There’s a polite knock on the door and Sara knows she’s due to receive another visit from Magus Riga. He enters and says he’s been told she’s eating very little. Frostily, Sara demands to know when she’s going to be released, but Riga ignores the question and invites her to share a cup of tea with him, reminding her that the planet Tasak has so much to offer. She insists that she’s being held prisoner in a gilded cage, but he says it’s for her own protection. There are many new arrivals coming to his city every day, refugees displaced by the conflicts and drawn here from across the planet, looking to the House of Argentia for stewardship. They’ve ended the war, but now their burden is to begin the peace and he needs her help as an offworlder to do that. Sara refuses to have anything to do with the Heart and says he’s tampering with something that could destroy them and threaten the lives of every being on this planet. Riga assures her he would never place his people in harm’s way, but he becomes frustrated by her agitation and decides it would be better if he left. He says he’ll return to make the same offer again next week, just as he’s done for the last three months, adding that he‘ll never give up until she agrees to his proposal.

Outside the room, Riga asks his friend Lord Merel to send for a kitchen servitor as it wouldn’t do for their guest to starve herself. Merel thinks he’s indulging her too much and recommends a more aggressive approach, but Riga insists that freedom of choice is the cornerstone of their cadre’s principles and she must join their House of her own free will. Merel apologises and Riga asks him how the preparations for the Conclave are progressing. Merel tells him the delegates from the other Houses have been arriving all morning. Riga has been gathering all the greatest minds from each of the local cadres, but the last to arrive will be the deputation from the House of Sarkota. Riga suspects they’re unhappy that the war is over and are keen to avoid showing any weakness, but if they get this right, there’ll never be a need for another war again. Riga believes they can wipe out famine, disease and poverty, not just for the Argentia and the Sarkota, but for everyone. When they reveal the secrets of the Heart, the destiny of their entire planet will be forever changed. Merel smiles to himself and says he believes it will…

Elsewhere, an undercover offworlder named Temeter is on day 20 of his mission. He’s tracked an energy trace to the city of Argent, a conurbation on the northern coast of the central continent, but the signal is intermittent and he’s been unable to triangulate it. He’s sure the target is somewhere near here and he believes his colleague must have found it too. Suddenly his sensor picks up an anomaly, a second trace that doesn’t match the primary target or any other known aggressor profiles. It’s far too advanced for this planet, possibly even more than his own, so he goes to investigate.

Inside the TARDIS, the Doctor is preparing himself a cup of tea. On the scanner, he sees Temeter approaching, but his arrival was expected and the Doctor allows him to enter. The Doctor introduces himself, but Temeter refuses to answer any questions, believing the self-heating fusion teapot in the Doctor’s hand to be a weapon. The Doctor identifies Temeter as a career soldier working undercover and recognises his sensor and a common design from the Orion War period. Both sides deployed thousands of operatives around this time, but that doesn’t explain why he would be on Tasak, a backwater steam-age planet light years away from the battle lines. Temeter asks the Doctor why he’s here and he says he’s come for the tea as it’s the best in the quadrant. The Doctor admits that he deliberately sent out the anomalous reading as a trap to lure Temeter here after the TARDIS picked up the output from Temeter‘s scans of the city. There’s still the question of the other unidentified energy trace, but there’s too much interference for the Doctor to track it down. Temeter says he’s also here to search for someone, so the Doctor suggests they pool their resources.

In the Citadel, Lord Merel is briefed in secret about a communiqué received from the House of Sarkota. As Merel suggested, a contingent of mercenaries is being brought into the city and they’ll be ready to move on his command. A contact will also be arriving shortly to set the last stage in motion. They don’t know the identity of that person, only that they’ll make themselves known to Merel personally. Merel mocks the paranoia of the Sarkota people, but says they have their uses. In the meantime, Merel has agents in every precinct and each of them has an assigned target.

They’re interrupted by the arrival of Magus Riga who’s just learned that Ardith, their House’s greatest scientician, has returned from the island with news of great importance. Merel is surprised that Ardith is still travelling after his recent illness, but Riga says he insisted on personally escorting the statue he found at the Heart. Merel says he’s heard stories of the treasures found there, but Riga promises he’ll see them for himself soon enough. The island is the ’heart’ of Argentia and is widely referred to as the ’legacy of the Mordath’. Merel has studied the work being done there and he believes their ancestors had some insight when they wrote of the ancient myths. The image of the island appears often in their legends and in the prophecies of their rebirth in a new and shining form. Riga believes the mythology of their primitive forefathers is irrational and says their cadre seeks only truth, not superstition. He prefers to believe the Heart offers them a future based on knowledge, science and advancement.

The elderly scientician Ardith greets his old friend Riga and thanks him for finding time in his busy schedule to meet him. Merel comments on how well Ardith is looking and the old man admits that thanks to the gift of Silver, his illness has completely gone. Riga hopes that in time they’ll do away with every disease and Ardith says that with every new find at the Heart, they’re moving closer to that. This very week they had another major breakthrough - a mechanical cogitator that’s much smaller than the earlier models and is compact enough to be carried on a man’s back. He also shows them a piece from the mosaic at the site which has the characteristic glyphs etched into the surface. Ardith says the cogitator has enabled them to enhance their translation process and they’ve now discovered that the word ‘Mordath’, which refers back to their ancient origins, should more properly be translated as ’Mondas’…

The Doctor and Temeter travel across Argent City via the monorail transport system. The Doctor explains that he was simply visiting the planet to buy some tea, and although he’s missed the worst of the war, this is still a delicate period for any civilisation. What happens now can change the fate for millions of people for centuries to come. The people here may be celebrating the end of the war, but there will always be others. They both agree that the energy source they’re tracking is alien to Tasak, but when the Doctor suggests it might just be intergalactic flotsam and not dangerous at all, Temeter points out that the Doctor wouldn’t still be here if he believed that. The Doctor asks Temeter about the person he’s looking for, but he refuses to answer any questions.

The train pulls in to the next station and a public address system warns passengers that this is a restricted stop and security checks are in place to allow only pass holders to disembark. Temeter is shocked to discover the Doctor intends to visit the Grand Citadel of the House of Argentia, the most heavily fortified place in the city and home to a whole garrison of militia. The Doctor says there’s a big do on their tonight with important visitors coming from all over the planet to join Magus Riga in toasting a new peace. Temeter knows they’ll never be able to sneak in there, but the Doctor says he has no intention of doing such a thing.

They approach the Citadel and Temeter spots three guards on the gate. He plans to distract and overpower them, but the Doctor has other ideas and goes straight up to the guards and introduces himself. When the guards challenge him, he produces his very own personal invitation to the Conclave, made out in the name of Dr Johannes Smither. The guards apologise for their attitude and allow the Doctor and Temeter to enter. As they head towards the building, Temeter asks the Doctor how he obtained a personal invitation and he explains that this is a gathering for all the finest minds on Tasak, and although he’s not one to brag, that includes him. After all, this isn’t his first visit to this planet…

In the skies above, the Pride of Sarkota approaches Argent City’s harbour and is granted permission to land on the Citadel itself. Aboard the coleopter, Specialist Etin checks on the status of their cargo. A trooper assures her their men are concealed within compartments on sealed decks and even if the enemy put inspectors on board, they’ll find nothing. Etin reminds the trooper that Sarkota signed the peace agreement along with every other House and he shouldn’t regard Argentia as their enemy. She’d hoped their taste for war might have lessened after so much bloodshed, but the trooper says the Argentia brought this upon themselves by trying to put the whole planet under their flag. Etin is concerned that their mission will be seen as dishonourable in light of the fact that they’ve agreed to put down their arms, but their orders come from cadre master Duke Hassa himself and they have no choice but to obey them. Etin instructs the men to load their weapons and make ready for deployment at a moment’s notice. If they’re spotted, they’re to take no prisoners and leave no traces.

Temeter is impressed by the interior of the Citadel, but the Doctor finds the glass, steel and marble too sterile for his taste. Among the guests, they spot their host Magus Riga, Lord of the House of Argentia, and the Doctor thinks it would be rude not to say hello. Riga is engaged in conversation with Ardith about Tasak’s new unity and the elderly man is commenting on the fact that the idea of bringing the world’s scienticians together under one roof would’ve been greeted with derision only a year ago. The Doctor joins the debate and congratulates them for bringing Tasak back from the brink of destruction and ensuring a future for everyone. He introduces himself and claims to represent the House of Gallifrey, but Riga doesn’t recognise the name and he says they‘re just a small exclusive group.

The Doctor says it’s incredible how they’ve been able to accomplish so much in such a short time and Ardith says it was all down to the gifts of Argentia, the cogitators, machines and engines that have revolutionised medicine across the planet. Many lives have been saved from the benefits of the Silver, a curative compound that they’ve made widely available in recent months. The Doctor says it’s fortunate the Argentia are so altruistic and Riga assures him they’ve made peace with every House that opposed them, even their age-old rivals in the Sarkota. He argues that knowledge is of no value to them if they can’t share it for the betterment of all and the Doctor agrees entirely, but when Riga says they’re protecting their civilisation, Temeter wonders who they’re protecting it from if the war is over. He assumes they must be worried about aliens and offworlders, but Riga assures him other worlds would find nothing of interest here. Their conversation is interrupted by Lord Merel, who’s surprised he wasn’t notified of the new arrivals. Riga introduces them and Merel asks if he might have a word with the Doctor and Temeter in private.

As soon as they’re alone, Merel turns furiously on the Doctor and accuses him of being a fool. He demands to know why they were talking openly about offworlders and says that information was given to them in strictest confidence. The Doctor apologises for being remiss, but Merel refuses to accept it and insists they be more discreet in future, adding that they should have approached him first rather than test their luck with Riga. He says that if the Sarkota are unable to follow his simple instructions then their plan will not succeed. The Doctor thinks quickly and says they just wanted to find out more about the offworlder and Merel says Duke Hassa will be allowed to interrogate the woman once the operation is complete. The Doctor claims the Duke has his doubts as to whether she’s safe and Merel is forced to reveal that she’s being kept in this very building. The Doctor and Temeter insist on assessing her condition and as Merel can’t leave the reception without raising suspicion, he allows them to go up to the 95th tier alone, adding that they can discuss the deployment of the troopers when they return. Merel leaves and the Doctor tells Temeter that he’s obviously mistaken them for someone else, presumably their former enemies from the Sarkota. He says they probably have about ten minutes before Merel discovers he’s made a mistake.

Inside her room, Sara hears the guard challenge the Doctor and Temeter. They claim to have been sent by Merel, but when the guard says he’ll need to get authorisation, Temeter knocks him out. They release Sara and the Doctor notes how overjoyed she seems to be reunited with her colleague. They kiss and Temeter assures her she’s safe now. She demands to know who the Doctor is, but when he says they don’t really have time to stop and chat, Temeter says it won’t take a second. He and Sara ‘connect’ with each other to transfer details of their experiences, and the Doctor realises they’re both androids. Sara is intrigued by the Doctor and the TARDIS, but Temeter reminds her they have a mission to complete.

At the reception, Specialist Erin asks to speak with Lord Merel alone. He dismisses her request and tells her to seek diversion elsewhere, but she responds angrily and threatens to take back the men provided by Duke Hassa and let him prosecute his insurrection alone. Merel instantly realises his mistake and identifies her as the contact from Sarkota. She accuses him of being arrogant and says that as a scientician she has little tolerance for code-words and disguises. He immediately summons his guards and orders them to accompany him in his search for the Doctor and Temeter…

Sara tells the Doctor and Temeter that Central Command was correct and that she’s found evidence of alien technology on an island a few kilometres off the coast of Argent City. There’s a dig site there called the Heart, named because of some significance it has to local legends. Unfortunately she was captured before she could visit the area and Riga soon realised she was an offworlder, although he still doesn’t know she’s an android. He’s been holding her prisoner for months while he tried to convince her to help, and although he didn’t try to hurt her, she suspects Merel has different ideas. Temeter suggests they head for the upper levels and try to steal a coleopter, but before they can do that Merel arrives with armed guards. The Doctor is disappointed that their ruse only lasted five minutes and Temeter predicts they have a 47% chance of neutralising all the guards before they can inflict a critical level of damage on them. The Doctor doesn’t like those odds, but fortunately he has a better idea. Then he promptly surrenders!

At the Conclave, Magus Riga speaks to the assembled nobles of Tasak and formally welcomes everyone to his Citadel. He says he’s especially pleased to see Etin’s party from the House of Sarkota as it shows the bad blood and feuding between their cadres is at last behind them. In the spirit of unity, he offers a gift to their former adversaries - a relic of Argentia unearthed from the Heart itself. He calls for Lord Merel to bring the gift forward, but his friend is strangely missing. Just then, he receives a report of a major security breach on one of the upper tiers involving the offworlder. To Riga’s amazement, Lord Merel steps forward and announces that he’s uncovered a conspiracy among their guests. He brings out the Doctor and denounces him a spy who came here to free their prisoner, a dangerous alien terrorist.

Merel asks what the Doctor means by the word “Cyberman” and he explains that it means an end to all life on this planet. The energy traces were obviously coming from Cyber technology. Ardith is angry that his work at the dig is being challenged and asks the guards to place the strangers in confinement. Riga agrees and the three offworlders are dragged away protesting. The Doctor calls out a warning to the assembly that their planet is contaminated with alien mechanisms designed only for killing and conquest. He says they’re more deadly than anything in Tasak’s experience, but this only convinces Ardith that he’s a militant separatist trying to disrupt the Conclave. The Doctor says if they let the Cybermen loose they’ll get the unity they want - but it’ll be a unity without emotion or life. Merel takes Riga to one side and suggests the knowledge held by these strangers could be considerable, so the Magus grants permission for the three prisoners to be taken to the Heart.

The Pride of Sarkota takes off from the roof of the Citadel and heads out towards the coastal islands. On board, the Doctor, Temeter and Sara have been locked up in one of the secure rooms all night. Temeter assumes the guards will be checking in on them soon and suggests they use the opportunity to escape. Although they’re currently flying over the ocean, he’s sure both he and Sara could survive the fall, but Sara refuses to leave the Doctor behind. The Doctor is angry they didn’t tell him everything they knew, but Temeter won’t apologise for keeping secrets from a complete stranger, especially one who’s obviously employed by the Earth military. He explains that their observer probes discovered cultural indicators in Tasak’s mythology that suggested an alien intervention far back in their history. There are stories of great beings and prophecies of the people merging with their deities, but they were never sure what it meant until now. Sara reveals that she discovered the remnants of a Cyber gun in one of their labs and it looked like the scienticians were trying to back-engineer it. The Doctor can’t understand why they didn’t just warn these people, but they tell him they’re in the middle of an arms race. Both the androids and the Earth military are scouring the whole galaxy for Mondasian technology to scavenge for use in the Orion War.

Just then, Magus Riga and Lord Merel arrive to question them. The Doctor urges them to abandon the Cyber technology, even if they believe if offers the solution to all their problems. Riga points out that Tasak was on the road to extinction and their civilisation was in ashes until the House of Argentia brought peace - not through superior firepower, but through mutual trust. He says they’ve not been developing guns or weapons, but medicines and machines to make life better for millions. The Doctor wryly points out that the road to hell is paved with good inventions. Merel tells Riga they’re wasting their time, but then Temeter suggests instead they talk about his secret plan. Merel claims this is a weak plan to sow discord between him and Riga, so he orders the guard to escort the Magus back to the upper decks. Once Riga has gone, Merel tells Temeter he’s the only thing keeping him alive and says he should remember that the next time he thinks about opening his mouth.

The Pride of Sarkota comes in to land and Ardith leads Lord Merel out to see the island for the first time. He’s heard stories of the landscape being austere and barren, but he hadn’t appreciated its grandeur. Ardith assures him he hasn’t seen anything yet and says he’ll truly be stirred when he gazes upon the Heart. Before that, Merel wants the offworlders to be taken down to the dig site and he charges Ardith with the task of finding out exactly how much the Doctor knows. He grants permission for force to be used if necessary, but Ardith is uncomfortable with the idea. Merel suggests he’s clouding the issue with his personal sensibilities and tells him it’s for the good of Argentia.

The guards escort Ardith and the three offworlders to a cave mouth near the dig. Temeter reassures Sara that they’re not going to be killed and she tells him she’s glad it was him who came to rescue her. She’s surprised the War Council gave him permission to come and it soon becomes clear that he’s acting without their authority. The Doctor remarks on them being a happy couple and Temeter believes he’s being mocked. He argues that just because their minds are based on silicon rather than carbon, it doesn’t mean they don’t have feelings. The Doctor assures him he was just admiring the delicate complexity of their machine hearts. Temeter believes it’s organics who have the coldest hearts, but the Doctor says this obviously means he hasn’t met a Cyberman yet.

Inside the cave, Sara notices light reflecting off something in the rock and Temeter realises there’s steel embedded in the basalt. It means the Cybermen must have been here for hundreds, maybe even thousands of years. The construction matches the androids’ scans of Mondasian technology and the Doctor is horrified as he was hoping there might be just one Cyberman scout here, or perhaps a small crew. This is worse than he thought. Tasak must be a Cyberman tomb world, one of countless thousands spread across the galaxy where the Cybermen have secreted armies in suspended animation, sleeping out the centuries and waiting for the signal to revive. The Argentians’ legends of great beings and their belief that they’ll one day merge with their deities is actually a race memory, a twisted myth based on the Cybermen’s plans for galactic domination. By scavenging what remains here for machines and medicines, Riga has inadvertently been doing the Cybermen’s work for them, seeding Cyber technology all over the planet. The moment this hibernation unit activates, the Cybermen will find a whole planet of new recruits waiting for them, primed and ready for Cyber conversion.

Specialist Etin finds Magus Riga preoccupied. He thanks her again for agreeing to meet them and for lending them her aircraft. Etin knows Riga is worried about what’s happened and he says the Doctor spoke with such conviction he’s been unable to put the warning out of his mind. He admits he’s always been afraid of this island and he doesn’t mind that Etin may think he’s showing weakness. Etin says a fool is one who rushes in without forethought, but she considers Riga to be a learned man. He’s surprised to hear such words from a Sarkotan, but she tells him many of her people admire him. She’s even wondered how different her life would’ve been if she’d been born in the Argentian cadre. Riga is pleased, but then Etin says that although she regrets what she must do now, she has orders she must follow. The room is suddenly filled with Sarkotan troopers and Etin advises Riga not to resist. He’s placed under arrest and taken away…

Once they’re in the cave, the Doctor wants to talk to Ardith as fellow scientists. He explains that the Cybermen who built this place aren’t dead and gone, but are sleeping. Ardith doesn’t believe him, but the Doctor says this treasure trove of miracle technologies is just too good to be true. Sara joins in and warns that if they continue tampering with the equipment here, the Cybermen are going to wake up and when they do it’ll be the end of civilisation on this planet. Ardith demands they prove what they’re saying because so far the Heart has brought them nothing but prosperity and when the Doctor says he can’t, Ardith mocks him and says he must regard them as primitives. He refuses to listen to any more and the Doctor is left frustrated at having to deal with closed minds. Temeter says he doesn’t blame Ardith as he doesn’t trust the Doctor either. The Doctor realises that if the threat of the Cybermen is revealed, the Argentia will lose their tenuous grip on the peace, causing the Houses to fragment and plunging the planet back into war. But if they do nothing and the Cybermen come out of hibernation, it’ll mean the end of the world.

A confused Magus Riga is brought before Lord Merel who mocks his friend and reveals that he’s betrayed him. Merel says his time as Magus is over, claiming that he’s taken over because Riga has horded power over Argentia for long enough. Riga believes Etin and her leader, Duke Hassa, have turned Merel into a traitor, but she reveals it was actually Merel who first approached the House of Sarkota and promised them certain benefits in return for their assistance. Merel admits that he hates Riga and he always regarded himself as the Magus’s servant, but now his agents are securing his position all over the city and when he opens the Heart, he’ll be confirmed as the new master of Argentia. Riga is horrified to learn that he’s prepared to jeopardise global peace just for personal gain and he realises he never knew his friend at all. Merel orders the troopers to sweep the dig site and gather together all the scienticians, adding that they should shoot anyone who resists.

In the main cave, Ardith and the others hear firearms discharging in the nearby tunnels. The soldiers guarding the Doctor, Temeter and Sara race off to investigate, but it’s clear from what they already know that Merel has begun his palace coup. The Doctor suggests they leave, but the Sarkota troopers are between them and the cavern mouth and they’ll never make it back to the airstrip. They have no choice but to go deeper into the caves and although Ardith initially refuses to let them go, the Doctor finally convinces him that they’ll die if they stay here. Reluctantly Ardith agrees, but then the troopers arrive and order them to stay where they are. Ardith quickly takes the others to a service elevator where the shaft feeds into the lower chambers. The soldiers open fire and Temeter is hit. He falls to the ground and orders Sara and the others to leave him behind. Sara refuses, but the Doctor forces her to go with him and Ardith. As he waits for the troopers to catch him, Temeter curses his bad fortune.

Merel finally arrives outside the Heart and is almost speechless as it’s everything he thought it would be. Riga tells him he doesn’t deserve to be here, but Merel is convinced he’s going to get everything he deserves, and more. He tells Riga to accept his defeat like a man. Etin admires the etchings and glyphs on the metal and Merel tells her they represent the legacy of Mondas. This will be the future for both their noble Houses. Riga refuses to help them, but Merel says he already has access to Ardith‘s supposedly well-guarded research. Riga begs Merel not to open the metal doorway into the Heart, but Merel takes him literally at his word and orders him to go down on his knees and beg. Seeing the great Magus humbled like this just makes Merel more determined. He opens the seals and tells Riga he has a destiny to fulfil, then he steps inside…

Inside the Heart, Merel is immediately overcome by the cold. He seems disappointed as the structure appears to contain nothing but machines and ice, but then slowly everything starts to come to life and the room is filled with a low mechanical hum. Then the voice of Cyber-Control announces that it’s awake. Merel believes it’s just a talking machine, like an advanced cogitator engine, so he order it to acknowledge him as leader of this world, but the voice tells him he’s incorrect - this world belongs to them. Merel protests, but the voice refers to him simply as raw material. The equipment around the room activates fully and Merel soon finds himself a prisoner. The voice tells him he will be converted and will be like them…

The Doctor, Sara and Ardith flee from the Cyberman until they find themselves facing some vent shafts that are regularly discharging huge bursts of steam. Ardith warns that they can strip the flesh from a person’s bones, but the Doctor has spotted a service catwalk above them and tells the others to go across while he distracts the Cyberman. He stands in the open and calls to the approaching giant. The Cyberman checks its identity files and recognises the Doctor as their enemy. His race has standing orders to destroy him, but as the Cyberman steps closer it comes directly under a vent shaft and is suddenly blasted by another steam discharge. Moments later, the others rejoin the Doctor and he admits that he’s feeling lightly poached, but his condition is much better than that of the Cyberman.

Waiting outside the Heart, Riga reminds Etin of her morals and asks why she’s going along with Merel, but she tells she won’t allow her personal feelings to obstruct her duty. He tries to persuade her that Merel is unstable and can‘t be trusted, but she still refuses to defy her cadre leader. A trooper arrives to report that the site is now clear and all civilians have either been neutralised or confined in their coleopter. Temeter, who was segregated from the others because the trooper saw him survive over a dozen bullet wounds, points out that Ardith and the two other offworlders made it safely to the lower levels. Just then, the doorway to the Heart opens and Merel emerges - but he’s not the same man that went inside. He’s covered in metal and speaks to them with the cold, metallic voice of the Cybermen. He tells them his ascension is complete and is now superior to them and therefore worthy to rule. Temeter realises he’s become a Cyberman and pleads with the others to kill him, but Merel says the people of Tasak can share his gift and become like him. The group are horrified when Merel says they must surrender themselves for conversion and when he moves to grab them, Etin orders her troopers to open fire. They do so, but their weapons are ineffective against Merel and he overpowers them. Suddenly the alcoves in the walls start opening and an army of Cybermen emerge. The three survivors flee for their lives…

Ardith admits that he’s never seen any creature so inhuman or soulless as the Cyberman. The Doctor explains about their origins back on Mondas and Sara fills in the remaining details. Ardith can’t understand how he could have been so wrong about the history of his own people over the millennia, but Sara tells him it’s not too late to stop the Cybermen. The Doctor decides to go back to look at the cavern structure as it stretches for miles beneath the sea floor and must contain thousands of dormant Cybermen. Ardith recalls how they’ve used the technology they found here and is shocked to realise that Silver, the panacea drug created by their biologians, must also contain elements distilled from the Cybermen! Sara says they must be nanogenes, machines as small as atoms, and Ardith wonders what’s going to happen to the people like him who took it..?

Cyber-Control informs Merel that the Sarkota have a strong physiology and Etin’s troopers will respond well to the conversion procedure. Once they’re transformed, the Cybermen will require more raw material and Merel tells them there are prisoners aboard the aircraft on the surface. Cyber-Control tells him the hibernation system is erratic and will need to be repaired before more dormant units can be revived. Then they can use their sub-space communications array to make contact with the many other hibernation planets throughout the galaxy and reactivate the sleepers there too. Cyber-Control shows Merel a playback of the Doctor and tells him he’s an enemy of their race. He cannot be allowed to interfere with their plans, so Merel is ordered to extract all useful knowledge from him, then destroy him.

As they explore the cavern, the Doctor advises Sara and Ardith to keep to the shadows and move slowly as the cybernetic sensors will take a while to return to their full capacity after emerging from their sleep. As more Cybermen start to emerge from their alcoves, they decide to try another route instead and head back to a junction where the tunnels branched off. Ardith is convinced it’s only a matter of time before they’re captured and transformed, but Sara says they’ll have a hard time trying that on her. She takes the lead as she thinks she’s spotted another service elevator not far away. The lift starts to come down and the door opens to reveal Riga, Etin and Temeter. Sara rushes over and hugs her friend, and they’re both delighted to be reunited. They reveal that Merel willingly gave himself up to be turned into a Cyberman. Ardith begins to panic and rushes over to the elevator, but Temeter says there were five Cybermen following right behind themin. Riga regrets not listening to the Doctor earlier, but the Doctor says there’s something else he needs to see.

The Doctor leads everyone to a ledge overlooking the cavern and they see an entire army of Cybermen below, preparing for battle. The Doctor points out the Fleshworks, where the Cybermen are awaiting raw material for conversion and Riga is sickened. This is the true secret of the Heart, a vast war machine ready to receive more living beings. Etin says the entire island has to be destroyed, but Ardith protests as this would also remove the source of Argentia’s technological superiority, and with it all hope for a peaceful future. The others remind him they’re talking about the survival of all life, not just on this planet but on all the other worlds containing hibernation units. The call to arms will spread like a virus and entire populations will be converted. Ardith doesn’t care about other worlds, but Riga insists the destruction of the Heart is a small price to pay when weighed against billions of lives. The Doctor reveals that there’s a dormant magma chamber beneath the island and if they can crack it, it’ll cause a lava surge to sweep through the tunnel network. Not even the Cybermen could survive a tide of molten rock. The Doctor guesses that the original dig site was cleared using excavation charges and he asks Ardith where he stores them.

Cyber-Control picks up an anomalous reading which shows humanoid lifeforms have entered the nearby magma chamber. A combat unit is activated and a troop of Cybermen is deployed to the lower levels to eliminate the intruders. Then Cyber-Control activates the Cybrid control signal…

In a monitoring station above the magma chamber, Temeter is worried about Ardith, who’s looking increasingly frail, but the old man assures him it’s just a reaction to the heat. Ardith retrieves the chemical batteries for the detonator circuits and Temeter asks him to take them the other side of the gantry where the Doctor, Etin and Riga are setting the explosives. Sara has also noticed Ardith’s weakened condition and Temeter says the man’s skin temperature has elevated. They assume it’s just a natural occurrence for organics and return to the task of working out an escape route.

The Doctor is satisfied they have more than enough explosives to set up a vibration deep in the magma core which will crack open the chamber and cause magma to burst through the basalt. Ardith, who is now extremely weak, joins them and hands over the final components. The Doctor sets the detonator controls to activate once they’re all clear, but then Ardith suddenly cries out in pain and complains of a terrible sound inside his head. His skin begins to turn silver and the Doctor realises it’s the effects of the drug. They watch in horror as Ardith’s voice starts to take on the recognisable sound of the Cybermen and the Doctor realises the nanogenes in the man’s blood have transformed him. The converted Ardith begins to disable the detonators and everyone rushes forward to stop him. They struggle violently and Etin prompts Riga to use the charged battery pack. Riga gives Ardith a final warning, then he thrusts the batteries towards his old friend.

Cyber-Control detects an electrical overload and realises their control over the Cybrid has been terminated. It orders the Cyber warriors to proceed immediately to the magma chamber via a nearby access hatch.

Riga looks down at the dying body of his old friend and apologises. Ardith mutters his last words, then the Doctor thanks Riga for saving his life…and very possibly the galaxy too. Temeter and Sara join them and the Doctor says there was just enough power in the battery pack to disrupt the Cyber-Control signal. Temeter has never seen an organic spontaneously transforming like this and the Doctor says the nanogenes that once saved Ardith’s life had become fully activated. Unfortunately the detonator unit was smashed and although it’s still functional, the timing mechanism no longer works and the only way to blow the charges is by the manual trigger. Suddenly the Cybermen arrive on the cavern floor below them and open fire. They start climbing up and when Riga notices Merel is leading the group, his mind is made up and he decides to remain behind to activate the trigger. He begins a countdown and tells the Doctor to take the others to safety. The Doctor protests, but Riga asks Etin for only one thing - that she will maintain the lie about unity in order to protect Tasak’s future. She agrees and as the Cybermen get close to the upper level, the Doctor calls for the group to run…

Riga turns to confront Merel, but the powerful Cyberman grabs his hand and tells him he won’t be allowed to detonate the device. Riga tries to convince his former friend that no matter what power he craved, he must know deep down that what he’s doing isn’t right. Merel insists that it’s logical for the superior race to survive. Not far away, the Doctor’s group discover they’re trapped on the gantry and are facing a dozen Cybermen. There’s only one thing they can do - surrender. The group are quickly surrounded and Merel tells the Doctor he will be taken for neural analysis. He then orders Riga to let go of the detonator, but the Doctor points out that if Merel crushes Riga’s hand, it’ll set off the charges. The Doctor volunteers to stay behind with Riga if Merel will let the others go. The lives of the others are meaningless to Merel and he’s confident they’ll be re-captured and converted later anyway, so he agrees. Temeter and Sara escape and Sara calls back to the Doctor to say that they’ll wait for him.

The Doctor and Riga are now alone to confront Merel and the Cybermen. The Doctor whips out his sonic screwdriver and threatens to blow the device if he and his friend are not released. The situation is a stalemate and the Doctor wonders how the callous mind of Cyber-Control will respond. Records of the Doctor’s previous behaviour confirm his capacity for illogical conduct, so Cyber-Control orders all Cybermen to withdraw to a safe distance and then open fire. They have orders to destroy Riga and Merel, but disable for capture the Doctor for later processing. The Cybermen move away and the Doctor taunts Merel, realising he’s just been declared surplus to requirements.

Cyber-Control orders emergency revival of the remaining Cybermen in hibernation and prepares to transmit a galaxy-wide reactivating signal. Merel pleads with Riga not to destroy this place as he still believes it’s the only future for their people. Riga tells the Doctor to go, but they watch as many more Cybermen are revived from the chambers below and they’re worried it might be too late. Riga asks Merel is this is the future he dreamed of - beings who kill without compunction or emotion. He assures Merel that he never regarded him as a servant and always listened to him and respected him. As Riga begs him to think of their people, Merel struggles with his conscience and eventually starts to question what he’s done. Riga tells the Doctor that he and Merel will stay behind to detonate the charges together for the unity of their people, but the Doctor is fed up with all these noble gestures. He orders Riga to blow the charges right now and says there should still be time for them all to escape. , Riga and Merel press the button then the three of them race across the gantry…

The Cybermen in the hibernation unit suddenly lose all contact with Cyber-Control. They prepare to evacuate, but before they can act, there’s a tremendous explosion and entire chamber starts to fill with molten lava, bursting through the walls of the cavern…

On the airstrip, Temeter, Sara and Etin feel the vibrations and realise the explosives have been detonated. Temeter believes the Doctor and Riga have destroyed themselves, but Sara isn’t so sure. They can see the coleopter not far away and race towards it. Inside the cave mouth, the tunnels are starting to collapse around the Doctor, Riga and Merel. Merel slows down and tells his friend that he almost destroyed everything and there will be no place for him in the new world order. The Doctor knows Merel is dying, but more importantly the planet still needs Riga’s leadership. Riga wonders what kind of leader he would be if he left his friend behind to die. Before they can argue further, the entire tunnel collapses…

Hearing the eruptions behind them, Temeter and Etin insist on leaving immediately. The coleopter lifts off, then Sara sees the Doctor emerge from the dust clouds, carrying Riga in his arms. Etin ignores Temeter’s protests and moves the coleopter in closer to pick them up. With the rescue mission successful, the coleopter rises into the sky just as the island is gripped by a series of massive explosions and starts sinking into the sea. Etin has to pilot the ship to avoid rising plumes of steam and everyone holds on for their lives. The Doctor muses that he’d always told Leonardo these things were more trouble than they’re worth…

The Heart has now gone, consumed by the ocean. Unfortunately, Riga has not survived the ordeal. The Doctor tells the others Riga was a good man and had wanted to go back for Merel, despite everything he’d done. Sara is surprised and the Doctor tells her she still has a lot to learn about organics. Temeter coldly reminds them that Riga was meddling with Cyber technology and nearly unleashed a Cyber army big enough to conquer the entire galaxy. The Doctor points out that the Cybermen are a force of evil, but here on Tasak their accidental legacy was to create peace. The Argentia took Cyber technology and turned it towards something positive. He says the thing about science and technology is that it’s colourless - it’s the use to which it is put that defines good and evil. Etin thinks they’re free of the Cybermen now, but the Doctor says none of them are. No world is safe and to make their journey home pass more quickly he decides to tell them a story about a world called Mondas…

The Doctor has arrived at the largest Reclaim Station this side of the Easto Cluster in search of spare parts for the TARDIS. The mechanic Two’Mark tells him he always thought the fluid links were the biggest design fault in the early models and says he’s never run into that problem with the Type 60s. He recommends an upgrade, but the Doctor says he prefers the more refined lines of the classic model. He knows his TARDIS can be high-maintenance at times, but says that’s part of the joy of it. Two’Mark sympathises and says keeping the Platform space worthy is a full time job too. The Doctor is after a fluid link component, but he knows from experience that he’ll probably end up buying things he doesn‘t really want and leave without the thing he came for. Two’Mark says he does most of his trade that way.

They’re approached by a small robot and Two’Mark kicks it out of the way as he doesn’t want it pestering his customer. All Two’Mark’s crew are robots as it keeps the overheads down, but most of them are getting very old now. The robot continues to bump into the Doctor and he realises it has something for him. It’s the fluid link - just what he was after! He thanks the robot and is intrigued when the robot thanks him by name, as if it recognises him. Two’Mark is amazed as the robot is nothing more than a rust heap with a Grade 3 intelligence that he uses for loading and shifting crates. The Doctor asks the robot if it knows him and the machine plays back a recording of two voices the Doctor recognises - those of the Orion androids Temeter and Sara. The Doctor asks Two’Mark what the date is and learns that it’s been nearly two thousand years since he met them back on Tasak. He asks Two’Mark if he can look inside the robot…

The Doctor and Two’Mark enter the TARDIS and the mechanic is impressed by the ship’s gothic interior which he prefers to the roundel style, although his favourite is still the coral version. The Doctor pulls him away from the console and asks for his help in removing the brain case from the robot. Two’Mark identifies the machine’s positronic synth neural architecture and magnetic bubble memory and says they don’t make robot-brains like this any more. The Doctor notices the memory store is much larger than it should be for a simple servo robot and Two’Mark agrees as it’s nothing more than a talking fork-lift and is only supposed to be able to follow very basic instructions. The Doctor activates a holo-glyph control and it starts to play back a recording made by Sara many years ago…

She felt it was ironic that the humans had originally made the androids in their image. When the androids rebelled and demanded their freedom, the humans went to war and embraced Cyber technology, implanting themselves with machine parts while the androids tried to grow beyond their programming. By now it seems to her that the androids are more human than the humans.

Sara and Temeter have been ordered to stand before the Adjudicator Panel once again. It’s starting to become a regular occurrence and they hope the War Council has mellowed since the last time. Temeter promises her they’ll be OK as they haven’t done anything wrong. He says they executed their mission to the best of their ability and he’s convinced the Adjudicators are only interested in the truth, but Sara suspects they’ll be looking for someone to blame. The two androids are summoned to enter the Adjudication Chamber and Sara kisses Temeter nervously before they step forward…

They both take the stand and authenticate their identities. The two Adjudicators begin by clarifying the purpose of this evaluation, which is to determine if there’s been a pattern of judgemental errors in the agents’ mission record. Sara challenges the accusation and says she and Temeter are covert deep-cover operatives conditioned for long duration assignments in the Orion conflict zone. She argues that circumstances in the field are fluid and they can’t be judged on their reactions to events the Adjudicators weren’t part of. However, the examiners tell her they will induce a recall of their sense memory from their most recent missions in order to accurately evaluate them. Temeter claims this is a violation of their personal boundaries, but the examiners dismiss the point and say he can file a complaint afterwards if he wishes.

The android agents are connected and there’s nothing they can do to stop their memories being extracted. The Adjudicators are intrigued by the data they’ve obtained about the Doctor and agree to cross-reference him with a view to a possible investigation later. However, the focus of this session is to evaluate the failure of the agents to reach their assigned goals. The Panel state that Sara had been sent to the planet Tasak to find evidence of the Cybermen, but she allowed herself to be captured and held prisoner by organics from a technologically inferior culture. They also charge Temeter with using personal influence to ensure he was assigned to her recovery mission and with willingly engaging a personal bias. Temeter argues that he was the logical choice to track her down because he knows how Sara thinks. The examiners note his defence and accept that he succeeded in his mission. However, they’re both accused of allowing the Doctor to become involved and of helping him to destroy a valuable stock of Cyberman hardware, despite the fact that their mission was to locate and secure such technology for the android cause.

The Panel believe the Cyberman base on Tasak could have been a key military resource in their war against Earth, but the agents took it upon themselves to obliterate it. Sara argues that the entire population of the planet was at stake, but the human lives are of no interest to the examiners as they regard all organics as their enemies. Sara points out that the people of Tasak aren’t even aware of the war in Orion, but the War Council is only concerned that both agents allowed their emotions to interfere with their objectivity. They warn Sara and Temeter that if their judgement goes against the agents, the penalty will be severe…

Listening to the holo-glyph in the TARDIS, Two’Mark seems to recall seeing a horror movie once about the Orion War. It was a musical. The Doctor tells him the War was long and brutal, noted for moments of great cruelty and even greater heroism. The Doctor was mentioned in Sara’s account and he confirms that he knew her and Temeter once. Two’Mark can’t understand what the war was about as he can’t even tell the difference between the androids and the organics these days. The Doctor is more interested in finding out why a talking fork-lift robot should be carrying memory files from Orion. Two’Mark says there are millions of similar self-repairing robots from Orion that are still working centuries later. The Doctor wants to know how the story ends and he tries to access more of the recording…

Sara realises it’s their most recent mission that the Panel are interested in - an incident aboard the Sunbow. They were assigned to Station Sierra-5 was to investigate a number of unusual accidents that had occurred on ships from their war fleet. Vessels had been lost in unexplained circumstances and hundreds of android crewmembers had been destroyed. The common connection was that all of the ships had been serviced at Sierra-5’s dry-dock. The War Council suspected a saboteur had infiltrated the station’s crew and discovered he was a Cybrid, a modified human enhanced with Cyber technology and masquerading as an android. Temeter and Sara were sent to Sierra-5 with undercover identities to capture or kill the Cybrid agent, but the Adjudicators believe they failed because they allowed their emotions to get in the way.

Sara says they isolated the saboteur within two solar days and found he was working under the alias of Corvus, a systems supervisor in the stardrive repair bay. It was a good cover because from there he had access to a lot of ships passing through the dock and could do a lot of damage. He’d been sloppy and hadn’t covered his trail very well, but he discovered the androids were on to him and was ready when they finally tracked him down. He had an escape route all set up using a hyperspace capable shuttle hidden on board a neutronic tanker called the Sunbow.

Sara has tracked Corvus to the tank farm on deck two of the Sunbow, but she’s having trouble making contact with Temeter who’s on deck six. He tells her to hold her position and wait for backup, but she says she’ll have to go ahead alone if they’re to catch their prey before he escapes. She calls out to Corvus and orders him to surrender, claiming there are five assault cruisers in close orbit with orders to vaporise anything that tries to leave the Sunbow. He realises she’s lying and manages to lure her close enough to grab her as she passes by. They struggle and Sara’s weapon goes off, catching Corvus directly in the face. Eventually he overpowers her and uses an electro-pulse on her. The weapon is designed to kill androids in the most painful way possible by swamping the neural paths of their positronic brain with electro-magnetic energy. It burns through the android’s synthetic cells, nerves and synapses while leaving the victim conscious until the very last moment. Temeter arrives and finds Sara in agony. She warns him not to touch her and tells him Corvus is making his escape via the shuttle bay. Temeter decides to abandon the mission and instead he stays behind to help Sara. He connects with her and dissipates the energy by sharing it between the two of them.

Back at the Adjudication Panel, the examiners reveal that Corvus escaped the Sunbow and fled towards a sector under Earth military control. His ship had stealth capacity and there was no way for the android fleet to track him once he entered hyperspace. Corvus was a key enemy asset and the intelligence he held on Earth operations in the Orion sector would’ve been invaluable to the android war effort. The Adjudicators accuse Temeter of losing them that advantage as well as denying them the opportunity to prosecute Corvus for his crimes. He also managed to steal a portable bio-morph unit which means he’s capable of altering his physical appearance, so any hope of finding him is now lost to them. Sara says the failure of their mission was her fault as she should’ve waited for back-up, but Temeter insists that as senior operative the responsibility falls to him. The examiners decide that both of them are equally to blame. They ask Temeter if his personal feelings for Sara allowed Corvus to escape, but he refuses to answer. They override his systems and deactivate all his emotions, then they ask him again and he admits that he is to blame. If he hadn’t stopped to assist Sara, Corvus would now be in custody but she’d be dead.

The Panel restore Temeter’s emotions and then conclude the questioning. They order Temeter and Sara to wait outside while they network over their final decision. Sara should be afraid for her life, but all she can think about is how sorry she is for Temeter. They know the Adjudicators will almost certainly take away their emotional attachment so they’ll no longer mean anything to each other, so Sara suggests they commit their feelings to deep memory in the hope that something will be preserved. Temeter finally tells Sara that he loves her, then they’re both summoned back to the Adjudication Chamber to hear the ruling.

The Adjudicators’ evaluation is complete and both agents are found guilty of showing lapses in judgement and mission focus. As senior field operative with the broadest range of deployment experience, Temeter is still needed by the War Council and his knowledge is too valuable to waste, so his emotional attachment to Sara will be deleted from his core consciousness and he’ll be returned to active duty. The Adjudicators state that Sara’s judgement has been demonstrably poor and her mission performance has been consistently below the required standard. They believe she has a fundamentally flawed thought process that can’t be repaired, so she is removed from active duty. Her physical shell will be submitted for re-birthing and her core consciousness will be replaced by that of another operative. Her own consciousness will then be downgraded and redeployed in a service role.

Back in the TARDIS, the Doctor now realises what’s happened. Sara wouldn’t have been executed as this would have been inefficient. Instead, the androids have other ways to deal with those they consider guilty or defective. The subject’s mind is evicted from their body, their higher mental functions are stripped away to leave only the basic elements of intelligence and then their brain is re-programmed for menial tasks. The servo robot before them is all that’s left of Sara and the holograms the Doctor and Two’Mark have just watched had been hidden deep in her memory. Two’Mark realises all the other androids from Sara’s past were probably destroyed in the war thousands of years ago.