An Obliteration module floats through space, as a voice speaks of its purpose. It contains Eldrad, the traitor, and destroyer of the barriers, who has been sentenced to obliteration. The voice comes from the planet of Kastria, a world ravaged by raging harsh winds. They intensely batter against the rocky landscape, including a white dome situated on the surface. Inside the dome we find two robed Kastrians dwelling within. One of them, named Commander Zazzka, walks around the frozen cold dome while the other remains still in a sitting position. The large booming voice of King Rokon sounds from Central Command, calling Outer Dome 6 for a module status report. Zazzka approaches his companion, who promptly drops dead to the icy floor when touched on the shoulder. Zazzka sits in front of the signal device where his deceased partner once was, answering Rokon's call, reporting to him of the obliteration module stasis, it is on course with normal function, computer time of detonation states it will reach designated point beyond all solar systems in six spans. Zazzka states the barrier condition is deteriorating, the King explains the North barrier has already collapsed and when the South does as well the temperature loss will intensify, and surface operations will no longer be possible. The King gives Zazzka orders to trigger the module now before control is lost completely. He questions this, computations indicate a one-in-three-million chance of Eldrad's survival if the module is destroyed at its present location. Rokon exclaims they have no choice, and Zazzka carries out his orders, obliterating the module with the touch of a button. It explodes in outer space instantly, disappearing from radar. Zazzka then receives his final orders from Rokon, to evacuate the observation dome, and as he does, he drags the body of his comrade out with him. The winds outside on the Kastrian surface increase dramatically. Pictures of galaxy clusters spin around on screen, signifying time passing on a massive scale.

Many years later on Earth, a group of workers prepare to detonate an area of a rock quarry in England. The explosives are set into place, the alarm sounds and the workers exit to a safe distance. Down in the quarry, the TARDIS materializes, and out steps the Doctor, followed by Sarah Jane, who points out they aren't in South Croydon. The Doctor ignores the blaring alarms, handing his coat to Sarah and having her watch as he tosses a rock across the ground into a larger rock in the distance. She isn't even sure they're on Earth, the Doctor informs her they're in a rock quarry, but doesn't know where they are exactly, spotting a man far down the way waving his arms and shouting at them. The Doctor and Sarah Jane wave back, unable to hear the man say to get out the way of the blast. He tries to get his coworker to stop before detonating, but his words fall on deaf ears thanks to the blaring siren. Sarah finally realizes what the alarm means, and pushes the Doctor into running. Just then, the explosives are detonated and the side of the hill near to where the Doctor and Sarah once stood is blasted to smithereens.

The man who attempted to warn them races down to the scene, and as the dust settles he finds the Doctor crawling from beneath the rubble. The man asks the weary Doctor how he got in there with all the signals going off. The Doctor staggers to his feet, gazes at the rock-slide and tells him that his friend is under that. The man shouts for his coworkers to get down there and call an ambulance, as the Doctor spots his coat half-buried under the rocks. He removes it in a sombre mood with no sign of Sarah anywhere. Soon, we find Sarah Jane Smith awakening under the rubble, barely moving thanks to the confinds of her stone prison. She calls out for the Doctor, reaching out for a rock that resembles a hand, and screaming in terror upon grasping it! This alerts the Doctor and workers to her location, pinned beneath two large slabs. She's removed from where she lay by the Doctor, who determines the unconscious Sarah Jane is still breathing. The ambulance arrives, a stretcher is brought over and Sarah is placed on it, still clutching the hand-shaped rock. The Doctor has her sent off the hospital, more concerned about the well being of his long-time companion than his own, following along with her but promising to talk to that rock quarry worker again later.

Later, the Doctor is being checked out by an intern at the hospital. The Doctor shrugs off some pain in his arm and asks the intern about Sarah Jane's condition, he's told she's still unconscious but there's no serious physical injury, paralysis isn't a concern as far as the intern knows. This sparks the man to ask him if he's a doctor himself, the Doctor says "sort of, yes...", the intern to ask where he qualified, he's told a place called Gallifrey, of which he's never heard of, thinking it an island. The Doctor is finally led to Sarah Jane's room, she's asleep and in a state of shock. Although not under sedation she was given an anti-tetanus for the considerable muscular contraction in her left hand and forearm. The Doctor feels the area himself to find it solid as a rock, while the rest of her arm seems perfectly normal. The Intern remarks it may be a psychological reaction to stress and the object which she was holding onto when brought in. The Doctor is told the object was sent to Dr. Carter in the pathology lab, so he heads off to examine it personally.

Once there, he reviews X-rays done by Dr. Carter on the object Sarah Jane was clutching when brought in, a rock resembling a hand. The X-rays of what is described as a fossil reveal no indication of any living organism, unlike normal fossils. Carter shows the Doctor a microscopic screening of the hand, revealing a crystalline lattice Carter notes has nothing to do with clinical pathology at all, but geology. The Doctor recognizes it is a silicone based molecular infrastructure, Carter knows of no such living forms made of stone, calling this whole thing an elaborate hoax. The Doctor asks if Carter has an electron microscope, assuring his fellow Doc it is no hoax. Meanwhile in Sarah's room, the intern finds no change her arm's rigid position. He exits the room, and within moments her eyes begin to open, glaring down at her hand. She curls back her fingers, revealing a large metal ring that had once been on the fossilized hand. It glows an eerie blue, prompting Sarah to quickly hop up out of bed and begin changing from hospital gown to her normal clothes.

Back at the lab, Dr. Carter is amazed the Doctor was able to get an electron microscope so easily from boys down at theology. He told them they were investigating the possibility of certain extraterrestrial viral infections on this planet. Carter laughs, but the Doctor is quite serious, admitting it a remote possibility but that viruses can survive, although not for 150 million years as far as they know. The fossilized hand was found embedded in Jurassic limestone, which explains how long it had been there, Carter flabbergasted on how it got there given that man didn't exist in those times. The Doctor agrees that's true, stating he thinks the answer might lie in the quarry, asking Carter to prepare another slide for him as he goes back to said quarry in search of clues. Sarah has quietly redressed herself and is beginning to exit her room when she senses the Doctor nearby, closing the door just as he walks by. He stops and turns the sign on her door to "patient not to be disturbed", before walking away again, grinning. Once gone, Sarah Jane sneaks off towards the pathology laboratory.

In the lab, Dr. Carter prepares a slide with a chipping off the fossilized hand. He places it inside the electron microscope, unaware to Sarah entering the room until he hears her putting the hand in a plastic container. He turns around to ask what she thinks she's doing, until he realizes who she is, asking if she's feeling better. Sarah responds in a possessed voice that Eldrad must live, extending her right hand at Carter, with the metal ring on her middle finger. It beams out a blue energy, engulfing Carter for a moment until he collapses on the floor. Sarah's voice echoes in her head, saying that it is Eldrad's will that all shall obey and none must interfere. She picks up the container containing the stone hand, and departs the lab. Elsewhere, the Doctor meets once more with the worker at the rock quarry, where they inspect the strata the fossil came from. The Doctor asks if any objects were found which a spaceship could be made from, the worker thinks it a joke, but only questions how a spaceship could crash here 150 million years ago. The Doctor mentions that not all life forms exist at the same time, offering another possibility. It may have just came fluttering down, wondering why and from where. The worker seems to have enough interstellar pondering, leaving the Doctor to search by himself, wondering where the creature who's hand ended up there was going to begin with.

Back at the lab, Dr. Carter awakens from his attack, looking around wearily before picking up the phone. He calls down to reception to stop the dark haired young woman with pink striped Andy Pandy-like overalls who's on her way out having stolen something from his lab. They inform him she already left an hour ago, Carter checks his watch to his surprise, finding he's been unconscious quite a while! The Doctor returns to the hospital, peeking his head into Sarah's room, he discovers not a trace of her anywhere. He goes to the lab to ask Carter if he's seen Sarah anywhere, he tells the Doctor he has and asks if she always knocks people out. The Doctor is told the story of events as they happened to Carter, who doesn't remember anything after a flash, but thinks she must have hit him. The demeanor of possession present in Sarah seems to be surfacing in Dr. Carter, as he gains a sinister tone when asking the Doctor if he found anything at the quarry, suddenly returning to normal without even realizing it. The Doctor found negative evidence of fragments, meaning whatever it was didn't crash land, though the sample's fracture lines proves there was an explosion some million years ago. Dr. Carter says it still doesn't explain why Sarah wanted it, the Doctor suggests that perhaps it wanted her instead. She was the first living creature to have contact with her possibly since its explosive encounter. Carter mentions the fossil was petrified, the Doctor agrees it was inert but likely not dead. Dr. Carter suddenly realizes he thought there was a strange type of subatomic structure to the crystal formation, like that of a double helix DNA molecule. He and the Doctor rush over to the electron microscope, Carter is shocked to find the sample has changed from when he placed it there earlier, having lost the electron charge. The Doctor thinks the sample has been quietly absorbing radiation from the machine and regenerating itself, he hopes it hasn't siphoned enough to be dangerous, telling Carter to put it somewhere safely away from further radiation. The Doctor exits the room, but quickly returns to ask Carter where the nearest nuclear reactor is located.

The Nunton Complex control room is in a state of emergency. Alarms continue sounding loudly, as the man in charge, Professor Watson, enters and demands to his assistant, Miss Jackson, that the noise be quelled. Heeding the alerts of reporting to safe areas, many of the complex' workers begin to pile out, walking past the Doctor and Dr. Carter who're being lead into the complex by armed guards. The two doctors take advantage of the crowds of people passing through and slip away from the guards in the confusion. Despite the alarmed warnings, they head to the control center of the nuclear power plant. Near the core, the possessed Sarah watches the revived hand in the box, understanding as she hears it speak in her head, she obeys by walking over and sealing the door to the room. Up in the control center, Professor Watson gets the alarm quieted at last, before having Miss Jackson start emergency shutdown procedures, while he tells the remaining personnel that an emergency exists in the fission reactor. He calls this a deliberate act of sabotage by a suicidal maniac young woman, filling in details about how she knocked out two workers and locked herself in the outer chamber near the core. Miss Jackson tells the Professor the shutdown is proceeding but there's no info on the neutron core. He takes this as a sign the "terrorist" knows what she's doing and gives word to get a team of armed men suited up to get her out.

The Doctor and Dr. Carter enter the control room and begin to watch Sarah on the security camera screen with Professor Watson. He asks who they are, Carter states they're from the hospital she escaped from. Watson figures this proof of her lunacy, despite the Doctor's attempts to defend her good name. Carter goes silent during this time, as we hear echoing thoughts in his head that Eldrad must live! Miss Jackson finds out the radiation team can't get past the jammed manual locks on the fission room door. According to their sensors Sarah's soaking up enough radiation to kill a herd of elephants, with levels continuing to rise. Watson contacts Sarah through the speakers, to ask what it is she wants. She remains quiet, sitting on the floor with the box between her legs. When Watson walks off to order evacuations of everyone within twelve miles, the Doctor takes the microphone and calls out to Sarah. It gets a response, she states that Eldrad must live, causing the same words to play in Carter's mind. The Doctor asks who Eldrad is and what he wants, Sarah hears only the voice in her head, which speaks of Eldrad the creator and savior, and how he must live, saying that aloud once more.

The Doctor tells Professor Watson to keep her talking while he goes in there after her, and when asked how he'll get in, the good Doctor mentions the cooling duct. Miss Jackson notes the temperature inside the duct is over two hundred centigrade and he'll roast, the Doctor quips "not if I'm quick!" before bolting out the door. Dr. Carter's head-voice tells him he must go with the Doctor, as Sarah will need his help in assuring Eldrad's rebirth. Sarah hears the word of Eldrad, walking over and turning off the security camera so that Watson can no longer see her. The formally fossilized and now-living hand slowly creeps its way out of the box and across the floor, Sarah watching with an evil grin on her brainwashed face as she notices the gage reads critical! The Doctor and Carter head up a stairway towards the cooling duct, Carter lags begin with a possessed look on his face, stopping and taking a large wrench. Up in the control center, with all help exhausted, Miss Jackson is ordered by Professor Watson to get to a safe place, while he stays behind. In the reactor room, Sarah follows the unheard orders of the hand and unlatches the locks to the nuclear core. On the stairwell, the Doctor halts when Dr. Carter leaps in front of him and shouts that Eldrad must live! The Doctor witnesses his madness with shock, as Carter says that there must be no interference with the design, swinging the wrench around twice. The second swing causes him to lose balance and fall over the railing, splattering on the ground far below, the Doctor unable to prevent it.

The Doctor reaches the roof on the complex and enters the cooling systems control. Down in the control room, Professor Watson calls his wife and lets her know he's staying at the complex for a while, lying about there being nothing wrong, as lights from the consoles flash about the system reaching critical mass. In the reactor room, Sarah prepares to ring blast the core door open, when the Doctor suddenly slides in through the cooling duct! He lands abruptly on the floor, turning and stating that Eldrad must live before Sarah can fire the ring at him. She lowers her guard thinking he's possessed as well, allowing the Doctor to grab her head and knock her out with a quick twist he then apologizes for. He drags her to the door, looking back and spotting the revived hand crawling around before exiting, not noticing that the ring has slipped off Sarah's finger and fallen on the ground. In control center, Watson says his final good-byes to his wife and children. After he hangs up to await fiery death, all alerts cease and the Doctor contacts him through the intercom. The surprised Watson informs him that everything appears to be fine, asking if Sarah is still alive, the Doctor hopes so and says him they're in decontamination now. Watson tells him he did well, saying he'll be down to see them after he finishes a few things up there, including calling all staff to return to their posts, announcing the reactor is under control.

In the decontamination room, Sarah finally awakens to her normal self, having thought she was still buried alive. Her body aches but the Doctor tells her she's alright, asking what the last thing she remembers was. She says someone held out a hand to her, which she thought was his, and that it was cold to the touch causing her to think he'd been crushed too before passing out, remembering nothing else. Professor Watson storms in angrily with Miss Jackson and demands an explanation for Sarah's actions, the Doctor standing ground on the fact she doesn't recall a thing. To help further prove his point of how abnormal the situation is, the Doctor runs the radiation scanner over Sarah's body, showing no traces of radiation whatsoever! Sarah wants to know what she's been up to during her blackout, as does Watson. The Doctor states that Dr. Carter is dead as he starts to answer their questions, starting at the beginning, with Sarah finding the hand in the quarry. By the time they reach the control center, the Professor is going over the last bits of info, about the hand of Eldrad having absorbed radiation like humans do oxygen, leaving Sarah unharmed. The Doctor explains the hand is not only living, but regenerating. Another assistant, named Driscoll, motions them to the core room monitor, where the hand can be seen crawling towards the reactor door. Watson sends Driscoll to retrieve the hand, so he heads to the room in a radiation suit and picks the hand up in tongs, placing it back into the container. On the way out (and out of monitor range), he spots the ring Sarah dropped. He picks it up, and it glows blue, putting Driscoll in a trance.

Soon in the decontamination room, the Doctor and Professor Watson are inspecting the hand, which has returned to a rigid, fossilized state. Aside from the expected surprise of no radiation emitting from it, the Doctor notices the ring finger has regrown from last he saw it. Watson has Driscoll put it in storage before it absorbs any more radiation. The Doctor questions Sarah on what happened to the ring she had on, but she draws a blank, so he asks Driscoll if he found anything. The bewitched Driscoll recalls seeing nothing in the core room but is sent back to go back to check, as Watson goes the control room to monitor his progress. This leaves the Doctor and Sarah alone in the room, allowing him to perform his usual hypnotism trick on her. She tries to protest it, but fails against his powerful voice, falling into a trance where the Doctor begins to ask her about Eldrad.

Professor Watson contacts Driscoll from the control room, who states he's still discovered nothing in the core room. Back in decontamination, the Doctor has Sarah back in her "Eldrad Must Live" mode, question the why's of the matter. He learns from her possessed ranting that those who've seen the Light Of Kastria from the ring, such as she and Dr. Carter, must obey the word of Eldrad. Driscoll affirms Watson that there is no ring in the core room, and when ordered out, Driscoll opens his hand, revealing the ring in his palm, and the word of Eldrad echoing in his head. Elsewhere, the Doctor gets as much info as he can from Sarah, and eventually whispers to her that she's free of Eldrad, before taking her out of trance. On the way back to the control center, she utters an "Eldrad must live", the Doctor is shocked, until she taps her head and tells him she was just testing. Watson says Driscoll had no news on the ring, but the Doctor is sure he was lying, considering the ring's ability to control wills, like it did with Dr. Carter. In decontamination, a security guard hears a thumping sound coming from in the storage safe. He calls up to control about it, and Watson realizes what a grave mistake Driscoll made in putting the hand in there with other radiated materials and samples. The Doctor rushes down to take care of it, but this leaves the guard alone with the entranced Driscoll. When the guard turns attention to the safe containing the hand, Driscoll's Eldrad programming kicks in, and he knocks the guard out! Driscoll then opens the safe, and removes the again-living hand.

Professor Watson recovers from his flashfiring, and stumbles out of the wrecked control center. Outside the core room, Sarah, still clutched in the Doctor's arms, asks if they're dead. He looks around and tells her no. They then get up and hurry into the reactor room, the Doctor quickly resealing the chamber door. Sarah asks what happened, and he says a sort of unexplosion occurred. Watson keeps distance from the room, thinking it contains lethal radiation levels, but the Doctor tells him to come on in, as there's no radiation in it at all as proven by the wall gauge. Watson enters and asks where Driscoll is, the Doctor tells him he's in the core, something that should have caused a nuclear explosion but was fortunately absorbed. Fission took place but instead of exploding outwards, it exploded inwards. Driscoll is likely vaporized, and the hand now has access to the nuclear core's full potential and is probably feeding on the radiation as they speak. Watson is stunned by this, as the Doctor explains the hand is regenerating and growing, matter is being created from energy, and Eldrad is rebuilding himself. He suggests that Eldrad may then target other power plants until he reaches maximum growth, whatever that may be. Watson is ready to fight fire with fire, going off to call in the armed forces before Eldrad can cause more harm. The Doctor expresses to Sarah that he doubts missiles will do much good against Eldrad, just as noises begin to come from the chamber door.

Watson returns to the control room and calls for red alert on his special nuclear strike phone. Sarah urges the Doctor to leave the core room, but he's waiting around with intent of communicating with Eldrad, she asks how, joking about hand signals. Watson enters and tells them they have ten minutes until the air command tactical nuke strike squad levels the whole complex! The Doctor tries reasoning on behalf of the destruction of something intelligent they don't know much about, Sarah arguing that it kills people. He goes over what he knows, an alien crystalline radiation regenerating life-force shipwrecked on a strange planet, and figures it's probably afraid. Sarah only knows that she's afraid if it gets out of the core, taking off with Watson with only nine minutes left until the bombing raid. The Doctor stays behind, staring at the door, fascinated. The Doctor returns to reality upon noticing everyone's gone, running for safety himself, as the door starts to melt as Eldrad appears to be escaping.

The Doctor ends up the first one out of the gate, hopping into a truck along with Sarah, Watson and a few guards. The truck takes off into a relatively safe distance from the Nunton Complex, as inside, the reactor chamber door melts even further. Watson, Sarah and the guards get off the truck and crouch on the ground, watching the skies as two jets stream through the sky. The Doctor remains standing on the truck, doing nothing but smiling devilishly when Watson tells him to get down. The jets fly into firing positions, those on the ground duck from possible blinding flashes behind the truck. The Doctor anxiously watches as the missiles are fired at the complex, no explosion is seen nor heard! Watson and Sarah stand up, confused as to why nothing's happened by then, the Doctor explaining the missiles have been neutralized, pointing out how something that can survive a nuclear pile would hardly be harmed by primitive missiles. He suggests they try a much older weapon, such as speech, diplomacy and conversation, before ordering the driver to take them back to the complex.

Meanwhile in the core room, Eldrad speaks, wondering what place this is and where he has come to. Upon exiting the chamber door that was melted enough to allow entry, Eldrad steps out in full crystalline humanesque structure female form! She glances as the harsh, warped reflection on the core room walls, pondering if this form is that of those who found and now seek to destroy her. She gives this no matter, they will fail as the obliteration module failed, and despite this strange form, Eldrad lives and shall gain rule of Kastria!

The truck approaches the gate of Nunton Complex, the Doctor enters and decides to go into the place alone, telling Sarah to stay with Professor Watson. She tries slipping by Watson, but he grabs her and tells her she should stay here with him. Sarah agrees she should, but isn't going to, running off and returning to her longtime companion's side. The Doctor stops in his tracks when she pops up next to him, she says she worries about him and brings up the fact she found that thing, it could have been her in Driscoll's place, and she's from Earth and the Doctor isn't! He agrees that's all true, but he worries about her too, Sarah tells him to just be careful, and he states that they'll both be careful. The Doctor and Sarah walk into the core room, finding the chamber door melted open, and getting surprised by Eldrad, who appears from behind them. She orders them to come foreword, threatening by aiming her ring, but the Doctor ignores certain death and puts on a happy face, giving Eldrad a how-do-you-do. She asks if he was responsible for the stupid attempt to destroy her, he denies it and asks how she prevented the missile explosion. She explains absorbing the nuclear energy into herself and using it to complete regeneration, which the Doctor expected was the explanation. Eldrad realizes he's not from this primitive planet, asking what he's doing here, he introduces himself and Sarah. Eldrad does the same, calling herself the creator of Kastria as Watson watches this on the monitor in the control room. Eldrad asks why they tried to destroy her, the Doctor crosses both his hearts and tells her she had it wrong as they're the ones who saved her. Eldrad's eyes glow blue and she scans the Doctor's mind, seeing that he speaks the truth, causing pain to him in the process.

Eldrad asks why they used their primitive weapons on her, the Doctor explains humans are stubborn, violent and sometimes try to destroy things they don't understand. She says they must be taught otherwise, the Doctor mentions she wouldn't be the first one to make that mistake as others have tried. Meanwhile, Watson thinks Eldrad extremely hostile, reaching for and loading a gun kept in the control room, before rushing down after the menace. The Doctor asks Eldrad how she ended up here, Sarah mentioning they found her hand in the quarry. Eldrad states she was betrayed and now she must return to avenge herself, the Doctor notes that she's lain dormant on Earth 150-million years. She does another brain scan to seek the truth, Sarah screaming for her to stop it. Eldrad discovers he is a time lord, having heard of such a race of time and space masters. She releases her hold, causing the Doctor to collapse to the floor, Sarah comforts him as he recovers. He tells Eldrad she had no need to do that as he would have told her, but she states that she's learned to trust no one. Eldrad says they must help her, as a time lord he's pledged to uphold the laws of time, and prevent alien aggression, the Doctor finishes the statement off as best he recalls by explaining only when such aggression is deemed to threaten the indigenous population. Eldrad tells him he must help her in her struggle, Sarah asking why he must help someone so destructive.

Eldrad explains Kastria is a cold, inhospitable planet ravaged by the solar winds, she made the spacial barriers to keep out the winds and designed a crystalline silicone form for their physical needs. She also built machines to replenish the earths and atmosphere, basically bringing Kastria to life. Then, two warring alien worlds made Kastria their battleground, destroying the barriers, the winds dehydrated the planet again. The alien invaders made puppets of the Kastrian leaders and discredited Eldrad, sentencing her to obliteration. Sarah asks why her people turned against her if she did all those things, Eldrad mentions they didn't, begging for help in saving Kastria again by taking her back through time. The Doctor tells her that would contravene the first law of time, a distortion of history, he can't do such, but will take her home in the present time. Eldrad silences him, detecting another lifeform in the building, despite Sarah and the Doctor's assurances otherwise. Professor Watson sneaks around with gun in hand, and Eldrad restrains herself from reading minds to be sure and takes their word for it. She also accepts the terms of agreement in going home, left with no choice. They exit the core room, and Watson begins to fire upon Eldrad, discovering his gun empty without having done any damage to her. Eldrad blasts towards him with her ring, chasing after Watson as he flees to the control room to reload.

Eldrad barges into the control center and her eyes glow blue with rage, Professor Watson is in agony as she prepares to send him to his the fate. The Doctor whispers to Eldrad that if she doesn't release Watson, he swears she'll never return to Kastria. Watson is let go, he collapses, Sarah checks him out, finding he's still breathing. Eldrad wants to be going, promising that the Professor to whom she owes her regeneration will recover. Watson awakens, shaken up but quite alive, Eldrad tells the Doctor she's not as cruel as he thinks her to be. They both exit the control room, Sarah Jane asks Watson if he's fine before hastily stating she has to go, rushing off to join the Doctor and Eldrad. Watson stands up and stumbles over to the gun, shocked he put six shots into the unharmed creature. Miss Jackson enters and witnesses the state of ruin the control center is in, asking Watson what happened. He's at a loss for words on the laws of physics, mumbling that the atomic energy commission won't believe this. Miss Jackson asks why planes were bombing the complex, Watson realizes the RAF won't believe this either. She says she saw the Doctor in his car, nearly running her over, Watson asks if she saw who was with him, and the only person Miss Jackson spotted was Sarah. Professor Watson slowly sits down, distraught over the unbelievablity of the truth. The phone rings, Watson answers and tells them they don't know what happened here, but there's going to be an inquiry!

The Doctor, Sarah Jane, and Eldrad return to the rock quarry, where the TARDIS still resides. He enters the copper colored room and welcomes Eldrad to the place, asking her what she thinks. Eldrad congratulates him on his people's achievements in temporal engineering being as impressive as she heard. She asks where its armaments are located, the Doctor points to his head and tells her they're in there. She tries again to probe his mind, but he informs her that her weapons won't work in this a state of multidimensional temporal grace. He reiterates that in a sense they don't exist when in the TARDIS, she can't hurt them and they can't hurt her. Sarah feels safer thanks to that, asking the Doctor why he's helping Eldrad, he says that he's helping Earth by preventing her from smashing up more nuclear power stations. Besides, he wanted to see Kastria, Sarah asks what for, and the Doctor tells her that travel broadens the mind. He then asks Eldrad's help in locating the whereabouts of Kastria. Sarah storms off in protest of the Doctor aiding her former brainwasher.

The TARDIS dematerializes from the quarry, Eldrad mentally entering the codes on a viewing screen. The whole room tilts sharply and the Doctor tumbles over, Sarah enters eating a banana, smugly asking if they're off course. He doesn't think so but asks Eldrad for her coordinates to check himself, she's insulted that he doubts her, but he just wants to make sure they get there, Sarah telling her that for once she'll have to trust someone. Eldrad remarks only fools trust as she once trusted the people who tried to obliterate her, the Doctor argues that she'll achieve nothing on Kastria until she learns to overcome her paranoid obsession with treachery and cooperates with them, and if she miss-set the coordinates, symbolic resonance will occur in the trackor time crystal and there'll be no chance of them landing anywhere. The TARDIS materializes on Kastria, amid the fierce winds and a large dome. The Doctor says they've landed, Eldrad smiles and mentions they'll have to trust her also. The screen displays what lies outside, the harsh surface of the solar wind devastated planet, Eldrad vows to reclaim it again. The Doctor notes that hurricane has been blown for nearly 150-million years, Eldrad asks how the atmosphere is constituted and he tells her it's near enough normal. The radiation count is a bit high, these things are all she'll need.

Eldrad gasps out in pain from the protruding projectile in her chest, the Doctor rushes to her aid but she yanks the arrow out herself. He asks what was in the tube and she tells him it was an acid designed to neutralize a molecular bond, something she developed years ago, there's no antidote and it's only a matter of time before she disintegrates. There's one last hope, a regenerator chamber on level-306, but the Doctor and Sarah will have to help Eldrad get there before the acid shatters her crystal matrix. They enter the door to the thermal caverns, setting off an alert somewhere else in the underground lairs. King Rokon, sitting in his silver strewn suit before a flashing blue screen, announcing that intruders have penetrated the levels and are descending. The Doctor holds the nearly unconscious Eldrad as they and Sarah ride down an elevator lift, finally reaching level-306. Eldrad points out the direction of the chamber, The Doctor drags her along and Sarah walks ahead. She ends up struck from above by an intense spray of gas, causing her to fall to the ground screaming and coughing. Eldrad notes the traps are only effective against silicon based lifeforms. The Doctor walks by Sarah, telling her to stop making a fuss and get up off the floor as she's a carbon-based lifeform from South Croydon. She staggers up to the Doctor, having nearly frightened herself to death a moment ago.

Sarah helps the Doctor support Eldrad as they journey through the tunnels, his "superb sense of direction" leads them right into a crevice filled with collapsing debris. They leap out of danger in time, and reenter a moment later to discover the vicinity ahead of them filled with chunks of rock, and no sign of life as far as they can see. The Doctor ponders the possibility that the aliens wiped all of them out millions of years ago, as the trio presses on their descent to the regeneration chamber. Sarah figures if they were all dead they'd have seen some bodies by now, the Doctor tells her those made of rock would crumble to sand, they're likely walking on their remains! She's disturbed by this, hurrying out of the area as fast she can. Meanwhile, Rokon continues detecting the two alien and one Kastrian intruders, awaiting orders to activate automatic defense procedures.

Sarah has the feeling they're being watched, the Doctor assures her nothing's been disturbed down there for years. She says someone set those boobytraps, he chalks that up to having been done long ago presumably for the warring aliens. This means they were silicon based too, the Doctor calls them very rare and notes the coincidence of two of them in the same galaxy. While dragging Eldrad further into the caves, Sarah doesn't look where she's going and nearly falls into an abyss. She clings to the Doctor's hand and is pulled to safety, a few rocks knocked into the pit proves it's a long way down. No other way to the regeneration chamber, they'll have to cross it via a crystalline log. The Doctor crosses it, finding it sturdy, hoisting the ever-erect body of Eldrad over his shoulders, reaching the other side without problems, while Sarah crawls on her knees over it. They finally reach the regeneration chamber door, the dying Eldrad attempts to place her ring-wearing finger into the slot to open it, but the Doctor stops her. He takes the ring instead, bracing himself and telling Sarah to get back as the door slowly opens. Checking for traps, he pulls out a magician's wand from his coat, popping it out in range of the open door. The wand is blasted into pieces with a loud flash. The danger over, the Doctor picks up and carries Eldrad into the regeneration chamber and places her body on a slab in the center of the room. King Rokon is still apparently monitoring the situation with the unidentified Kastrian, attempting to eliminate the intruders by any means necessary.

As Eldrad lies unconscious in the regeneration chamber, the Doctor finally understands the workings of her ring. The crystal carries her genetic code which enables her to reconstitute herself from any suitable radioactive material. He then takes the ring and plays a guessing game with the control panel of the regenerator, trying to get it working as best he can. Eldrad's body is starting to succumb to the effects of the acid, cracks forming along her crystalline structure. Sarah screams for the Doctor to hurry, he's going as fast he can, using maximum irradiation and hoping it'll work. Rokon discovers Eldrad's identity, mentioning she was sentenced to obliteration. Attempting to halt her regeneration causes a malfunction in the machinery for reasons unknown, as down in the chamber, the large block above the slab Eldrad is on drops! Sarah screams about it crushing her, and the Doctor is unable to stop it from apparently flattening Eldrad. Sarah calls this horrible, the thing that was to remake her instead killed her, and it was all their fault. The top block rises, revealing only an imprint where Eldrad once was, the Doctor realizes they've been used, her people were determined to get her one way or another. Left with no other options, the Doctor suggests they depart.

Suddenly, the chamber door opens, Sarah and the Doctor quickly hide behind the stone slab. A large Kastrian male exits from it, calling out for his two friends to come see his successful new form. They do so, stunned that not only did Eldrad survive, but now looks completely unfamiliar. Eldrad proves who he is by saying the Doctor's doubtful attitude makes him sound like Professor Watson as he should be well acquainted to the process of regeneration, being a time lord and all. Sarah asks why she's a he now, Eldrad states he had to assume a form that would be acceptable to those on that primitive planet. The Doctor figures out that Eldrad had modelled himself after the first "primitive" he came in contact with, that being Sarah, she gives a disturbed thanks to him. Upon the notion that Sarah feared he was destroyed by the slab, Eldrad calls his people fools in trying to obliterate him using the regeneration chamber he gave them, having designed it to recognize his cell pattern. Eldrad laughs maniacally about how no one can do anything to eliminate him, and now he shall control all of Kastria, crushing his enemies with his renewed might.

The voice of King Rokon rings out, calling to Eldrad, and even appearing before him on a glass screen nearby. Rokon says he's a traitor, and he may think he has victory within his grasp, but he's won nothing but defeat. The image fades, Eldrad explains it was the King who ordered his obliteration, boasting evilly about how they dared to betray the barriers' architect, so he then destroyed the barriers in return. The Doctor is a bit surprised by the truth, there were no warring alien invaders, a lie Eldrad made to help get him to his home planet. He asks Eldrad why Rokon would have him destroyed in the first place, Eldrad talks about how Rokon was afraid he'd take his place, being youthfully strong compared to the ancient ruler. Plus, Rokon had no appetite for conquest as he does, and with Eldrad at the new Kastrian King, they shall conquer the galaxy! Eldrad exits to have an audience with his former King, leaving the Doctor and Sarah behind. Sarah notes they've been taken for a ride, but the Doctor isn't about to be made a fool for much longer, taking Eldrad's ring from the control panel on their way out.

Eldrad enters the King's chambers, finding Rokon sitting in the middle of room. He speaks to the King about being a successor scorned, and upon pulling down the body tarp he wears, Rokon's body falls over, shatters against the floor and crumbles into a pile of dirt. Eldrad cries about being cheated of his long awaited revenge. Sarah can't understand how they saw Rokon speaking, the Doctor says it was a recording from the past, knowing there was a chance that Eldrad would return he left it with the boobytraps. Eldrad refuses to be denied his rightful place as ruler, but as the Doctor points out, he has no subjects to preside over. Eldrad speaks of their being able to be restored once placed in the regeneration chamber, and once reborn, he'll have them build a new Kastria from the ashes. He enters the room where the race banks are stored, from which he'll be able to recreate his people, but finds the room with little of nothing inside! Rokon calls once more for Eldrad, appearing on the view screen he explains that after the module was prematurely destroyed, the Kastrian race knew there was a chance he'd return. With the barriers gone, life on the surface was finished and the alternative was a miserable existence underground. They all decided on final oblivion instead, and fearing Eldrad might return to attempt eternal war throughout the galaxy, they elected also to destroy the race banks. King Rokon gives Eldrad his due as the new king of Kastria from the dead, hailing Eldrad before disappearing forever from the screen.

Eldrad whines about having created this whole world and how this is his reward for all the good he did them. The Doctor remarks to Sarah about how Rokon seems to have solved this problem for them, albeit a drastic solution. Sarah shakes her head, saying she wouldn't want to live down here and be ruled by Eldrad either. This seems to spark an idea into Eldrad's head, he'll go back to Earth and rule them instead. They may be primitive but they have the needed aggression for galactic conquest, he'll be their God and king! The Doctor turns down this plan, it wasn't a part of the one-way ticket deal, his obligation to Eldrad is finished. He rubs the ring with his finger, Eldrad spots it and asks for it back. The Doctor refuses, but after Eldrad calls it his "key to eternity", he changes his mind by rolling it along the ground into the race bank room, quickly telling Sarah to move as they escape in the other direction. Eldrad searches for his ring and then chases them towards the abyss. The Doctor hands Sarah one end of his multicolored scarf, telling her to hide and to pull on cue. The Doctor goes over to the other side of the walkway, hiding behind a rock and leaving the scarf draped across the floor. Eldrad marches in, shouting that they will take him back, when the Doctor shouts for Sarah to pull, the scarf whips up, tripping Eldrad! He tumbles into the endless pit, dropping out of view in the inky blackness below. The Doctor and Sarah crawl over to where Eldrad fell, looking down into the abyss, the Doctor notes the gravity of the law finally caught up to him. Sarah smacks his arm, saying it was a bit rash in giving Eldrad his ring back, the same ring the Doctor holds in his hand now. She's stunned by his sleight of hand, asking what he gave Eldrad instead, he says it was his magician's stick, before tossing the ring into the pit to join Eldrad in literal oblivion.

The Doctor and Sarah Jane return to the TARDIS, taking off from the cold, dismal planet one last time. Sarah asks if he thinks Eldrad is really dead, something the Doctor doubts. Sarah says she quite liked her, but couldn't stand him. The TARDIS swiftly tilts, the Doctor figures the temperatures must have effected the thermal couplings, and starts checking the console panels. Sarah knows how she feels, still wearing the Doctor's coat and rubbing her hands, she thinks Kastria has to be the coldest planet in the galaxy. The Doctor says he's been to much colder places, Sarah thinks this alright for him but she's human, they're not so thick-skinned. The Doctor starts to check the under-panelling of the TARDIS console, asking for a few tools from Sarah and what it was she last said. She repeats the thick-skinned part, and comments she might as well be talking to the moon considering he doesn't even listen to her. Proving her point, he ignores this and asks for more special tools, she sighs and takes them out from the coat.

Sarah sits down next to the busy Doctor, saying to herself out loud that she must be mad. She's sick of being cold, wet, hypnotized, shot at, savaged by bug-eyed monsters, and never knowing if she's coming, going or being. The Doctor calls out for more tools, she just continues ranting, saying she wants her hair washed in a bath, just to feel human again. He sits up and changes his mind about the last tool, a Zeus Plug, he'll just have the Sonic Screwdriver. Sarah is annoyed, but hands it to him anyway, saying she's extremely sick of that Screwdriver as well. She shouts that she's going to pack her goodies and she's going home. The Screwdriver is noisily blaring from inside the TARDIS console, so she repeats herself just as loud, trying to make him hear. Shunned still, Sarah storms off to her room in a huff. The Doctor finally finishes his tasks and asks what it was she wanted, but she's gone. He doesn't know why she goes on like this as there's really nothing a matter at all.

The Doctor stands up and is stricken by an overwhelming sensation in his head. It's a call from Gallifrey, after all this time away from his home world. He speaks of not being able to take Sarah with him, and must reset the coordinates to get her back home. He enters the location of South Croydon, just as Sarah enters the room, having packed her goodies just as she threatened. She loudly clears her throat, and the Doctor calls her a good girl, but she says it's too late, she's got everything packed to go. The Doctor turns around and asks her how she knew, he tells her he got the call from Gallifrey and has to return alone. Sarah suddenly changes her mood, telling him she was joking, didn't mean what she said and can't miss a trip to Gallifrey. Sarah asks if he's going to regenerate again, he says no but isn't sure what's going to happen this time. She gives a nervous smile, thinking this another one of his tricks in trying to make her stay, but the Doctor tells her seriously that he received a call and as a time lord he must obey.

The TARDIS materializes on a modern street, with a Golden Labrador laying on the sidewalk nearby. Inside, Sarah accepts he must go alone, telling him she'll give his love to Harry and the Brigadier, and she'll tell Professor Watson he's alright. The Doctor tells her they've landed, at Sarah's home in South Croydon on Hillview Road. She says that she'll be off, handing him back his coat and putting on her own. She tells him not to forget her, the Doctor exhales with a smile, telling her the same. Sarah shakes her head and fights back any tears with a smile, telling the Doctor bye and walking off. She reaches the exit and turns around, saying that travel does indeed broaden the mind. The Doctor grins and agrees, bidding "Until we meet again, Sarah", to which she nods and smiles, before departing the TARDIS.