On a fog shrouded planet called Solos, the Marshal, a black uniformed man with blaster rifle, drops his oxygen mask as he chases a Mutt, an old man with a long beard in ragged clothes. His two aides, black man Cotton and Stubbs want to leave this world, fed up with hunting Mutts. They find the Marshal over the hunted man... who is dead. They have been killing Mutts like this before. Mutts are humans on Solos who seem to have twisted spines. To the superiors on a space station above the planet, Stubbs reports that this Mutt died of unknown causes.

At UNIT, the Doctor is making a minimum intertia super drive for Bessie. Jo is commenting on the fact that it is way past lunch time when a strange multi sided container appears on the table in front of them. This has been sent by the Time Lords for the Doctor to deliver to someone. Jo wonders if they have no other way to deliver it. The TARDIS comes to life via the Time Lords. The Doctor takes the container and his cape and tells Jo she cannot come -- it is bound to be dangerous. To her exclamation of the fact that he needs her, he says, "...a lot of rubbish." He enters the TARDIS but Jo jumps in before the doors close.

Skybase One, a station orbiting the misty planet is where the TARDIS materializes. The pair exit the TARDIS. Jo finds a round porthole where they see the planet below. The Doctor comments it is like a tradesman entrance. A voice calls over a loudspeaker that visitors must be put through a decontamination unit which Jo calls cheek after the Doctor calls it delousing.

But the visitors that are expected are not Jo and the Doctor but primitive and proud warriors from Solos who arrive via transfer cubicles. Ky, a more noble native arrives to find another chief leader named Varan there first... Varan who works with the Overlords. Younger Ky was summoned to the conference. Ky wants his people to stop being murdered by the Overlords. Varan tells Ky that the mutants are evil and must be rooted out. No one knows what the mutants are... Varan believes they are diseased, told to him by the Overlords. The Solos people were once farmers and hunters but since the Overlords arrived, the planet, which was clear of mist and full of sweet air and rivers, changed. Ky and Varan argue and almost come to blows but an Overlord guard Stubbs stops this and tells Ky and other primitives to go into decontamination units. He also sends Varan to the Marshal. Varan's bodyguard remains behind, not going to a decontamination unit. A guard tries to make him go too but the man's hand is revealed to be a crustacoid claw and the guard is attacked.

Jo thinks the area the TARDIS is in is a broom closet. She asks where they are time wise. The Doctor tells her not to use expression like that. They are in the 30th century of the Earth Empire. Once the solar system was sacked, the Earth people moved on to other planets. He asks if she's ever read Gibbon's Decline and Fall of the Empire. The Doctor takes out the sonic screwdriver to break the circuits of the door.

Varan reports to the Marshal, who has other plans for Ky than allowing Varan to kill the younger leader in a combat situation.

Jo reminds the Doctor to bring the box and they get out of Storage Area 4. A computer reports the door being opened as a malfunction. The Doctor is not sure he likes being described as a malfunction. He has given Jo the message box.

The computer reports that Cotton and Stubbs should investigate the malfunction but they will do it tomorrow. They are playing chess in Storage Area 5.

The Marshal gives Varan a pass and a gun to dispose of Ky at the conference.

Jo and the Doctor arrive at Storage Area 3 where the Doctor starts to open a door but Jo points. The mutating bodyguard attacks them! The Doctor gets the door open and they race on through, the door shutting on the man's arm. The Doctor holds the door shut but can't much longer. Cotton and Stubbs arrive and shoot the man dead with blasters.

Marshal receives news of this. The Administrator comes to talk to the Marshal about giving the Solonians independence. They are pulling out of the area. Earth is exhausted politically, economically, biologically... it cannot afford an empire anymore. The Marshal's atmospheric experiments might be the reason for the mutations but the Marshal refuses to believe that. The Marshal has put years of his life into this planet and experiments. He will now be reduced to working behind a desk on Earth at the Bureau of Records. When the Administrator leaves, the Marshal uses his staff to call for that "fool" Varan.

The Doctor and Jo are in a room where they can look out and see the planet at daylight now, through a porthole, Jo using a pair of large sun glasses. The Doctor tells Jo about Earth in this time: grey seas, grey cities linked by grey highways, grey deserts, slag, ash, the products of technology. The Administrator arrives to take their statement. The Doctor claims the Overlords sent him but the Marshal arrives to refute the Doctor's claims. The Doctor shows them the message box but it doesn't open. The box will only open for the person the box is meant for... he cannot open it. It will only open for the person it is for. Stubbs puts the box on a stall and the Marshal fires his blaster rifle at it. It does not open. The Marshal thinks the Doctor and Jo are from Solos or are saboteurs or both. The Marshal leaves Stubbs to watch Jo and the Doctor.

In the conference room, Varan tells the Marshal that the man at the door is his son, named Vorn, someone who they can trust. Vorn, the one Varan has chosen, understands the new instructions from the Marshal. Varan gives his son Vorn the necklace. The Administrator arrives at the conference room. On a round clear ball, the Doctor, Jo and Stubbs watch the conference begin; the Doctor commenting negatively on the music beginning an Overlord Telecast of the conference, "Bombastic signature tune." The Solonian All People's Union, the primitive warriors, are there as well. Stubbs thinks the Solonians should be given freedom just so he can go back home. Jo asks him if he has family back on Earth. He hopes so and he gets the odd message and videos. As Jo asks if these are like letters, which Stubbs does not understand, the Doctor sneaks up behind him and uses a double neck pinch on him. "A pity he was rather sweet," Jo says. They leave, Jo giving the Doctor the box.

The Administrator makes a long speech about a pact, a treaty made between the Solonians and the Earth people: united together to create a new society, a new and richer world. 500 years after the scientific aide, the Earth has steered them to the verge of independence. Ky shouts that they have steered them to disaster. Unrest occurs among the Ky followers as the speech goes on. Ky shouts for freedom and now.

The Doctor and Jo find segregation notices on a hallway marker for transporter units -- Overlords, Solonians, and notices for oxygen masks. Cotton stops the Doctor and Jo at the door. He will not let them in to the conference without a pass even though the Doctor tells him the box contains important documents about the signing of the agreement.

Ky makes a scene and is arrested even while the Administrator yells that they have no need to quarrel -- he is giving them their freedom. The gun given to Vorn is used by him and fired -- a strange dart like device enters the Administrator's neck and he dies. The Marshal yells that it was Ky, who resists and runs out, passing the Doctor and Jo along the way, Ky calling the Doctor an Overlord. Ky inadvertently touches the strange box of the Doctor's and it starts to open, "Wait! This is for you!"

Ky and Jo appear in the transfer station on the planet, where Ky tells Jo his life is more important than hers. He cannot stop to help her if she gets in trouble but he does spend time to get her out, away from a guard, whom he fights down to the ground. They get to an airlock door and close it just as the fallen one also fires his blaster. Finally they get out to the surface of Solos.

The Marshal tells the Doctor during daylight humans must wear oxygen masks. A nitrogen isotope in the soil combined with ultra violet rays from the sun cause a deadly mist which cannot be breathed in by humans from Earth. He gives Jo an hour. The Doctor insists that he have his men continue to search for Jo. "I can do as I please," the Marshal tells the Doctor. Solos is under Marshal law. The box is closed again and unless the Doctor shows him what is inside it, he will call off the search for Miss Grant. The Marshal at the Doctor's insistence also orders that the girl must not be harmed. In return, the Doctor will try to open the box but he must do so in a laboratory.

On the planet, Jo collapses, not able to breath. Three men in masks hunt Ky and Jo. Ky carries the unconscious Jo off.

The Doctor calls the lab adequate. He was hoping for a particle reversal set up to turn the box inside out. The lab is more for atmosphere modulation or weather control. The Doctor stops a circuit from overloading just as Germanic Professor Jaeger enters, protesting about this intrusion. The Marshal leaves the Doctor with Jaeger.

Ky hides Jo in some bushes, then watches the guards. One moves in his direction so he jumps the guard and gets his mask off. Ky uses the mask for Jo.

In a cell and dismissing the guards to wait outside, The Marshal goes to Vorn who asks why he is a prisoner. The man he killed was told to him by the Marshal to be an enemy of his people. The Marshal gives Vorn his reward -- to be shot with the same gun. He shoots him in the stomach. Varan arrives moments later and does some quick ceremony over his dead son. The Marshal tells Varan that his son was in league with Ky and that he attempted to kill him, too. Varan knows this is a lie and draws his sword to kill the Marshal and tries to get another weapon. Varan is forced to flee. The Marshal uses his staff to tannoy (loudspeaker call) an order to have Varan captured... telling them Varan has gone mutant. The Marshal orders either Stubbs or Cotton up to the station as Varan evades the guards. Stubbs returns to the station.

The Doctor talks to Jaeger about why the humans came to Solos. The Doctor switches some wires for Jaeger in order to hold the proton beam steady. Jaeger calls the reversal procedure a waste of time but the Doctor finds it a most useful tool. He starts it and the box begins to turn inside out.

Varan hides from guards, entering the Herbarium.

Solos: Ky brings Jo into a disused, slightly green glowing mine cave, one of many on Solos, where there is less mist. He grasps her hands from behind and presses them both down into her chest to get her breathing better and conscious. Ky nearly left Jo to die. Jo worries about the guard Ky took the mask from. Ky tells her the man will be all right if he takes it easy. It is dark now and he will be taken back. Jo explains as best she can about the message and the Doctor but she cannot answer a great deal of his questions because she honestly doesn't know.

The box has flashed back to its regular form, the inside now again inside. The Marshal interrupts the Doctor and Jaeger to ask about Varan. The Doctor asks to go on the Mutt hunt; Jaeger's help gets the Marshal to agree. Jaeger gets the Marshal alone to tell him that with the Doctor's help he could have the entire atmosphere regeneration set up on the planet within a week -- the Doctor understands particle reversal. The Marshal makes plans to keep the Doctor here.

In the cave, Ky tells Jo his people will tell him when the Doctor comes: the dispossessed, the outcasts, the terrorists. He asks Jo to tell him about the caverns of Earth. He thinks maybe she lives in a sky city. He has seen pictures of Earth: no one can live on the ground any more. Jo tells him it is a rather long time since she left earth. Ky's people are slaves in factories and mines; the mutations are caused by the pollution from the atmosphere experiments and the draining of his planet by the Earth people. A shadow of a strange creature is seen on the wall of the cave.

The Doctor and Stubbs enter the Herbarium, the seed propagation area. Stubbs intends killing Varan in cold blood since he thinks he is a mutant. Varan comes up behind Stubbs with his short sword but before he can kill Stubbs, the Doctor uses the fallen blaster rifle on Varan's sword to knock it out of his hand. The Doctor stops Stubbs from killing Varan since there is no sign of any mutation and gets Varan to tell him what he knows about the Marshal's lies. We also see a stage light from the roof enter and leave the scene. Cotton tells the Marshal that the girl Jo has the guard's mask. The Marshal tells Cotton to tell the Doctor that the girl is in a hospital on Solos: another lie.

In the hallway, Stubbs is convinced to play along with the Doctor and to hide Varan until he can gain more knowledge and time. They return to Cotton and the Marshal, who lies some more about Ky leaving Jo to die and about Jo being in hospital. The Marshal will not let the Doctor see Miss Grant unless helps Professor Jaeger. The Doctor and Jaeger work now -- but the Doctor realizes the rocket barrage onto the planet might kill all the Solonians. Jaeger wonders if the Doctor has an alternative solution. Cotton asks the Professor to go see the Marshal, who cannot wait. After Jaeger leaves, Cotton tells the Doctor about his mate Stubbs and their talks. Cotton tells the Doctor the truth: they haven't found Miss Grant. She is still with Ky. Guards have orders to stop the Doctor from leaving. The Doctor wonders about the emergency power supply to the station and wants to blow the power supply while Cotton switches in a secondary supply to the transfer station down to the planet. In the chaos, the Doctor wants to escape to Solos. Cotton will try to do this. Jaeger returns, wondering who told Cotton that the Marshal wanted to see him: the Marshal didn't. "One of us must be mad," he tells Cotton. The Doctor explains to Jaeger about the experiment about Particle Reversal and needs to link up to the main power supply. Cotton beings his aide to the Doctor. In the herbarium, Stubbs tells Varan that he will leave the door unlocked. When the power failure comes, Varan is to escape. Varan doesn't trust Stubbs but he must if the plan is to work for him to escape. The Doctor starts his experiment and his device set up blows up, knocking Jaeger down. The Doctor gets the box and his cape and flees as Jaeger recovers.

The Doctor manages to get himself and Varan into the capsule and they are teleported to Solos. The Doctor frees himself using his karate, then tries to explain as he uses a hold on Varan. Varan still believes the Doctor is an Overlord. A firestorm at night is hitting and this will work for their advantage. The Doctor will not be affected by the atmosphere on Solos, asking Varan, "Did I say I was an Earthman?" There were no firestorms before the Marshal, Ky tells Jo in a cave. Jo sees a monster at the end of the cave. Ky tells her it is harmless... these mutant monsters were his people before the experiments. Ky uses a flame torch on it to drive it off. The storms can last many hours or a very short time. Ky and Jo move further back into the cave. The mutants are usually not aggressive. A group of them come at Jo and Ky. Ky puts Jo into a rock outcropping to hide her while he drives the others off.

The fire storm seems to rage around the space station. Marshal chides Jaeger about the booby trap the Doctor set. Marshal wants the atmosphere on the planet changed now. He calls Stubbs and Cotton to his office. He tells them they will pay their last visit to Solos "to kill three birds with one stone... the Mutts, Varan, and the Doctor."

Varan and the Doctor find a dying fire Ky made at the mine entrance where they see a mutant bug like humanoid -- another mutant. One leaves. The Doctor will go on to help Miss Grant. When Varan refuses to go on, the Doctor asks if he is afraid of the dark. Varan follows him to a surrounded Ky. One mutant makes its way to Jo, who leaves the outcropping to avoid the thing. Ky falls backward over some rock and the mutants seem to converge on him. The Doctor arrives and drives the mutants away from Ky.

Jo is running in the caverns, moving deeper and deeper into strange glowing areas. The sound and the lights flashing make her hold her head and pass out but not before seeing a silver suited figure in the haze. The silver suited being, in a space suit of some kind, moves to her to pick her up.

The Doctor introduces himself with a hand shake to Ky. Varan tells Ky that the Overlords are his enemies too now. He will lead his people in battle against the Overlords. The Doctor gives Ky the strange Time Lord box which fully opens. Inside are tablets and sketchings. Both Solonians wonder how these can help. The inscription is the language of the Old Ones which he cannot read. He's seen signs like these carved in rocks all over Solos. The Overlords destroyed all culture on the planet so that no one remembers how to read these. The Doctor refuses to leave without Miss Grant but Varan leaves to fight the Overlords. As Ky takes the Doctor toward where he left Jo, Ky tells the Doctor about a man from Earth who came to study Solos. This man, Sondergaard, came to study their culture but he disappeared, all arranged by the Marshal.

The Marshal gives Cotton and Stubbs some gas grenades made by Jaeger.

Ky and the Doctor find Jo gone from the rock cubby and being searching for her.

Jaeger tells the Marshal that Solos is getting hotter and hotter. It has been spring for 500 years but now it is summer. It may becomes too hot for humans to stay. There are also more Mutts than ever, all moving toward the target area.

Nearing the exit of the cave, Varan sees an Overlord guard in the cave but he runs out past the others. He is shot at but he manages to avoid the blasts. Four guards chase him but turn back to the cave.

Ky and the Doctor must research for Jo. Ky tells the Doctor the Mutts are watching them all the time. The Mutts have not attacked them in the tunnels, only in the chamber. Ky feels an awareness of being drawn to the center of this place, a warm, safe place. Some instinct in him makes him want to.

The Marshal tells Stubbs and Cotton that no one must know about the gas grenades. They protest that they should find the Doctor. He lets them go into the cave to find the Doctor, telling them they will kill Ky.

Varan returns to his village and finds one old man, who tells him the fire from the sky is responsible. Many people are gone. The old man is told to summon his council. The old man rings a gong.

In the caves, Cotton and Stubbs call for the Doctor. The Doctor suggests he and Ky return to the glowing chamber. They hear Jo Grant inside someplace and find her. She wakes up. The Doctor stops Ky from running from Cotton and Stubbs. The Marshal hears their reunion as well as Cotton and Stubbs claiming they are hear to help them. He orders his men to plant the grenades. The Marshal makes the other men pull out and he plants a seal pack explosive on the cave wall entrance.

The old man tells Varan that the warriors have become mutants. All who can walk have gone to the mines. Varan wants to know why this curse has come upon them. He sees his hand is turning into a Mutt hand. A voice tells him to go to the place of darkness and light, the place of sleeping. Varan draws his sword, yelling he will not die sleeping.

Gas is filling the tunnels of the mine. A Mutt in the mine is gassed. Jo explains about the lights and the silvery figure. The Doctor doesn't want to find himself in the Marshal's hand again. Cotton tells them they should all move out now before the attack begins.

The Marshal thinks the Mutts go to the mine to die -- the old idea of the elephants' graveyard.

The silver suited figure arrives and leads the five to a lead door just as blasts rock the cave and gas fills the tunnels. At a lead door, the Doctor and Jo realize radiation must be involved. The man removes the space helmet to reveal a bald Professor Sondergaard -- the Doctor already guessing the man's name.

The Marshal orders every exit sealed. He will go back to Skybase so he orders a garrison back to the caves once the gas clears.

Sondergaard explains the thesium radiation is natural in the caves but Jo was in a dangerous, unstable zone area. He rescued Jo, who thanks him. The Mutts' numbers increase and their aggression grows. The Doctor stops Ky from yelling about the situation and the Overlords, telling him that all of them here are on his side. Sondergaard tried to inform Earth Control about the Marshal's doings here. The Marshal intercepted the message thus Sondergaard escaped him to reach the caves. The mutants helped him to survive -- they stole clothing and food for him but now they do not come near him, as if he was diseased. The flora and fauna of Solos has changed; at first natural changes. Jaeger's experiments accelerated the changes and something has gone wrong. The Doctor wonders why he was sent here. He asks Ky to touch the box and it opens when the young man does so.

The Marshal tells Jaeger that this is war and that he should prepare his rockets to bomb Solos.

Sondergaard examines the tablets and tells them this is the story how civilization on Solos began -- a lost Solos Book of Genesis. Blasts rock the lab -- the mountain itself is falling into the tunnels. Sondergaard tells them there is an exit to Varan's village. The Doctor will stay with Sondergaard to figure out the answer to the problem. He urges Miss Grant and the others to go, leaving him with Sondergaard.

The others avoid the Mutants in the caves.

The Doctor and Sondergaard find sun symbol but figure on different theories. Life in some form will always go on, the Doctor tells Sondergaard. "I hope so," the bald man claims.

A Skybase guard comes from a tunnel to Jo's group and fires on Cotton and Stubbs. Ky knocks one out so the group can continue.

The Doctor figures the four recurring symbols are the four seasons of the calendar. The seasons must be 500 years long; the planet does not tilt on its axis relative to its sun and takes 2000 years to go around its sun. Stick men mark the spring. Radiation or sun symbols mark the summer. They will go to the chamber to find more answers. Sondergaard worries for the Doctor's life, "You spend any time in there and you will die. Any man would." The Doctor hands him the space radiation suit, "Any man perhaps."

At the tunnel upward, Jo spots a figure with a helmet above. The four have to climb upward. Cotton begins and calls down that it is safe. Ky, Jo, and Stubbs follow.

Varan, his face mutating, addressed a small group of his warriors when Cotton calls to him. One of Varan's men calls him an Overlord from the place of darkness.

The Doctor and Sondergaard arrive at a section of intense radiation. The Doctor reminds the Professor to put his helmet back on. The Doctor finds it magnificent, like a cathedral.

Ky's group enter the empty village but are surrounded by the ragged group of warriors and disarmed.

In the cave, blasts of energy make Sondergaard collapse. He urges the Doctor to leave him and go on. The Doctor checks him and moves on, deeper into the caves of light and radiation. He finds a large glowing crystal on a strange white glowing stand. Taking the crystal, he returns to and helps Sondergaard, carrying the man over his shoulders.

Jo, Stubbs, Ky, and Cotton are tied up and on their knees before Varan. They will act as shields in their attack on Skybase: Varan's revenge.

Skybase: Jaeger reports to the Marshal, telling him the rockets and the engineers have deteriorated almost similar to each other. Deep Space message saying that an Earth Council Investigator is coming. Jaeger tells him that the Marshal's activities will give the investigator quite a report. Marshal wants him to step up the experiment even though the missiles have not been tested. The Marshal wants to meet the Investigator on a breathable surface on Solos.

Varan's attack on Skybase begins with guards knocked down.

On the planet in the lab, the Doctor finds that the life cycle unique to this planet -- the change is natural. The mutants are just an intermediate part of the cellular change. The crystal shows no radioactivity at all. Jaeger's lab is the only place to examine it more closely. The Solonians change to fit the environment and this crystal the radiation in the chamber must help them to do this.

The rocket launch countdown starts and all guards are called back to the station. The Marshal insists that even unauthorized personnel in the station will not stop the countdown.

The Doctor and Sondergaard take the crystal toward the tunnel exit shaft that moves upward out of the mine but Sondergaard collapses before they can reach it.

The Marshal is sucked closer to the opening as the others form a human chain to prevent being sucked out into space. The Doctor and Sondergaard take cover from blasts caused by "that fool" Jaeger as the Doctor calls him. Cotton and Stubbs help Ky and Jo out of the room as the pressure stabilizes. Huh? The Marshal is already out in the hallway with a group of guards he calls to bring the foursome.

On Solos, the Doctor takes out a telescope from his pockets and looks at Varan's deserted village. He and Sondergaard make for the transfer station.

The Marshal, in his office, tells the four prisoners their crimes but can trump nothing up for Miss Grant. The Marshal calls in his firing squad. Ky tells him he will fail anyway. Before the firing squad can kill the four, Jaeger comes in, complaining about the rockets... total failure. Every rocket failed. The ionisation crystals, hitting the planet's surface, have contaminated the planet and will turn it into a lethal desert. No one will be able to set foot on Solos for centuries after the crystals take effect. Jaeger asks how the Marshal will cover up his bout of megalomania from the investigator. Stubbs tells the Marshal he and Cotton are not the only guards who disagree with the Marshal. Jo recounts the Marshal's crimes: the Administrator assassinated, native leaders dead or missing, and a contaminated planet. The Marshal hopes to destroy the investigator's ship. Jo bluffs about her knowledge of the investigator, lying that she and the Doctor were sent ahead before him. The Marshal says, "The Doctor always the Doctor. Where is your precious Doctor now?"

Sondergaard weakens as the Doctor notes the first signs of contamination. Sondergaard urges the Doctor to go to Skybase alone. He will take cover in the lab caves. The Doctor will go on alone. A masked guard takes readings.

The Marshal tells Jo that he knows where the Doctor is. Jaeger explains that they plan to use the Doctor's knowledge of particle reversal to cover up the contamination before the investigator arrives. The Marshal sets guards on Solos out after the Doctor. The guards use tracking devices. They chase the Doctor through the misty woods. The Doctor runs right into one of the men but uses karate on him and flips him, "Goodbye." The Doctor makes for the transfer station and succeeds in getting back to Skybase, waving to two guards just before he vanishes. The guards fire their blasters at the device and then follow the Doctor back to the Skybase. The Doctor hides from the pair and they pass by.

The Doctor rushes into the Marshal's office and begins using the sonic screwdriver to free Jo. The Marshal tosses the key to him, he and his guards now present. The Marshal takes the sonic screwdriver. He wants the Doctor to put the planet right. The Doctor tells the Marshal he is quite mad. "Only if I lose," the Marshal laughs. The investigator's spacecraft is on the way.

Guards bring the Doctor to Jaeger's lab. Jaeger tells the Doctor this was not his fault. The Doctor says, "...you were simply obeying orders. That's a common excuse, Professor." The Doctor wants to use the macrothizer to train it on each affected area like a searchlight. This will be used to transfer the effect from the lab to the areas. The macrothizer is the one used in the transfer booth. If they don't come up with an effective control system they will particle reverse Skybase and be in more of a mess than before.

Jo, Ky, Stubbs, and Cotton watch a sleepy guard who sits down. Jo fakes a collapse to get him over to them. When he leans down, Jo grabs the blaster from the man. She gets the keys from him and frees the others. She also gets the sonic screwdriver. Ky, now free, covers the guard. Cotton gets the Marshal's staff-communicator open to call the investigator's ship, the Hyperion while Stubbs guards the doorway. Ky ties up the guard. Stubbs warns the guards he will not shoot unless he has to. The Marshal takes one of their guns and fires at Stubbs. Cotton reaches the Hyperion but it is up to Jo to tell them what is happening. Stubbs gets hit from a shot by the Marshal and is wounded but then he is shot even more badly. He dies in Jo and Cotton's hands on the floor. Cotton, Jo, and Ky race out another door and out a secret corridor of the Marshal's to try to reach the transfer station.

The Doctor hears shots being fired as Jaeger readies to switch off the macrothizer isolated from the transfer station to the lab. They do this just as the trio reach the transfer station. It will not work and the Marshal and guards arrive, firing. A guard kicks the blaster from Ky and the three are captured. Cotton and Ky are taken to the radiation chamber. The Marshal takes Miss Grant.

Jaeger almost drops a tube as the Doctor explains that the slightest accident at this stage of the procedure will reverse everyone to anti matter instantly, blasted out to the other side of this universe in a flash of electromagnetic radiation and, "...we'd all become un-people doing un-things un-together... fascinating..." They are ready to lower the beam away and see what un-does. The Doctor explains about the mutations.

Sondergaard reaches the glowing red caves but a Skybase man finds him there and is ready to shoot him. A mutant monster comes up behind him and knocks the guard down. More arrive to confront Sondergaard, who tries to communicate to them. They understand him as a friend. One mutant talks to Sondergaard who tells them they are supposed to mutate like this... the change was made to go faster but they can help him to find the Doctor and make the mutation occur as it was supposed to.

The Doctor's beam builds up to full power. Solos is no longer contaminated. The isotope level is exactly as it was before the rockets landed. The Doctor wants to switch it off now but the Marshal comes in and insists the Doctor operate the device until Solos is liveable but only by humans. When the Doctor refuses, he has a guard bring in Miss Grant at gunpoint. The Hyperion is about to dock and is announced. The Marshal orders Jo taken to be put with the others.

The Marshal's entire crew is on suspension making him angry. The Investigator arrives and does not shake hands with the Doctor but asks the Marshal if the Doctor is his scientific advisor. The Investigator demands answers about the crimes the Marshal is accused of. The Marshal trumps up more lies about rebellions and terrorists, prisoners allowed to escape aided by Stubbs and Cotton, now deceased. He also lies that his actions were reasonable to quell the rebellion and the plague. The Doctor calls the supposedly dead thousands of Mutt natives really just accelerated genetic metamorphosis. The Investigator asks what the Doctor is qualified in. "Practically everything." The Doctor has to go along with the lie that the Mutt condition was incurable as he believes Jo's life would be spared if he does lie.

The Hyperion fuel injection pump moves toward an opening while the fuel storage pump opens. Cotton tells Jo and Ky to escape via the hole in the few seconds before the probe tube injects in as the glow fills the room. With Ky unwell, the trio escape via the hatch just before the insertion.

In the Marshal's office, Jaeger whispering, tells the Marshal about the trio of escapees.

Jo, Ky, and Cotton get out of the tube and climb up a ladder as the green glow of theasium seems to follow.

Jaeger tells the Investigator, who is flanked by two dressed just like he is. Jaeger tells him the experiments making the atmosphere of Solos breathable by humans but not by the natives is nonsense. He tells the Investigator Solos is as it was before.

Just as the Investigator is about to declare the matter closed, Jo, Cotton, and Ky storm in. The Doctor explains them to be the missing witnesses. The Doctor is allowed to make the truth known against this travesty of justice. He accuses Jaeger and the Marshal of "the most brutal and callous series of crimes against a defenseless people it's ever been my misfortunate to encounter."

Sondergaard enters the cubicle transfer station to go to Skybase but the Mutts are afraid to follow him. He leaves them behind to go find the Doctor. Sondergaard goes to Skybase.

The Doctor debates the Marshal, who yells that the Mutts are mutants, a disease to be wiped off the face of the planet. The Doctor asks the Investigator, "Need I say more, Investigator." The Doctor does not have any of the tablets which the Investigator asks to see. The Mutts are one of a series of adaptive changes as the seasons on the planet are changing. Sondergaard is brought in by a white dressed Investigator guard. He attempts to explain, calling the mutational change a mistake, the process gone wrong. This makes the Investigator question the Doctor's claims of the mutants being part of the natural process.

As one of the Mutts enters the cubicle, being brave enough, Sondergaard tries to explain that the Doctor is correct. One of the Mutts enters the Marshal's office. It hits one of the Investigator's guard's small guns out of his hand. Left alone it would not hurt any of them but the Marshal gets a blaster rifle and follows it into the hallway, shooting it down. The Investigator sees this monster and puts the Marshal in charge again -- even of his own men.

The Doctor, Jo, and Sondergaard get out of the office in the confusion of guards and race to the lab where they have the key and lock the lab. Cotton and Ky are to be brought back to the radiation chamber. Jaeger explains that the chamber is much hotter now with more radiation now and after more time... the Marshal tells him that is the two men's problem.

The Doctor is analysing the crystal which he finds is a bio catalytic agent for the mutants. It might help the Doctor to accelerate the process. Thesium radiation is part of the process for a successful mutation. The Doctor wants the Professor to take the crystal to Ky. The Marshal blasts into the door and orders that girl and Sondergaard to the radiation chamber. The Doctor confronts the Marshal, who still wants an Earth like atmosphere. He tells the Doctor to hurry as his friends are in the radiation chamber for as long as it takes the Doctor. Ky is mutating... due to the radiation doses. Before Jo's eyes, Ky changes, his hands becoming crystal like and then claws.

The Marshal orders Jaeger to watch the Doctor, telling the scientist that that is all he is good for. The Investigator with his head dress off, comes to the Marshal and asks why his ship is to be detained and he and his men are being held as virtual prisoners. The men and the Investigator will be put down on Solos... now renamed New Earth. The Doctor, in the distraction of this, changes some wiring without Jaeger seeing. The Marshal tells the Investigator that he and his men will be reported dead and other ships that arrive will also have their crews added to the population of his new world. The Doctor tells the Investigator it is all right, the Marshal is quite mad. "Oh no, Doctor, I told you, madmen lose, I've won." The Marshal will release the Doctor's companions as soon as the job is done.

In the chamber, the radiation seems to vanish into Ky, the form of Ky changing from mutant into a beautiful humanoid, glowing and in blue-white robes with many colors, angel-like. He thanks Sondergaard for saving his people and showing them the way. He communicates with thought transference and can float. "Goodbye," he says after telling them there is little he cannot do. Ky vanishes backward, passing out of the wall. Cotton laments that Ky just flashes off and leaves them here. The door opens for them and the trio escape.

Ky's form knocks out two Marshal guards without touching them physically.

The Doctor asks the Marshal if he realises he is about to destroy a unique life form. He doesn't care and makes Jaeger operate the machine, the Doctor moving off to join the Investigator. The device overloads, the Doctor pulls the Investigator down, and the device blows up and Jaeger falls, dead. The Marshal aims his gun at the Doctor and the Investigator who rushed to the door. He means to kill the Doctor. Ky arrives and tells the Marshal to die, to end the torture of his people. A ray from Ky makes the Marshal vanish completely. Ky thanks the Doctor and vanishes, "Goodbye," his mind sends.

Sondergaard will stay on Solos to see the others go through the mutation process. The ones in the caves were prematurely changed -- like butterflies who hatch out on a cold spring day. Ky is on the planet to help them using the crystal. Sondergaard and Cotton will help. Cotton will then return to his home -- to Earth -- something they should have all done a long time ago. Cotton is acting as leader of the base.

The Investigator wants the Doctor and Jo to come with him in to the Hyperion to find out a full enquirement into the Doctor's involvement in this entire affair. The Doctor tells them that his assistant is not well at the moment, is feeling rather faint, and has been overdoing it. A questioning Jo is lead out by the Doctor. After the pair leave, the Investigator asks, "Doctor... who did you say?"

The pair find Storage Bay Area 4 locked. He reaches into his pockets to find the sonic screwdriver but Jo has it, taking it out of her pocket. "Give me that," he laughs and smiles at her and snatches it from her. He shields her from the blast as he uses the sonic screwdriver to blow it open.