at it, making it flee down the tunnels. Ogmunee fires after it. Mesanth is visibly upset by the screams of pain that he can hear. Donna pleads to have the damaged robot transmatted to the ship but Ogmunee shoots it in the head and then moves on, leaving it dead in the passage.

The Doctor beams into the tunnels with Li’ian and the robot called Mother. They find the charred and very dead remains of the monster and the smashed robot. Mother is visibly upset at the robot’s demise while the Doctor checks the smashed body over for any back-up circuitry containing signs of life. There are none.

Donna is wondering at her change of heart regarding robots; she is angry that Mesanth and Ogmunee are so callous in their attitude to mechanicals. Meanwhile, their arrival in the tunnels has been spotted by the Jaftee who are now in a bit of a panic. While they were happy to accept the cultists as their new gods (and therefore agree to look after the device for them) this didn’t actually mean much to them as they tend to adopt new religions on a regular basis and have been through quite a number in the intervening years. Their current religion, worshipping The Chicken of the Apocalypse, is wearing thin after only two weeks. Enchikka, High Priest of What We Believe Today, is worried that the cultists will want their artifact back and is fairly certain that it is stored with hundreds of other redundant religious items in the caves. He sends someone to look for it, more in hope than certainty that it will be found.

Donna and her two cultist companions step onto a ledge in a cavern. The Jaftee are gathering in the pit below but look less awed than Mesanth hoped for. The lizard uses a device to make his voice boom out in godlike fashion but the Jaftee seem underwhelmed. Impatiently, Donna snatches the device and demands to meet the High Priest. Her strident tones bring En chikka out onto a platform in the pit and the rest of the Jaftee begin to kneel in devotion. Heartened by this Donna demands the return of the Sacred Artefact but Enchikka tells her that the Jaftee have moved on and found new gods, including the Chicken of the |Apocalypse. Donna denounces the chicken, and all the old gods, as false idols and declares herself as their new goddess, calling herself The Ginger Goddess. Unknown to her, the Doctor is watching, along with Li’ian and Mother, from a cave on the other side of the cavern.

Enchikka is excited beyond measure at the concept of the Ginger Goddess, not least because many of the Jaftee have similar hair to Donna. The Jaftee began to bow down and praise the Ginger Goddess. Donna commands that they have no other gods and that all the relics of their previous religions must be brought to her and cast out. While the Jaftee race off to scour the caves for every religious item they can find (including, Donna hopes, The Sacred Artefact that the cultists are seeking) Enchikka stays behind and asks Donna to descend into the pit. She does so, followed by Mesanth and Ogmunee. Eventually a huge collection of relics is produced and largest among them is the Sacred Artefact. Donna says she will take them all to the heavens for disposal but Enchikka says there is no ne ed as the Jaftee will destroy them all, there and then. He adds that the false gods will die, too, and indicates Mesanth and Ogmunee. Together with the false icons the false gods will be burned. The Jaftee swarm over to the three aliens and disarm Ogmunee. There is a sudden crash as a large rock drops into the pit. Mesanth is frozen in fright but Donna urges him to use this distraction to produce a transmat augmenter that takes the three of them and the Sacred Artefact out of the cavern. In the split second before her departure, Donna catches sight of the Doctor on the ledge from which the rock fell.

The cult ship leaves the Karris system soon after, followed at a distance by The Sword of Justice. The Doctor is in his room with Mother. He produces the memory core from the robot they found in the tunnels. In deference to Mother’s wishes, the Doctor changes the word ‘robot’ to ‘mechanical’. He tries to access the memories to see if there is anything that can tell him what the cultists are up to or even if there is anything that would allow the mechanical to live again. A section of Mother’s chest opens up and a tendril extends and attaches to the memory core. While this is happening, the Doctor spots something out of place inside the machine’s chest cavity. Mother causes a virtual screen to hover in the air. Words appear, telling the Doctor that the memory is irreparably damaged. A visual record of Donna appears, from the point of view of the dying robot as she offered comfort and help in the last moments of life. Mother asks why Donna is with the cultists and the Doctor replies that she was kidnapped.

As the conversation continues with the Doctor speaking and Mother transcribing her answers on the screen it appears that Mother was created in research laboratories and attached to virtual war simulators. Her most efficient responses were implanted into other war machines, her ‘children’, hence her name. When she found out what was happening she tried to end her existence but failed. Boonie found her on a scrapheap and repaired her but she refused to have her speech returned and is mute as a reminder of what happened to her.

On Dark Light Mesanth tells Donna that the cultists, Garaman in particular, are disappointed by the sympathy she showed to the robot in the tunnel. Donna finds it incomprehensible that they do not se e the robots as sentient whereas Mesanth is similarly puzzled by her attitude.

In the greeny-yellow province of Junk on yet another world an unidentified vessel transmats three figures onto the control tower of a vast scrap yard. 77141, the controller, an overweight alien, is horrified when Mother smashes in through his window, followed by Boonie and the Doctor. It is the Doctor who cheerily asks if they can poke around in the huge piles of junk that tower into the sky. Leaving the mechanical to keep an eye on the controller, the Doctor and Boonie follow a trace and make their way to one of the distant piles.

On the way the Doctor tells Boonie what he has learned from Mother about her start in life in the weapons industry. Boonie is amazed that Mother confided so much to an apparent stranger. When they reach the pile the Doctor uses his screwdriver to set up a scrambler field to stop the cultists from simply beaming up the next segment. They call Mother down to climb into the pile and locate the segment for them. As she tosses down old bits of redundant machinery around them they are unaware that the cultists have arrived in orbit above them. 77141 is in contact with Garaman who tells the controller to call up some large reinforcements.

Boonie is ruminating on his two year wait for the Cult of Shining Darkness to reappear and his month of surveillance as they recovered the first three segments of the device. His thoughts are interrupted by the arrival of two vast robots, striding towards them. One is made up of large spheres, arranged in a humanoid shape, and the other is slender but equally as tall, about the size of two houses. They order the two men to stop what they are doing. The Doctor engages them in conversation and finds that they have been sent by 77141 to kill the intruders. The robots introduce themselves as Chuck and Crusher. Crusher’s job is to compress the junk and Chuck’s task is to throw the junk into the nearest sun. The two of them are prevented from carrying out their orders by a tendency to slip into frequent squabbles, usually over Crusher’s poor grammar. The Doctor interrupts another domestic row to explain that they have a mechanical with them who can vouch for the fact that they are trying to prevent the activities of the Cult of Shining Darkness. When he points out that the sector they are searching has lain dormant for two years, far longer than is the norm in the scrap yard, it is enough for the robots to realise that 77141 is in the pay of the cult.

A small army of scuttling appliances approaches them, obviously sent to finish the job that Crusher and Chuck are not doing. The two robots gleefully fight them off while Mother appears from the tower of scrap with the fourth segment. Chuck flings one of the appliances at the control tower, bringing the structure crashing to the ground. He then taps into the recent planet-to-ship communications and finds that 77141 is following cult orders. The two giants then stamp away, leaving the Doctor to investigate the segment. A small robot suddenly starts to speak to the Doctor. It is being used as a conduit to facilitate communications from elsewhere. Using the robot’s voice someone tells the Doctor not to interfere with the segment or Donna will die.

The Doctor replies that the easiest way to make him start interfering is to threaten Donna and then starts to jump up and down on the segment. Ogmunee and one of the supermodel robots beam down so he jumps even harder until Donna also appears. The Doctor throws her the screwdriver which puts the interference field around her. The oth er two newcomers and the segment are transmatted away, and at the last moment the Doctor leaps onto the segment and goes with them. Donna switches off the screwdriver. She finds herself being transmatted with Boonie, Mother and the little robot.

Ogmunee is alarmed to find the Doctor with him on Dark Light. He searches the Doctor and pulls a small red sphere from his pocket. The Doctor advises him that it would be better if he simply returned it but Ogmunee pockets it and says that he will investigate it later. Garaman has the Doctor brought to the bridge of the ship and tied up for questioning. Mesanth, however, intervenes. He points out again to Garaman that such methods are unproductive and has the Doctor released.

On The Sword of Justice the little robot, Weiou, is questioned to find out why it was arguing with the Doctor but it proclaims its innocence. Donna argues that force should be used to extract information from the robot which outrages the rest of the crew. Again she is perplexed to be so out of step from the thoughts and feelings of those around her, but this time from the opposite point of view from the cultists. When its internal clocks are checked they find out that eight minutes of Weiou’s existence are missing; it was clearly under the power of somebody else. Later, Donna finds herself alone with Mother. The large mechanical sets up a screen to communicate; Donna learns that it was Mother who pushed the rock that scared the Jaftee and saved her companion’s life. Mother is puzzled that Donna was so caring for the robot that died in the tunnels (its Lipanov rating for sentience was only 23, much lower than the 40 required to be considered truly sentient) and yet Weiou (with a rating of 68) was seen as a mere object. Mother accuses her of judging mechanicals by their appearance. At first Donna reacts aggressively, saying that she is not a racist, but ultimately concedes that Mother as right about her.

Dark Light enters the Sentilli system where the planets orbit a black hole. The four segments of the device are fitted together and put into space where the Doctor recognizes them as a ‘can-opener’ for opening black holes. When the device is activated it releases an enormous spaceship called The Torch from the singularity. Garaman tells20the Doctor that it took an army of robots many years to build The Torch. Garaman is interrupted by Ogmunee saying that someone or something has arrived safely. This triggers an attack on The Sword of Justice, which has tracked them to this system.

On board The Sword of Justice the crew is frozen in panic when high energy beams start to rip the hull apart. Only Donna seems to have the presence of mind to order the crew, organic and mechanical, to evacuate the ship in escape pods. Boonie refuses to go; he intends to fight back. As the escape pods are ejected, leaving Donna, Kellique, Mother and Weiou on the bridge, Boonie starts the engines in order to power the ship towards The Torch. He tells them that there are eight minutes to impact. Survival seems impossible until Donna remembers that the TARDIS is in the hold of the ship. The five of them run through the corridors of the disintegrating vessel, only to find that the hold has gone, and with it the TARDIS. In one last gasp attempt for survival four of them squeeze into an escape pod which had malfunctioned while Mother climbs out into space and rips it free from its clamps. They use thrusters to leave the proximity of the wrecked space ship just as another high energy beam from Dark Light slices The Sword of Justice in two. Donna is appalled to think that she has managed to lose the TARDIS.

Garaman, Mesanth, the Doctor and a blonde robot transmat onto The Torch. Garaman tells the Doctor that the vast ship was paid for by wealthy philanthropists who will have the satisfaction of seeing robots put in their place and organics returned to the top of the heap. Mesanth is troubled by the deaths aboard The Sword of Justice. The Doctor points out that millions more deaths are likely to happen as a result of the planned revolution in the galaxy. Mesanth says that he merely wants true life, not mechanical, to take its rightful place but the Doctor wonders aloud about the possibility that humans might decide the revolution has not gone far enough; perhaps reptiles from Lota, like Mesanth, will go the way of the robots.

They step into a vast space at the heart of the ship. Only the centre is lit, leaving the rest in darkness. Here, Garaman decides to reveal the true nature of the cultists plan. He hands an activator to Mesanth who makes his way up the stairs. Garaman tells the Doctor that in half an hour, once Mesanth has installed the activator, the plan will be complete. Before he can go further Donna, Kellique, Boonie, Mother and Weiou step into the light. Their escape pod has docked with The Torch and they have made their way through the ship to this location. The blonde robot turns to attack them but in a blur of movement Mother swipes it aside, destroying it. Suddenly, Li’ian steps out of the darkness. Boonie wonders how she got off the ship until the Doctor remarks that she is a double agent working for the cult. As Garaman claps his hands in triumph Li’ian produces a gun and shoots him between the eyes.

Li’ian puts the gun to Donna’s head. The Doctor remarks that she is the one who took control of Weiou on Junk. He also works out that The Torch is a giant transmitter and that the activator is going to link into the mechanet (a robot internet) that will give the cult access to the brains of every robot in the galaxy and switch them off. Li’ian says that was Garaman’s plan, but she wants20to go further; she wants to take control of all the robots. She starts to lead Donna up the stairs to join Mesanth who (she admits) is oblivious to this revised version of the plan. As she departs she orders the lights to be switched on in the area. The anti-cultists realise that they are surrounded by dozens of the blonde super model robots. When Li’ian reaches the top of the steps she orders the robots to kill all of the newcomers.

Mother fights a losing battle to keep these robots at bay. Weiou tries to access the robots’ inputs to see if they can be reprogrammed but Li’ian has switched off their audio and wireless links. All he can find is that they were made by the Meeta-Corrin company. Mother flashes up a screen to say that this is the company that created her, too, and these robots are effectively her grandchildren. The Doctor asks how to get access to their command pathways. She replies that it is a 256 character string of alpha-numerics. He says that he will understand if she does not want to use the code but she replies that this is not murder. With a Lipanov rating of 23 these are not sentient mechanicals and switching them off is not killing them, just shutting them down for good. Mother flashes the 256 numbers and letters onto her screen and as the robots come in s ight of it they become immobile.

In the laboratory at the head of the stairs Mesanth is installing the activator. Li’ian tells him about the change in plan. When he objects she informs him that these were Khnu’s own ideas. Suddenly, through the window, they see the others standing outside in the corridor. Li’ian says that there is no point in trying to break in, as only a bomb can smash through the reinforced walls and windows. The Doctor replies that it is a good job they have a bomb. He says that when Mother revealed her chest cavity to him on The Sword of Justice he noticed an antimatter bomb that would destroy the whole station. Mother has no idea what he is talking about but Boonie admits that he hid it just in case of such an eventuality. He apologises to Mother for not telling her and insists that he is still her friend. She says he would not have planted such a bomb on an organic life form. He responds by telling her that such a thing would be far harder to accomplish and, in addition, he always planned to be with her if the bomb had to be used.

Mesanth tell s them that the activator will be ready in five minutes. Boonie says that will give the Doctor time to try to find a shuttle off the ship but the Doctor says they are all in this together. Boonie tells Mother the code to start the bomb’s countdown and a clock starts to count down on her screen. Li’ian decides to cut her losses and grabs the activator. She says she can find another transmitter to complete her plan. She communicates with Dark Light and the transmat beams her and Mesanth away. Weiou asks Boonie to stop the bomb but Boonie replies that it can’t be switched off once the countdown has begun. This upsets the Doctor, too.

Li’ian and Mesanth arrive on Dark Light beside Ogmunee. He picks up the red ball he took from the Doctor and wonders why it has started flashing. The ship is vaporized in the anti-matter explosion.

After the countdown has reached zero and The Torch is still intact Boonie realises that the bomb must have been on the cultist’s ship. The Doctor tells them that he expla ined to Ogmunee, when he took the bomb, that he wouldn’t really want it. On the view screens they look at the blackness where the Dark Light used to be and see the TARDIS drifting in space. The Doctor says that he will use a shuttle from The Torch to pick up the TARDIS and then take their new friends home, after he has crashed The Torch into the black hole. He asks them where they want to be taken and Weiou begs to go to Pasquite where there is a robot theme park that can simulate what it feels like to be a Squidgie (organic life form).