The Doctor wakes up with a start and calls out for his friends Mel, Ace and Hex - then realises they’ve all gone! Time has passed and history has moved on. The sickness has disappeared and as he reaches out for a mirror, he’s relieved to see his own reflection staring back…

Kalendorf strides towards the chambers of Susan Mendes and insists on being let in. He turns on her and asks whether she knew about the arrival of the Daleks’ science division, but she assures him she only found out herself about 30 seconds ago. She accuses him of being suspicious, but he wants to know why they’re here on the planet Zaleria, but the Daleks haven’t taken her into their confidence. All she knows is that they expect to be greeted by the civil administration and she asks him to arrange this.

The Dalek spaceship comes in to land and the Zalerian leader asks Kalendorf what’s going on as his people are in fear. He urges them to stay calm as the last thing anyone wants is panic in the streets. The Daleks emerge and ignore Kalendorf’s greeting, demanding to know why Susan Mendes was not here to meet them as expected. He attempts to excuse her absence, claiming the ’Angel of Mercy’ is engaged on important work on behalf of the Dalek Empire. The Dalek insists that the needs of the science division supersede all other instructions and he’s ordered to make sure their work here is unimpeded. Additional security measures will be necessary and the Daleks have brought with them reinforcements in the shape of Ogron troops. Kalendorf is shocked by the appearance of the alien soldiers, especially when he’s told they have orders to crush all opposition without mercy. The Ogrons are instructed to establish an exclusion perimeter around the Dalek command block and to set up security checkpoints at all major interchanges throughout the city.

The TARDIS materialises in the jungle nearby. The Doctor knows he is on Zaleria and that this is the time of the Daleks’ second great occupation, but he has no idea what other secrets will unfold on the planet.

Susan Mendes has never heard of the Ogrons either, but Kalendorf tells her they’re having exactly the effect the Daleks wanted - the Zalerians at the landing pad were visibly quaking at the sight of them and the mood in the city is getting ugly. Susan can’t understand why the Daleks wanted extra security as this planet has a completely unblemished record of co-operation. Kalendorf mocks her for sounding proud of such an achievement, adding that the irony is that these new heavy-handed security tactics are the very thing that threaten to besmirch that record. The communications system is overridden by the Daleks with a special announcement to the citizens of Zaleria - three of their outer sectors are to be evacuated immediately and if this is not completed by the following morning, any remaining inhabitants will be destroyed. Susan realises these areas relate to the Elders’ settlements and wonders whether there’s any connection. The Daleks also order that certain Zalerian workers will be required for special duties and the people are to be selected by the Angel of Mercy herself. Susan can’t understand why the Daleks are risking chaos on an otherwise tame planet, so she decides to contact the Dalek Supreme. When Kalendorf mocks her again, she asks him to leave.

A fleet of transport ships is brought in to aid in the full-scale evacuation of the Zalerian Elders’ settlements. Away from any prying ears, the rebel leader Skerrill contacts Talamar and pleads with him to make sure there’s no bloodshed. He understands, but believes their people need to hear it from her personally as they’re losing faith in the Angel of Mercy. Skerrill reminds him that Susan Mendes is as much a slave as anyone else, but she’s sure Susan will had done her best for the people of this planet. Later, the Doctor appears behind Skerrill and advises her not to carry any of the rebel secrets with her when she leaves as everyone will be searched on arrival at their new home. She claims not to know what he’s talking about, but he knows she’s involved in organising an uprising against the Daleks. He asks her to let Kalendorf know that he’ll be arranging a meeting with him later.

The evacuation is proceeding on schedule and the Daleks place explosive charges in position around the settlements, set to go off in 50 rels, which will completely flatten the area. One transport ship reports engine malfunction and will not be able to life off before the detonation deadline, but the Daleks are ordered to go ahead as planned as only Zalerian prisoners and Ogron pilots will be killed. Dalek units are then informed that as soon as the designated areas have been obliterated, seismic probing can commence immediately so their objective can be located with all speed. Susan Mendes arrives outside Dalek control to discuss the selection of work shifts, but her request for entry is refused. She argues that the presence of their scientific division is causing unrest amongst the local population but the Daleks accuse her of threatening to go over their heads to the Dalek Supreme. She reminds them that they have only one mission here, whereas she is responsible for aiding the entire war effort to ensure that valuable Dalek resources aren’t wasted on the unnecessary suppression of people on worlds that have already been conquered. She’s spent years facilitating the peaceful occupation of over half the galaxy and it’s that which has freed up their armies to prosecute the war. The Dalek leader consults its standing orders and grants her permission to enter. At that moment, the countdown reaches zero...and the explosives detonate at the three chosen sectors!

The explosions can be heard from miles away. Skerrill tells Kalendorf she was on the last transport ship to lift off and she noticed that half the ships on this planet are burnt-out wrecks - it seems the Daleks couldn’t even be bothered to invade Zaleria with decent hardware. The people aboard the ship with the engine malfunction didn’t make it in time and although she’d hoped the Daleks would delay the detonation to give them time to get away, but Kalendorf knew this was never very likely. She mourns the people who died and she knew every single one of them. Kalendorf knows she’s angry, but he tells her it’s important she stops her people rising up against the Daleks as it‘s too soon and they wouldn‘t stand a chance. Suddenly the Doctor appears and Kalendorf tells him he‘d better explain his presence if he wants to get out of here alive. Skerrill tells him that she met the Doctor earlier and he asked for her help in tempting Kalendorf away from Dalek security, but now the Doctor wants to talk to the warrior alone.

Susan insists that the Daleks’ behaviour has been unacceptable - it would have taken less than an hour to have another transport ship fly into the detonation zone and replace the one that had broken down - but they say the lives of the people on this planet are of no importance to them. Susan points out that the Zalerians are a volatile people but the Daleks interrupt and order her to begin selecting worker shifts for special duties. She demands to know why, but they will only say that further work groups may be needed later once their science division has relocated to the areas where the explosions were detonated. She refuses to co-operate and they remind her that her sole function is to facilitate all matters of population order and control. She doesn’t believe they have the authority to execute her, but they confirm that in this instance that do. Susan has no choice but to make the announcement…

The Doctor and Kalendorf are alone in a private booth in Bar Zaleria. The Doctor knows that Kalendorf is a Knight of Velyshaa who he was on a secret mission to make a defence pact with the Earth Alliance when he got caught up in the Dalek invasion. But that was years ago and ever since then he’s tagged along with Susan Mendes and together the two of them have become traitors to civilisation. He’s interrupted by a city-wide announcement from the ‘Angel of Mercy’ and the Doctor can’t hide the contempt in his voice, describing her as the willing puppet of the Daleks. They listen as Susan instructs several work groups to go immediately to the Dalek command block. Kalendorf has heard enough from the Doctor and threatens to kill him, but the Doctor explains that he’s come here because the Daleks know him of old and his greatest fear is that one day they may win. He admits that he knows what Kalendorf and Susan are really doing and he knows they’ve been using his telepathic powers to plant the seeds of rebellion on every planet they’ve visited. More importantly, he knows that something is going to go horribly wrong if they don’t put it right. He reveals that there’s a terrible secret buried on Zaleria and the Daleks have been searching for it ever since they began their invasion of the galaxy - but they don’t know they’ve found it yet! They’re only just beginning to suspect that they’ve got it right and they’re about to unleash the most powerful Dalek army ever created. Kalendorf returns to Skerrill and when they hear the seismic probes being launched they realise the Daleks are looking for something. When they try to question the Doctor, they discover he’s disappeared again.

The Dalek command base orders a triangulation pulse to be sent, and within moments the results start coming back…confirming that their objective has been located! The drilling operation is ready to begin and authorisation is given to commence experimentation on the slave workers. In their cells, the Zalerian prisoners beg to be released and demand to know what’s going to happen to them. They’re ordered to enter a light-wave chamber and are positioned in front of special projectors. The Daleks then leave and the projectors are activated. The prisoners scream in agony…

Kalendorf and Susan meet secretly in the jungle, a little later than planned as it proved difficult to get out of the city without being seen. Susan is confident they can’t be overheard here and she refuses to let him make the physical contact necessary for his telepathy. She warns him that something big is going on, and in return Kalendorf tells her he’s received intelligence that there’s a massive Dalek army buried deep under the surface of the planet. She asks him who his contact is that provided this information, but when he tells her it’s best that she doesn’t know, she realises he doesn’t fully trust her any more. However, they can’t risk not trusting their contact so Kalendorf asks her to cover for him while he’s away. Their conversation is interrupted by a communicated message telling Susan that everyone in a three-block radius has heard screaming coming from the command block. The work shifts she nominated are in there and she thinks the Daleks are killing them! The stories are already travelling around the population and the first riots have started. She tells Kalendorf to do whatever he has to, while she goes to investigate.

The Daleks order the rioting Zalerians to disperse immediately or the Ogron forces will be deployed against them. People in the crowd start hurling abuse at the invaders, demanding to know why they’re killing the people in the evacuated zones. The order is given for the Ogrons to advance on them…

Kalendorf brings a small group of rebels as close as he can to the outer sectors. Skerrill reports that the whole area has been blasted clear and there’s nowhere left for them to hide. The Daleks are still digging so they can’t have found what they’re looking for yet, so she orders her colleagues Mendac and Aytrax to prime their explosives. Kalendorf suggests they wait until they’ve found the Dalek army and worked out a way to destroy them first. They’re hopelessly outnumbered and they haven’t even discovered a way down into the planet yet, so they’re in danger of running out of time and options. Skerrill promises him that none of her group will allow themselves to be captured alive, so they go ahead with preparing their weapons. Mendac and Aytrax have found one of the old ice volcanoes which they think will be a good place to start, and Kalendorf suddenly realises they knew about the buried Dalek army all along. Skerrill says that only the Elders knew about the army, but everyone on the planet knows of the Daleks as they’re part of their history. It’s not widely known, but it’s written in their sacred texts that the Zalerians once worked for the Daleks. They spot a patrol approaching so they head off for the nearest ice volcano.

The prisoners in the light-wave chamber scream in agony and tell the Daleks they can’t take any more pain. The experiments are starting to alter the Zalerians’ cellular structure, but the readings indicate there is a pigmentation barrier and one more projection is needed to break through and confirm the Daleks’ prognosis. The next wave of the experiment continues and the prisoners start screaming again.

Susan contacts Dalek control urgently and demands that they stop whatever they’re doing to the Zalerian prisoners. Suddenly the door bursts open and the Ogrons enter and drag her out of the room, and when she struggles, they deliberately break her arm. She’s to be taken to the Daleks and on the way she hears the screaming at first hand. The Daleks order her to be silent, but she tells them they can break every bone in her body if they want, but she won’t stand by and let them continue torturing the prisoners. An Ogron steps forward and attacks her again. The laboratory Daleks contact the command room and confirm that the light-wave experiments have been successful. They show images of the test results from the chamber and Susan can’t believe what she’s seeing. They demand that the insurrection stops and Susan is ordered to speak to the rioters to tell them that further slaves will be needed to take part in the experiments.

Kalendorf and the rebels make their way into the planet through the ice tunnels, but they encounter a large overhang which means they have to jump across the last bit of the journey. They hear a distant rumbling which means either the Daleks have started drilling or the volcano is about to erupt. Kalendorf jumps across first and the ice on the other side seems safe enough so the others follow one by one. Skerrill is sure the sound is from the drilling, so they can only hope that the Daleks don’t rupture any of the localised caverns or they could find themselves swallowed up by molten ice.

The Daleks at the drill-head report to the command base that they’ve nearly reached their objective, and they’re ordered to increase power to maximum and proceed at full haste. The Ogrons continue to struggle against the rioting population, but the conflict is interrupted by the Daleks announcing a message from the Angel of Mercy. Susan addresses the people of Zaleria and tells them they cannot defeat the Daleks and if they continue to challenge their authority, they’ll all be killed. She tells them the Daleks have a secret mission here, and although she doesn’t know what it is, they’re prepared to go to any lengths to complete it. They would prefer the Zalerians to continue being an efficient workforce, but they’re willing to sacrifice them if necessary. She urges them to go back to their homes, and it appears that she’s about to offer them some words of hope for the future when the Daleks interrupt her.

The sound of an explosion fills the air and the Daleks report total success of their drilling operation. Their scans reveal one million and one hundred thousand Dalek units are submerged under approximately thirty thousand cubic gigathrons of molten ice beneath a ten gigathron skin of frozen ice. Dalek command orders that heat rays are activated and turned up to their maximum safe setting. The ice immediately starts vaporising and before long their scanners show the army starts the first phase of its reactivation.

Kalendorf’s group come to a stop when they realise the temperature in the caverns is rising rapidly. Without warning, the Doctor steps from the shadows and tells Kalendorf he was right all along. The Doctor thought he wasn’t meant to interfere, but now he knows he must - but first they have to get out of the scorching vapour. He asks Skerrill to confirm that there’s a Dalek army buried under tons of ice and she wonders how he knew. Kalendorf realises the heat is the result of the ice being melted, but the Doctor’s priority now is to get the Knight of Velyshaa to safety as he’s too important to die here. He leads the group to a safe hiding place and Skerrill realises he’s the same Doctor from her planet’s history. The Doctor reveals to Kalendorf that this planet was once known as Spiridon, but it’s more than just the name that’s been changed. Skerrill says they had no choice - their planet was occupied generations ago and the people were enslaved…or more accurately they agreed to work with the Daleks to save themselves. She explains that the Daleks came here for a specific reason. Not just to store a vast army, but also to gain knowledge.

Susan asks the Dalek command what happened to the Zalerians in the laboratory as it looked as though they were fading away. The Daleks reveal that invisibility is the natural state of the Spiridon people and ‘Zaleria’ is merely a deception to hide this planet from them - but now the Daleks have returned and their original mission will finally be completed. They tell Susan that she and Kalendorf will be transported from this planet to begin their next assignment, and although she insists that their work here isn’t finished, the Daleks refuse to listen.

Kalendorf has learned that the Daleks plan to reactivate their army and make it invisible! It would give them a terrible advantage, enough to tip the balance of history. The Doctor thought his role was just to make sure nothing went wrong here, but now he knows he has to interfere and become a part of history. Maybe this is his penance for not doing the job properly last time he was here on Spiridon… Skerrill reminds him that the Daleks never fully mastered invisibility because the massive amounts of energy they had to expend to maintain it caused light-wave sickness. The Doctor asks if she’s heard of reverse engineering - the Daleks plan to discover how the Spiridons made themselves appear to be visible and will reverse that process. Skerrill explains that her forefathers subjected themselves to many painful forms of treatment, mostly based on the ingestion of cell-altering chemicals, such as pigmentation and nutrients that decayed their non-reflective cellular properties. It’s become a vital part of the ‘Zalerians’ food chain, but even now their visibility is only skin deep and their genetic coding is constantly trying to reinstate their natural invisibility. The Doctor returns his attention to Kalendorf and says his aim is to make sure he fulfils his potential. Kalendorf is becoming frustrated by these cryptic comments, but his priority is to stop the Daleks…and fortunately the Doctor has a plan!

The heat ray equipment in the caverns is shut down as soon as the ice elimination is complete. The drill-head team report that the Dalek army has been freed from the ice and the scientific division prepare to join them at their location. The test results of the invisibility experiments are still inconclusive, but work will continue until the problem of light-wave sickness has been eradicated. Then the army will be made invisible and will proceed with the conquest of the galaxy and no force will be able to stand against them…

The Doctor leads Kalendorf’s party to the huge caverns where the Dalek army is beginning to awake. Fortunately they’ve been in deep-freeze for generations, so it’ll take time for them to fully recover. They’ve already passed through about twenty chambers and there are even more ahead of them, which tells the Doctor that the reports he heard last time he visited the planet of ten thousand Daleks were inaccurate. They’ve already seen about ten times that number so the problem is much bigger than he thought. Suddenly Aytrax spots a group of Daleks ahead of them, but before he can shout out a warning, they open fire and he’s exterminated. The others take cover and when Skerrill tries to help Aytrax, the Doctor warns her to keep still. An alarm sounds and the Daleks announce that intruders have been spotted. The Doctor realises they have to move fast to get their explosives to the heat emitters - if he can destroy their control units and override the activation controls they may be able to set the equipment to maximum and melt the Daleks!

The Daleks report that they’ve been unable to locate Kalendorf and they demand Susan tells them where he is. She says he didn’t inform her of his precise location, but she trusted him when he said he was doing important work for the Daleks. They accuse her of trying to deceive them and threaten to set the Ogrons on her again. They have access to data on her personality and physical responses and know she is afraid and will be unable to resist further pain. When an Ogron is ordered to break her left leg, she has no choice but to reveal that Kalendorf told her about the Dalek army buried on this planet. Just then, the Daleks at the drill-head report that intruders have been detected and Susan is forced to admit that Kalendorf is probably among them. She reminds them that the Dalek Supreme has issued standing orders that he is not be killed as he’s vital to Susan’s work, so they issue orders for him to be captured alive.

Airborne Daleks are sent to locate the intruders and when the hiding rebels overhear their orders not to kill Kalendorf, they realise Susan must have revealed his betrayal. The Doctor recognises the area they’re in from his last visit to Spiridon - but just then Mendac is spotted by a nearby Dalek patrol and before he can pass his explosives to the others he is exterminated. The effects of the ray triggers a huge explosion which brings part of the roof down in the tunnel. The Daleks report back to the command area that one of the intruders is dead and Susan is worried it might be Kalendorf. Fortunately the others have survived, although Skerrill is badly injured. The Doctor spots the heat emitters and races off with the other explosives, and before long the Daleks report that their heat wave projector has been disabled. They try to shut down the power, but by then the Doctor has already bypassed the controls.

Kalendorf notices it’s starting to get hotter in the main chamber so he helps Skerrill to her feet, but they’re immediately stopped by the arrival of more Daleks. They order him to accompany them back to their main base, but Kalendorf calls out to the Doctor and instructs him to melt them and destroy the army. He knows he and Skerrill will die, but as her race failed the galaxy before by collaborating with the Daleks, she’s willing to make amends by sacrificing herself to stop their army becoming operational. At the command base, the Daleks overhear Kalendorf calling out to their enemy the Doctor and they scan for him and detect his presence by the heat ray emitters. Susan urges them to exterminate the Doctor before he destroys everything in the cavern, but he already has control over the heat ray and they cannot override it. However, the Daleks know from their profiles on the Doctor that he has compassion, so they threaten to kill Kalendorf and Skerrill unless he ceases his sabotage. Kalendorf is surprised when the temperature starts falling. The Doctor emerges and admits that he’s switched off the heat rays because the most important thing is preserving Kalendorf’s life. The Knight of Velyshaa is furious and accuses the Doctor of being a coward and a traitor, but there’s worse to come. The Doctor informs the Daleks that he can cure their light-wave sickness and he agrees to help make their army invisible if they let their two prisoners go. He realises they’ve already assimilated a vast amount of data from the tortured Spiridons, but he also knows they haven’t been able to make any sense of it. They agree to his terms and the Doctor says goodbye to Kalendorf and tells him he’s working for the Daleks now.

The Dalek spaceship leaves Spiridon, taking Susan and Kalendorf to their next assignment. Susan tells he old colleague she was wrong about him and she now realises it’s not that he doesn’t trust her, it’s just that he hates her. He tells her they have a job to do and he doesn’t have time to hate her. She asks about the Doctor and Kalendorf admits that he doesn’t know who he was, he just hopes that one day everything they’ve done here and everything they’re going to do will make sense.

The Doctor remains a prisoner of the Daleks for many years. Most of the Spiridons think he must have been killed because nothing changes - the Daleks remain on the planet and the slave labour continues, but the army never emerges. Then one day, the Spiridons’ invisibility starts to return to them and centuries of cellular decay is reversed. No one knows for sure why, but Skerrill is convinced it has something to do with the Doctor…

In the underground cavern, an emergency alarm sounds and the Doctor proudly declares that after waiting all these years, he’s chosen this moment to release a contagion upon them. The Daleks move in to exterminate him, but before they can act they’re overcome by his “cure” which not only returns the Spiridons to their natural state of invisibility, it also infects everyone else - including the Doctor - with light-wave sickness. He explains that he did this after overhearing the Daleks talking to their command about a rebellion that’s started on Yaldos. This means Susan and Kalendorf have finally succeeded in their mission and the Dalek Empire is going to be defeated. History has been saved. The Daleks on Spiridon scream out in agony and die…