The TARDIS continues to buck and pitch, tossing its occupants about, completely out of control. The Doctor manages to reach the controls and flips switches frantically amid shouts from Ben.

After a few moments the ship begins to calm. The floor settles back to level and the lights return to normal. The engine noises settle back to their usual calm hum. Everyone is unharmed and the Doctor is back in control.

Ben notices on the scanner that they are approaching what appears to be a small planet. They prepare for landing while Jamie - the newcomer - soothes his bumps and bruises. Polly thinks they've reached Mars, just as the Doctor predicted, but she is wrong.

The surface on which the TARDIS materialises is barren enough to be Mars, full of craters and sharp rocks, but it's decidedly not red. The landscape is grey and ashy. Ben, seeing the outside view on the scanner, picks this up immediately. He has seen pictures like this on telly - the surface of the moon. The Doctor, abashed, confirms that he is correct. They've missed Mars by several million miles.

The Doctor is ready to move on quickly, but Ben and Polly wish to explore. From the perspective of their time, they've reached the moon before anyone else. Jamie is still a little perplexed as to how they got the moon, way up in the sky. The Doctor tries to hold them back, but Ben and Polly persist. Finally the Doctor agrees, allowing them just a half-our outside. But first they'll need space suits.

Jamie wonders if they'll meet the old man in the moon.

Shortly, all four of them are kitted out in space suits and bubble-like helmets and emerge from the TARDIS. A sky full of stars is overhead and the strangeness of the landscape captivates them all. It is all very desolate and forbidding.

Polly, who has gotten somewhat ahead of the others, notices a flash of light in the sky, something like a shooting star. She calls to the others through her headset radio but the flash is quickly gone and they see nothing. Ben dismisses it as an optical illusion and the group moves on.

Polly is still unused to the change of gravity and the space suit. The Doctor urges her to try a little jump and she does so, sailing higher and farther than she intended. She lands with a light bump. Ben and Jamie follow her lead and begin jumping around like kangaroos. They are having a great time but the Doctor warns them to be careful of their suits. One tear and they will die in the airless atmosphere.

The three young people comply with the instructions but continue to jump. However, Jamie gets too energetic and jumps too far. He disappears over the edge of a ridge and out of the sight of the others. The Doctor, Ben, and Polly hurry to see where he's gone.

But their attention is drawn by a striking sight below the ridge. A giant complex of buildings rises from the grey dust in a semi-circular pattern, low buildings radiating from a central hub. At the centre is a giant clear plastic done enclosing what looks like a large telescope or a gun of some sort. The Doctor cannot fathom its use.

Finally Polly spots Jamie lying unmoving on the ground at the base of the dome. He must have taken a bad fall. The Doctor and his friends watch helplessly as two spacesuited figures emerge from the base and carry Jamie inside. The Doctor leads Ben and Polly down the ridge to the complex. They reach the door and must bang on it to get it to open. When it does, they hurry inside.

Inside the moonbase, a group of scientists from many nations work together on a large-scale weather control programme. In this time, the Earth's weather is controlled from the moon by an international coalition, all of whom are represented here. The leader of the moonbase crew is an Englishman called Hobson. Middle-aged and a bit blustery but a patron to his men. A fine leader and scientist. His No. 2 is Frenchman Roger Benoit, a bit fussy and more highly-strung than Hobson, but smart and quick and loyal. No. 3 is a Dutchman called Nils. He is even younger but more calm. He is a stabilizing force.

At the moment, the crew faces a crisis. The main control room is made up of two sections, a large central chamber under the dome and a smaller control area that is heavily shielded and surrounded with thick glass windows. Inside, there are just two men on duty, both wearing protective clothing and headgear. It is clearly a dangerous assignment. The crisis comes when one of the men in the inner room suddenly collapses, stricken with a virulent disease.

Hobson and Benoit rush in to help him and see that a strange pattern of black lines has appeared on his skin. Benoit and another man bear him off to the medical unit, carefully avoiding contact with his skin. But they all know that this is a futile effort. The infectious disease has been on the base for some time and their doctor has already been stricken. They are helpless to stop the disease until the relief doctor arrives from Earth.

Hobson realises they are facing a manpower crisis and has Nils call a meeting of all base personnel. In the entranceway, Hobson runs into the Doctor, Ben, and Polly. They don't look out of the ordinary in their spacesuits sans helmets and he initially mistakes them for the crew of a relief shuttle. He is concerned to learn of Jamie's injury, worried about more difficulties on the base. The Doctor assures him they will be no trouble.

Polly is allowed to go to the medical unit to check on Jamie, escorted by Benoit. Hobson takes a better look at the Doctor and Ben and becomes alarmed. These are no ordinary astronauts and no relief shuttle crew. The Doctor particularly looks like he could use additional decontamination! Hobson tries to extract some information as to where these two have come from, but the Doctor stays vague.

The men are assembled in the main control room and Hobson must address them. Deciding the Doctor and Ben are harmless, he invites them to join in. Once inside, the Doctor looks around interestedly at the technology. He realises the base's function as a weather control centre and recognises the large telescope-like device as a Gravitron. It was the Graviton's gravity disruption which must have affected the TARDIS and caused the bumpy landing. The Doctor's piloting skills are vindicated.

However, Hobson hears the Doctor discussing his realisations with Ben and becomes concerned again. Every school kid knows of the Gravitron yet it seems to be news to the Doctor. Where has he been the last 20 years? They are in the year 2070, only 20 years out from the Doctor's initial guess. The Doctor introduces himself formally and Hobson forgets his misgivings. The base is in sore need of a doctor right now, no matter how strange he may be. At this point, the Doctor seems to take over the meeting. He gets Hobson to introduce the gathered crew members and to explain how the Gravitron works. The talk of gravity, tides, and weather goes over Ben's head but the Doctor quickly acclimates himself.

Suddenly an alarm goes off in the control centre. In the glassed-in anteroom, one of the Gravitron operators has collapsed, another victim of the disease. A crewman called Francois dons protective gear and takes over from the stricken man. He is taken away to the medical unit.

The disruption of the Gravitron caused by the collapse of its operator is pronounced. The weather on Earth has changed suddenly for the worst, threatening many lives. The entire crew braces for the reaction, which comes quickly via the radio through an urgent message from International Space Control. The Gravitron was guiding a hurricane in the Pacific, keeping it harmlessly out at sea. It is now 45° out of phase and threatening Hawaii.

Hobson reports the sudden illness and assures ISC that they are back in operation. Unfortunately, the mention of yet another outbreak alarms the ISC staff and its controller is anxious to get answers. The base is put on hold while consultations take place on Earth.

While they wait, Nils announces that their radio signals are being monitored by an unknown party here on the moon. A playback proves it; and this is not the first time.

Nils is absolutely right. Not far away, in a spaceship, an unknown figure continues listening in as ISC returns to the radio.

Hobson is instructed to provide blood samples of the infected crewmen to be sent back to Earth on the next shuttle. Until then, the moonbase will be under quarantine. For at least a month! Hobson is of course livid. He reports the manpower shortage and tells ISC that he cannot wait a month. The response is less than supportive and Hobson demands to speak to ISC's controller, Mr. Rinberg. He is denied.

The conversation is overheard by the shadowy figure.

Nils reports this to Hobson, but he cares little about radio interference now. They have much larger problems. With the base's doctor himself a victim of the disease, they have little recourse to help themselves. The Doctor volunteers to help and Hobson jumps at the offer. As the Doctor and Ben are led to the medical unit, Hobson turns to his men. It looks like they'll be shorthanded for some time; they must all be careful to try and stop the spread of the disease while doing a lot of extra work.

The Doctor and Ben enter the medical unit to find a semi-circle of raised medical beds, many of them occupied by base personnel. Jamie occupies one of the beds near to the door and Polly tends to him. He is semi-conscious. With the Doctor's help, she attached a lead to him which will monitor his pulse, temperature, and breathing. It will regulate them and can also deliver medicine. He will be well taken care of by this "electronic doctor".

The Doctor says Jamie is concussed and feverish but will be all right with some rest. Jamie comes to for a moment, mumbling deliriously about "the Phantom Piper". It apparently is a legend of his clan - the piper appears to a McCrimmon just before he dies. It is clearly a worry to him; the Doctor promises to keep the piper away.

Polly is very concerned over the ill crewmen in the unit, especially without their doctor to help. The Doctor, Ben, and Polly check over the base physician, Dr. Evans, where he lays on his medical bed. He was first to become ill and is the worst of them all. All of them have the pattern of black lines on their hands and faces and the Doctor is disturbed by what he sees. Something about this doesn't add up - it doesn't even seem like a real disease at all. He needs to know more and he decides to send Ben back to the control room to keep an eye on things. He and Polly will stay here and try to learn more.

The crew shortage is already taking its toll. Hobson is weary but still up and working. The Gravitron - if functioning improperly - can do untold damage on Earth and the pressure on the remaining crew is increasing. Hobson is irritated by the attitude of ISC and he takes his frustrations out on Ben, whom he has just spotted lurking in one corner of the control room.

Ben quickly offers to help but Hobson is dubious. Benoit steps in tactfully and accepts his help. Still irritated, Hobson decides to go and lie down for a bit. He knows he needs rest, but his mind cannot leave behind the problems they face. He tells Benoit of two momentary pressure drops in the base. It has happened before and is still unexplained.

Benoit decides to send Ben down to the food stores to help a crewman called Ralph. He can be helpful there... and out of the way.

The food stores are in the bowels of the base, dark and creaky and full of sight-obscuring racks of food and supplies. Ralph works to clear up some supplies, concerned about a back of food with several inexplicable holes in it. He is startled by a noise nearby and becomes alarmed. There is no response to his calls and he becomes even more concerned.

Ben appears and calms him, wondering why the crew seem so on edge. Ralph seems to reinforce Ben's worry by asking if he is responsible for the damage to the food sacks. Ben becomes cranky but helps him anyway.

The two separate and Ralph works alone in the darkest corner of the store room, looking for milk and sugar to take upstairs. But his earlier fears were well-founded; he is not alone. He turns at a slight sound and finds himself facing a terrifying sight. A silvery, three-fingered hand reaches out from the gloom for him. He is trapped, unable to run. The hand touches his forehead and Ralph cries out in pain. He collapses and the hand catches him. Ralph is dragged away.

Ben hears the sounds indistinctly and goes searching for Ralph. There is no sign. Where could he have got to?

Later that evening, Polly dozes in a chair in the medical unit. All is quiet. The Doctor enters and startles her awake. He has been doing a little investigating around the base and he's found a small piece of silver material that seems to interest him, although he's not quite sure what it is. The lights in the room suddenly dim and the Doctor must explain to Polly the artificial day and night cycles necessary on the moonbase to keep everyone's biological time clock ticking properly. She doesn't quite get it, but the Doctor applauds her efforts to understand the concept.

Dr. Evans suddenly comes to and cries out. He is terrified of "the sliver hand" and begs for it not to touch him. He is extremely agitated for a moment and then collapses back on his bed. He is dead. The Doctor is stunned, unable to explain what has happened. The reference to the silver hand particularly disturbs him. He hurries off to report Evans' death to Hobson.

Ben has already returned to the control room to report Ralph's disappearance. Hobson is none too pleased by this. His quarters have been searched and there seems to be no way he could disappear in a place this small and isolated. When Hobson hears from the Doctor of Evans' death, he is devastated. Their crew is small...and dwindling fast. He decides not to report the death - the first from this disease - to International Space Control just yet. He wants more answers before he does so.

In the medical unit, Polly is trying to give Jamie some water but he is uncooperative. His attention is fixed on a point behind Polly. Finally she turns to see a door close but only glimpses the figure going out, seemingly carrying something. She screams at the sight. The Doctor, Hobson, and some other crewmen enter at this point and she tells them what she saw. An examination of the area beyond the door reveals nothing and Hobson chalks it up to nerves.

Hobson goes to check on Dr. Evans' body and is shocked to find his bed empty. The body is gone and he is livid. The Doctor is very worried by this turn of events, especially at the suspicion Hobson turns upon him. There is a commotion at the door as Benoit bursts in. Another man has contracted the disease and collapsed at the controls of the Gravitron. It is seriously out of alignment again, posing yet another danger to Earth. Hobson hurries out, but stops long enough to warn the Doctor to find Evans' body. Otherwise he and his friends will be expelled from the base.

Polly assures the Doctor that she saw someone or something leaving the medical unit through the far door. The Doctor may believe her, but there is something else worrying him just now. He must go and check on something, leaving her alone in the medical unit...just for a minute.

Alone and nervous, Polly returns to Jamie. He asks again for water and she is more than glad to leave the medical unit to fetch some.

However, as soon as she is gone, the far door opens again and the same figure that Polly saw before returns. It is a silver humanoid giant with a metal head, holes for eyes, and a slit for a mouth. Tubing runs down the length of it, centering on a large chest unit in front. It is a Cyberman! The creatures have been monitoring the base's communications and it was a Cyberman who captured Ralph in the food stores. Now it is here in the medical unit to take another body.

The Cyberman approaches Jamie's sick bed. Jamie continues to moan deliriously in resistance to "the Phantom Piper" he thinks is coming to take him away. The creature scrutinizes him for a moment and then moves away without taking Jamie because he is not afflicted with the disease. For the second time, he is spared. The Cyberman approaches another bed and snatches the man from it, carrying him out the rear door of the medical unit without apparent effort.

Polly returns with water just as the creature is leaving. She catches sight of it, this time very clearly, and she screams - loud and long. The Doctor, Ben, Hobson, and two crewmen rush in. Polly runs to the Doctor's arms for comfort, telling him through her sobs what she saw. The calms her down, listening with horror as she describes a Cyberman carrying a patient away.

Jamie renews his wailing about the Phantom Piper and Ben quiets him. Hobson knows that Polly definitely saw something because there is another patient gone, but he refuses to believe that a Cyberman was responsible. He sends the 2 men - Sam and Jules - to search the base thoroughly for the two missing patients. They must be around here somewhere. Hobson's anger grows as Polly repeats her story in detail, insisting that the silver man she saw was a Cyberman. She remembers them all too well from her first encounter.

Hobson still disbelieves this part of the story. He, too, recalls the history of the Cybermen attack on Earth almost 100 years earlier, but it has been reduced to a fairy story for children. All he knows for certain is that they were all killed when Mondas blew up. Ben can only hope he's right, but the Doctor now seems unsure. Hobson turns his attention to the strange Doctor and his friends. Their appearance in the midst of the mysterious disease and the unexplained disappearances is very suspicious to him. He certainly can't tie them to these events, but he decides that they must leave the moon immediately. One less problem for his crew to deal with.

This suits Ben fine, but the Doctor will not go. There is great evil at work here in the base - he is convinced of it - and evil must be fought. He will help, starting with the disease. The Doctor promises to find its cause and Hobson reluctantly agrees. He needs all the help he can get, no matter the source, but he will allow the Doctor just 24 hours to find the cause of the disease. If not, all four of them will be put outside. The Doctor accepts.

Hobson leaves and the Doctor sets to work right away. He gathers some supplies from the small lab area in the centre of the medical unit and takes a tissue sample from one of the infected men. Polly assists. She's a bit concerned as she never thought the Doctor was a real medical doctor. He thinks back and is fairly certain he took a degree in Glasgow, 1888, under Joseph Lister. She is dubious.

The Doctor gets his sample, but the removal of tissue causes the patient's arm to convulse, claw-like. The Doctor thinks it an unconscious reaction and is unconcerned. He returns to the lab area to analyse the specimen and sends Ben and Polly on a scavenger hunt. In order to find the cause of the disease, he'll have to examine clothing, boots, food, towels, soap, etc. They must work together to collect the needed samples. Together they'll find the cause.

After its last deviation, the Gravitron is proving difficult to get back under control. There's a distinct lack of coordination between the controls and the machinery. A test proves it - a planned 5 degree deflection swings far off course. The hurricane is barely holding under their control. If it is not kept steady, it could cause a major disaster on Earth. They try again, Hobson becoming more frustrated and the men more desperate.

Suddenly the R/T unit comes to life, International Space control calling. This time, Controller Rinberg himself wishes to speak to Hobson. Sounds serious. Rinberg is concerned with the weather fluctuations on Earth. Miami was just enjoying clear skies and a heat wave, now Hurricane Lucy is right overhead. He is insistent - in a narrow-minded bureaucratic sort of way - that they get the Gravitron back into balance immediately.

The crew however know that this is impossible to accomplish and Benoit proposes a dangerous solution - shutting the Gravitron down completely. Hobson is astonished at the suggestion but considers it for a moment...until Controller Rinberg vetoes it. The international coalition which came together to create the global weather control system is a delicate one. Many of its members are now calling for blood due to the current errors. Rinberg is in a politically unstable position and must depend upon Hobson and his crew to save his job... by accomplishing what seems impossible. They must get the Gravitron back into balance quickly and that is that. Rinberg rings off, leaving Hobson only one alternative.

He addresses all the men who have gathered around the radio and gives them new orders. They must do a full test on every system, every nut and bolt, on the Gravitron until they find the fault and fix it. Everyone is galvanized into action and they all start immediately. Into this frenzy of activity comes the Doctor, bearing surgical tongs and a specimen tray. He is looking for samples to test.

As the men work, he starts picking hairs and fibers from their clothing. Nils and Benoit work together to test a vital computer system and the Doctor's efforts provoke a stinging barrage of unflattering French from Benoit. The Doctor backs off. However, he follows them as they move on to another terminal and begins unlacing Nils' boot as he is absorbed in his work. When he moves off, the Doctor holds onto the boot and Nils continues his work with one stocking foot. The Doctor stands up holding his prize but runs straight into Hobson. He, too, is unamused by the Doctor's foolish-seeming behaviour and urges him to move his "investigations" elsewhere.

In the medical unit, Ben and Polly are busy preparing the Doctor's other specimens. Ben needs some more chemicals and heads out of the unit to ask Hobson for more. Jamie awakes, moaning slightly and Polly goes to him. She tries to calm him but he is certain he is dead, taken to this strange place by the Phantom Piper. Suddenly he reacts in fear as he sees "the piper" again. Poor Polly turns again to confront the Cyberman who has returned.

This time, the creature takes no chance. He stuns Polly with a flash of electricity from its hands. She crumples to the floor. Then it does the same to Jamie and snatches up another of the diseased patients, carrying him out the rear door. Jamie is again spared.

The Doctor returns to find Polly on the floor. He runs to her, trying to wake her.

Some time later, Hobson and Benoit consult on their individual work. Nothing out of the ordinary has been found in the mechanical equipment and Hobson is beginning to think it may not be a mechanical fault at all. On top of all of this, another unexplained pressure drop is detected. As with the others, it lasts about 5 seconds and then returns to normal. The compression chambers all appear empty and there seems no explanation. Hobson's frustration grows.

In the food storage room lies the answer to the pressure drops. Against one wall is a large pile of plastic food bags. Suddenly they are pushed over from behind and from a concealed hole in the wall, a Cyberman emerges. It piles up the bags again, taking about 5 seconds. The pressure drops recorded by the crew are the entries and exits of Cybermen entering and leaving the base unseen.

Hobson is glad that the pressure has returned to normal and starts to move off to check another piece of equipment when suddenly one of the crewmen detects what he thinks is the source of the Gravitron's problems. One of the probe control antennae appears to be broken, leading to a lack of coordination. That would answer the questions, but how did the antenna come to be broken in the first place. Hobson jumps to the conclusion that the Doctor and his friends are responsible. The Gravitron started playing up just after they arrived from "somewhere", and they were clearly walking around on the surface for a while before they were found. He sends two men out to fix the broken antenna and then decides to confront the Doctor. However, he is interrupted by Ben who has come to tell him about the latest disappearance. Hobson, enraged, hurries for the medical unit.

Jules and Franz, two crewmen, are quickly kitted out in their space suits and onto the surface of the moon. They realize the importance of their task.

In the medical unit, the Doctor concludes his tests and finds nothing. As far as he can tell, the entire base is sterile. He cannot understand and is dejected by his failure to detect the source of the disease. Polly tries gently to suggest that the relative age of the Doctor's degree (almost 200 years ago now) might have something to do with his inability to find the answer. The Doctor is very put out by this but has little time to worry as he hears the sound of footsteps approaching quickly from outside. Looking out the door, he sees Hobson and the others approaching and he can tell Hobson is out for blood. Theirs!

He hurries back to the lab area and pretends to work. Polly passes him samples he's already examined, attempting to look busy. Hobson bursts in with a million questions for the Doctor, but he is hampered by the strange little man's seeming single-minded absorption in his work. The Doctor doesn't look up from his microscope except to ask Hobson to close the door. Hobson is forced finally to order them all off the moon right away, but the Doctor is not listening. He has found something promising in his slide and must be left alone to work.

Hobson and his men are bamboozled by this bluster and retreat. The Doctor despatches Ben to keep them out. Unfortunately, the Doctor was only bluffing to keep Hobson away. He's found nothing at all and he must think of something else to tell Hobson to keep from being sent away. He suggests that Polly make some coffee to keep them all busy and distracted.

Outside on the surface, Jules and Franz reach the damaged antenna. Clearly it has been broken by something other than a meteorite. Sensing movement behind them, the two men turn. Two Cybermen have walked up behind them unheard. The Cybermen attack, clubbing the two men with their metal arms. Jules and Franz are knocked unconscious and sink slowly to the dusty surface.

Some time later, the Doctor emerges from the medical unit to find Hobson and several other crewmen waiting for him. Hobson pounces impatiently, demanding to know what he's found. The Doctor is finally forced to admit that he has failed to find the cause of the strange disease and Hobson is fully prepared to force him and his friends to leave the base. Their conversation is interrupted by the timely entrance of Polly with a tray of coffee and cups. The Doctor is very glad to see her. Hobson sends Benoit off to check on Jules and Franz and decides to get some coffee himself. He can always kick the strangers out later.

However, out on the surface, Jules and Franz are nowhere to be seen. Their empty spacesuits lie next to the antenna. Their bodies are gone.

The coffee is poured and Polly fusses around getting sugar for everyone who wants it. The distraction makes it hard for Hobson to make himself heard. Suddenly one of the crewmen screams and collapses on the floor. The Doctor rushes to his side and watches in horror as the black pattern appears on the man's face and hands. He has been stricken by the disease right in front of them.

Ben and another crewman carry him carefully into the medical unit and the Doctor and the others sit back down, worried looks on their faces. Just as he starts to sip his coffee, the Doctor suddenly realises what the key is. He leaps up and knocks Hobson's cup from his hand just before he drinks it. The disease agent is in the sugar! It affects the men differently because not everyone takes sugar, but Hobson was just about to drink some in his coffee! Hobson is astonished.

The Doctor uses his surgical tongs to pick up the sugar container and carry it gingerly into the medical unit. In the lab area, he quickly prepares a slide and places it under the microscope. And in an instant, the mystery is clear. Infecting the sugar is a large neurotropic virus that attacks the human nervous system. It follows the course of the nerves under the skin, hence the black lines that appear on the hands and faces of the victims. He is very certain that the Cyberman are responsible - they have deliberately infected the base.

Hobson is stunned. There's no way that the Cybermen could be on the base. It has been thoroughly searched and there's no place that even a single Cyberman could hide. The Doctor takes this in with mounting horror. His voice becomes increasingly urgent as he asks whether the medical unit itself was every searched. Hobson finally admits that it has not, although he thinks there's nowhere in here they could hide.

But he is wrong. Covered by a blanket, looking like just another patient, a Cyberman lays on one of the medical beds. Polly recoils in horror at the realisation and the Doctor tries to back everyone out of the room quietly.

The Doctor, Ben, Polly, Hobson, and the other crewmen stare in shock at the Cyberman. It orders them to remain still but makes no move to kill them. However, one of the men panics and is shot down while trying to escape. A second Cyberman has entered and holds the rest of the group under guard while the first communicates with their spaceship.

In the Cyber ship, parked on the lunar surface not far from the base, the message is received. A third Cyberman, the one who has been monitoring the base's communications all along, accepts the report that "operational system two" is complete. They will now begin operational system three.

The lead Cybermen addresses the Doctor first, acknowledging that they recognise him. The threat is clearly implied. While he is frightened, the Doctor does not hesitate to respond: he knows them as well. The Cybermen turn to Hobson, the commander, and announce that he "will be needed". Unfazed, Hobson demands to know what has happened to his missing men. They are not dead, but the alternative may be worse. They have been "altered" and "controlled". And they will return.

The Cybermen turn their attention to Jamie, noting that while he is in the medical unit, he has not received the neurotrope. The leader aims its weapon at Jamie but Polly steps in front of him, demanding that Jamie be spared. She tells the Cybermen that he hurt his head and this seems to work. Jamie is of no value to the Cybermen if there is something wrong with his brain. The Cybermen force Hobson to take them to the control room. The others go along, except for Ben and Polly who are left behind.

Ben ponders the meaning of the Cybermen's words, particularly about the crewmen being "converted".

In the Cyber ship, the meaning is very clear. The three missing crewmen have been fitted with three-pronged metal pieces over their heads and wires connecting them. They stand zombie-like, marked with the black lines of the neurotrope, until they are given an order. The metal pieces help the Cybermen control their minds completely. The creatures are pleased at their success. One by one the men are transferred into metal containers for transport.

Benoit tries to raise Jules and Franz on the R/T with no success. They have been lost on the lunar surface; further victims of the Cybermen. Benoit prepares to go out after them, but is stopped in his tracks when Hobson enters the control room propelled by the Cybermen.

Hobson shouts to his men to stay where they are and keep calm. The Cybermen are lethal and he wants no needless heroics. Benoit and the others are stunned to see the giant silver creatures. It is like a fairy story come hideously to life.

The Cybermen announces that they are taking over the Gravitron and they plan to destroy all life on Earth with it by altering the weather. Hobson believes the Cybermen are doing it to avenge the destruction of their home planet, but that emotion is unknown to the creatures. They are here to "eliminate all dangers" to them. Emotion is foreign to them. The Cybermen radio the completion of operational system four to their spacecraft. They have completed their entry into the base.

Hoping to gain more information, Hobson asks how the Cybermen got into the base. The creature cannot resist revealing the ease with which this was achieved: they tunneled in under the surface and cut their way in through the food storage room. This allowed them unseen access and the opportunity to contaminate the food supply. Hobson realises that these entries account for the pressure drops his men recorded, but that knowledge is of little comfort now that the Cybermen hold all the cards.

Ben and Polly cool their heels in the medical unit, Polly picking at her nails. Jamie awakes and tries to get up, feeling a bit better, but Polly forces him to lie back. Jamie seems aware of what has been happening and the fact that the "apparition" he saw was certainly not the Phantom Piper. Ben and Polly try to think back to their first encounter with the Cybermen at the Antarctic tracking station and how they managed to defeat the creatures. The only weakness they remember is the radiation which held them off, but there's no access to the moonbase's reactor. That won't work.

Jamie chimes in, trying to be helpful, suggesting that in his day witches would be sprinkled with holy water to ward them off. Ben laughs at this naïve thought, but the words give Polly an idea. The Cybermen are all metal, but their chest control units appear to be made of plastic, and she just happens to have the very thing to dissolve plastic - nail polish remover. Ben doesn't quite see the connection, but Polly is eager to set to work on an experiment she hopes will lead them to a solution.

The controlled crewmen are brought into the base, and the Cybermen order the men out of the inner control room where the Gravitron's power controls are located. They hesitate until Hobson orders them to comply; it is all he can do. When the men come out, the controlled crewmen go in, taking over the controls like the experts they are. Benoit is appalled as the controlled men are not using protective headgear to deflect the intense sonic fields generated by the Gravitron. They will all go insane within 12 hours. The Cybermen do not care. Their purpose will have been served long before then and the men will be "disposed of" at the end anyway.

The men begin working as directed to change the location of the Gravitron. On the world map, the changes are already reflected in the Earth's changing weather. The Doctor watches quietly as they go about their work, thinking furiously for a way to stop the Cybermen.

Polly's work has progressed nicely. She has moved on from simple nail polish remover to all of the bottles of chemicals in the lab area. When Ben reminds her that nail polish remover is a form of acetone and she finds and adds a whole bottle of it to her arsenal. With luck, the acetone will soften up the Cybermen's chest units and perhaps knobble their controls. Worried that the alien plastic might be harder to dissolve than regular plastic, Polly decides to hedge her bets and mix up all the solvents available to her in the lab - benzene, ether, alcohol, etc. It's the best they can do. Then the problem becomes how to deliver it to the Cybermen. Ben has an idea...

In the control room, the Cybermen make more modifications to the Gravitron, using the controlled men to do their work for them. In a corner with the others, Hobson wonders aloud why the Cybermen need the slaves to do the work; why not just do it themselves and save the trouble? The Doctor meanwhile is working on another question. He takes out his recorder and plays a long high-pitched note. The controlled men suddenly freeze at the controls and stand motionless. The Cybermen realise this and try to trace its source but the Doctor stops playing and hides the recorder in his pocket. The men begin to work again and the Doctor is pleased. Just as he thought - the men are sonically controlled. It should be easy for him to overcome.

The Doctor then turns to Hobson and repeats his observation about the controlled men, as if he hadn't heard Hobson before. He deduces that there must be something in the power control room that the Cybermen don't like. It's not pressure or electricity or radiation, but gravity. The Doctor is sure of it and now has yet another piece of information to use against them.

The Cybermen start to realign the Gravitron probe and turn it up to full power. The sounds of rising power fill the control room. Hobson and Benoit both realise the devastation that will be wrought on Earth as soon as the new gravitational alignment takes hold.

The R/T comes to life with a message from Earth Control. The base is supposed to send a routine signal on a schedule to indicate that all is well. The last signal was not sent and the authorities are worried. They are to fire a sodium flare rocket if they are having radio trouble. The Cybermen are concerned as to what the Earth authorities might do if the flare is not sent and no radio response is received. Hobson thinks quickly and lies, saying they'll assume the crew is dead and do nothing. He knows that after a second missed transmission, a relief rocket will be sent. Hopefully the Cybermen bought this lie.

In the medical unit, Ben is proposing to use a fire extinguisher to deliver their solvent concoction to the Cybermen. But first he must explain the fundamentals of the device to Jamie, who has of course never seen anything like it. Luckily, the lad takes this in his stride, as he has all the amazing things he's seen on his short travels.

They have enough solvent for a second extinguisher and Jamie decides to go and get it for them. Ben and Polly try to keep him on the medical bed, saying he's still ill. However, Jamie is feeling much better and ready for action. He even challenges Ben when he tries to stop him from getting up. Polly makes peace between them, only to be ordered to stay behind herself by Ben and Jamie as this is "men's work".

In the control room, Hobson watches as one of the controlled humans collapses at his post. The sonic fields in the power room are already affecting them. He shouts out to try and stop the Cybermen from killing the humans but is brushed off by the leader. It orders him to remain silent, then uses a hand-held control device to force the fallen man up and back to work.

Ben and Jamie, both armed with fire extinguishers, creep along a corridor toward the control room. They are trying to be as quiet as possible. Ben starts when he realises that someone is creeping up behind them. Polly! She of course refuses to stay behind and joins them in their mission. Jamie tries to argue with them but Ben silences them both so that they will not be heard by the Cybermen. They crouch down by the door to the control room as Ben gives Jamie some final instructions on how to aim and fire the extinguisher.

However, the Cybermen hear them outside and start toward the door. The Doctor realises what is happening and uses sound waves to freeze the controlled men in the power room. The Cybermen decide that control of the Gravitron is most important and so turn on the Doctor to stop him. Ben and Jamie burst in and begin spraying their solvent at the Cybermen. Their gamble works. The creatures lose control and stagger about the room, finally collapsing onto the floor, their chest units spewing foam as they melt.

The Doctor springs into action and rushes into the power room to remove the controlled men. He makes sure to remove the metal head pieces to release them from Cyber-control. Hobson and his remaining crew return to their places to try and regain control of the Gravitron and save Earth. The controlled men are carried off to the medical unit.

In the Cybermen's spaceship, contact has been lost with the Cybermen in the base. They must have failed in their task. The Cybermen decide that they must now invade in force and set about preparing their weapons.

Hobson and his men quickly regain a measure of control over the Gravitron and the disaster on Earth is averted. However, the coordination problem between the Gravitron and its controls still remains. Once it calms down, Benoit remembers the faulty antenna on the surface and the lack of response from Jules and Franz. Someone must go outside and Benoit decides it must be him. No one else can be spared.

He is quickly suited up and out of the airlock. The moon's surface seems even more barren than usual to him. He maintains R/T contact with the base and when he reaches the antenna, he must report finding only two empty spacesuits. He is certain the Cybermen have got Jules and Franz.

Hobson orders Benoit to return quickly, but it is too late. Nils, watching from the window, can see a Cyberman marching along the surface toward Benoit's position. He will be caught. Hobson radios a warning but Benoit cannot spot the creature until it is nearly on top of him. He is armed with a fire extinguisher of solvents but it doesn't work in the vacuum of the moon. Benoit is forced to dodge the Cyberman's reaching arms as he bounds toward the base. If only he can make it in time.

Inside the base, Ben and Polly are watching the deadly slow-motion chase. They realise that Benoit is helpless out there and they must do something. Ben has an idea and races to the airlock. Polly helps him into a spacesuit and he enters the airlock. Polly waits outside.

The outer door opens allowing Benoit to bound inside, but the Cyberman is right behind him and gets in the door before it can close. Ben has a small bottle of the solvent mixture with him and smashes it against the Cyberman's chest unit. It works as before and the creature collapses to the floor, dead, its vital control unit dissolving. The airlock door closes and Ben and Benoit are safe.

Hobson is buoyed by Ben's success and glad to be pro-active once again. He announces that the siege isn't over yet and that everyone should be prepared for further attacks. He orders all armoured doors to be lowered at all entrances and renews efforts to re-establish contact with Earth. Polly is to mix up more of the solvent in case it is needed.

The Doctor and Jamie welcome Ben back as Hobson tries to get a radar fix on the Cyberman spaceship. At last they find, about 3 kilometres away over a nearby crater rim. It is well hidden. The Doctor has an idea to stop them that involves the Gravitron, but it appears that the machinery cannot be lowered enough to suit his needs. He has no time to explain though as Nils reports seeing more movement outside. Everyone rushes to the dome observation windows to look out.

They see nothing at first, despite Nils' insistence, but shortly they notice sunlight glinting on silver in the distance. Hobson commandeers a telescope and is stunned by what he sees. More Cybermen! He pushes everyone back inside to relative safety. They all know that the siege is over.

As the Doctor and Hobson watch, the Cybermen continue to march toward the base. The Doctor fears they're planning to simply march inside but Hobson thinks that impossible. Their tunnel in the store room has already been blocked up and the doors are sealed. They'll have to get in by force.

The Cybermen have broken in on the base's radio frequency and issue an ultimatum. Open the doors and let them in or else. Hobson refuses to listen to the rhetoric and angrily shuts off the radio. He orders Nils to contact Earth immediately and request assistance but the radio issues only a piercing wall of sound. It's not a jamming signal, but somehow the Cybermen are blocking transmission. Hobson realises it is the aerial and the Doctor confirms his suspicion by looking out onto the lunar surface through the telescope. They can clearly see two Cybermen destroying the radio antenna.

Hobson is despondent, certain that the Cybermen have thought of everything. The Doctor is not so pessimistic. Watching them through the telescope, he says that everything's got a weak point; it's just a question of waiting until it shows up. The Doctor's philosophy of battle.

Benoit tries to think positively, sure that Earth control has already launched a rescue rocket. Their radio communications are now far overdue and no flare was sent. Standard procedure is to launch a rocket. Nils agrees, as does Hobson, although the commander wonders if there's a replacement for him on board. Polly thinks that solves it. As long as they can keep the Cybermen out, they can wait for the rocket from Earth to rescue them. But the Doctor is sceptical - their survival doesn't depend on them. If a rocket is coming from Earth, the Cybermen have already detected it and will surely be hatching up some plan to stop it.

In the Cyber ship, the incoming Earth ship has indeed been detected and plans for invading the base are put on hold. They must deal with the ship first. They have only 14 minutes to stop it.

On the ridge overlooking the base, the Cybermen produce a large, box-like control device. It is a long-distance sonic device which is used to control the altered humans. In all the excitement, they have been nearly forgotten for the moment in the medical unit.

At a signal from the Cybermen, Dr. Evans sits up, completely under their command. He picks up the metal head piece lying next to him and puts it on. He can now receive all instructions and will obey. Zombie-like, Evans leaves his medical bed and approaches the crewman on duty and clubs him viciously with a heavy piece of equipment. He is now free to move about.

In the control room, the waiting men are becoming tense. There is little to do but much to worry about. Benoit and Nils switch on the radio in hopes of listening for any more ultimatums from the Cybermen.

Evans picks up a discarded Cyber weapon before leaving the medical unit. He has received his final instructions and leaves the unit to carry them out.

Due to the crew shortage, Ben has been pressed into service to help at a monitoring station. Benoit says he's learning fast but it will take a long time for him to get the hang of it. Hobson is pleased; for the moment at least, the Gravitron is stable and they are safe. He's a little worried about Benson, the crewman in the power room. He's been in there alone for 6 hours straight and says he's fine. Hobson knows it is tough duty, even under the best circumstances. They all must make sacrifices to make sure that the Earth's weather stays under control. Polly brings some coffee to the crewmen, which is greatly appreciated, especially as it is served without sugar this time!

Unnoticed in the busy control room, Dr. Evans has slipped in. He enters the power room unseen and knocks Benson unconscious. He dons Benson's protective headgear, more for disguise than anything else. He sits with his back to the observation window and the headgear covers the control unit on his head. At a command from the Cybermen that only he can hear, he begins to change the field coordinates on the Gravitron.

Ben announces excitedly that the base's scanners have picked up the rocket from Earth and everyone gathers round the monitor. Nils and the other crew members are relieved, certain that their nightmare is finally over. They are surprised to find that it is very close, coming in from parking orbit, and should be down in mere minutes. They've got weapons that will deal with the Cybermen.

There is a long pause while everyone stares at the blip on the screen. It seems to move so slowly. However, it becomes clear that something is wrong. The ship continues moving, beyond the target on the screen and seems to be speeding away from the moon. The crewmen are stunned as the ship picks up speed. Nils tries to keep the blip on the radar screen but it is too far and moving too quickly. The screen is blotted out by a flash of white - the sun. The rescue ship is headed there on an ever-accelerating course. The saddened crewmen fall silent, knowing exactly what this means.

Polly is in the dark and begs for an explanation. Somehow the rocket has been deflected away from the moon and sent hurtling toward the sun. It will take a week for them to reach it, but they cannot escape its gravity now and the ship will be destroyed in its heat. All aboard are as good as dead.

Hobson is demoralised by this failure, knowing that they are once again on their own against the Cybermen. He wonders how it could have happened. The Doctor, of course, has the answer - a gravity beam like the Gravitron. In fact, the only machine which could have done what they have witnessed is the Gravitron itself. Turning toward the power room, they are all shocked to see Evans looking back at them from inside. He has manipulated the controls and deflected the rocket under Cyber control.

Hobson tries to get inside but the door is locked. They can do nothing to stop him. The Doctor is pleased to see that Evans seems to be the only one of the controlled men reactivated and he sends Ben and Jamie off to the medical unit to barricade the others inside. They must keep the men from getting out.

In the medical unit, the remaining control headpieces come to life, bringing the men out of their catatonic states. They all sit up and put on their headpieces.

Ben and Jamie reach the door to the medical unit, glad not to have encountered any more of the controlled men. They are confident the men are all still inside. Ben and Jamie look for heavy enough objects to bar the door, but they are too late. Just as they pick up a metal table, the doors open and the first man starts to exit. Thinking quickly, Ben leads a charge and they use the table to ram into the man. He falls back into the medical unit, knocking into the other zombies. Jamie closes and locks the door but that won't hold them for long. Indeed, as he and Ben pile up more furniture against the doors, a metal bar slips through the gap in the doors and starts to pry them apart.

Hobson addresses Evans over a speaker system, trying to get him to resist the Cyber control and open the doors. No response. Evans brandishes the Cyberman weapon, but makes no further move. Hobson is furious. Benoit is even more concerned over the Gravitron. Apparently the Cybermen are moving ahead with their plans to disrupt Earth's weather. As it stands now, the wildly swinging gravity field will flood half of Europe unless they stop Evans.

Hobson decides to break out the rest of the captured Cyber weapons and rush the power room. He starts to address the Cybermen via the radio, but they tell him they have overhead everything he's said. He vows that they will never get inside the base but the Cybermen remind him that they are in it already.

Suddenly there is a pop and a great outrush of air through the dome. The lights go out and chaos breaks loose. Evans has fired his Cyber weapon and there is now a hole in the dome and their oxygen (along with any other loose bits) is escaping violently into the vacuum. Everyone races for the emergency oxygen masks, except for Hobson and Benoit.

They crouch on the floor, gasping for breath and trying to think of some way to plug the hole. They try using Hobson's coat but that only works for a moment. It, too, is sucked out due to the intense pressure. Benoit spots the plastic tray with which Polly served coffee and they place that flat against the dome. It is larger than the hole and seals it effectively, the pressure keeping it in place. The base is saved.

Polly has been particularly hard-hit by the pressure and oxygen loss and she struggles to recover. The Doctor helps out. The crewmen pick themselves up as Nils releases reserve oxygen supplies to compensate for what was lost. Everyone takes stock of the place, making sure that they are still functional.

Polly is first to notice the total silence in the base. The ever-present background hum of the Gravitron has stopped. The Doctor looks toward the power room and sees that Evans has collapsed due to the oxygen loss. He was unable to or uninterested in reaching an emergency mask. The Doctor acts quickly and has Ben and Jamie help him get Evans out before he comes to. Ben is a bit put out to be given the task of "taking care" of Evans. The Doctor cares little for what he does, just as long as Evans is out of action and won't come back here. Nils helps him and Jamie remove Evans from the control room. The Doctor takes the precaution of removing Evans' headpiece and putting it in his pocket.

The Doctor joins Hobson at the dome wall, examining the burned and jagged edge of the hole there through the clear plastic tray. But they have little time to contemplate their next move as Benoit announces the arrival of more Cybermen. A second and then a third spaceship land on the surface outside.

The radio comes to life once more with another ultimatum. The Cybermen have brought up reinforcements with other weapons. The humans must open the doors of the base or else they will be completely destroyed. The message ends, leaving the crew to worry anew about what the Cybermen are hatching up.

Outside, another group of Cybermen have joined the first on the ridge overlooking the base. They carry with them a large box that it takes several Cybermen to lift. From it, they begin unpacking some machinery. It looks very like a large cannon. The humans are given a count of 10 before the Cybermen will fire.

Hobson takes all this in from his observation point and orders everyone to get down away from the windows. All comply except for the Doctor. He stands defiantly at the telescope, either stubbornly or stupidly refusing to take cover. As long as the Gravitron is still switched on, he will stay put.

The Cybermen reach the count of 10 and fire. The weapon projects a powerful laser beam toward the base, recoiling the gun and the Cybermen holding it. However, the beam deflects off the dome, directed by the intense gravity field of the Gravitron. A second shot yields the same results. The Cybermen begin re-packing their weapon.

Inside the base, the Doctor reports this to Hobson and the others, his surprised voice betraying the fact that he was more scared than he let on. But they have all survived and they have another chance to stop the Cybermen. And the Doctor has an idea.

Returning to a question he had asked some time earlier, the Doctor tries to determine how far down the Gravitron can be aimed. He wishes to point it at the surface, using the gravity control directly against the Cybermen, but Hobson tells him that the safety system imposes a limit on the downward angle so as not to affect the base. The Doctor, energized by the belief that this plan will work, convinces Hobson to give it a try anyway. They've got very little to lose at this point.

Benoit takes over the controls in the power room while Hobson and Polly form a visual relay of instructions from the Doctor to Benoit. Once they are set, Benoit begins lowering the Gravitron probe to its lowest position.

Out on the surface, the Cybermen appear to be massing in some sort of formation on the ridge. There is no effect from the gravity.

The Doctor knows that the beam is far above the Cybermen's heads. They must try to manually move the probe further down if it is going to have any affect on them. He and Hobson heave on the massive machine from several different points. It won't budge beyond its current point and the Doctor is on the verge of giving up. But Hobson remembers the angular cut-out which stops the probe's downward movement to reinforce the safety. It is a small switch at the base of the machine. He opens it and trips it. That should allow them to lower the probe as far as they wish. They don't even need to do it by hand.

Benoit, via relayed orders, activates the Gravitron again and lowers it further.

The reaction outside is almost immediate. First one and then another and then another of the Cybermen are picked up off the ground as if by invisible hands and flung into the air. They and their spaceships are repelled from the surface and out into space, farther and farther away. The Cyberman invasion of the moon is over.

Cheers ring out in the moonbase as the crew realises they've won. But their victory is short-lived. They realise that they must get the Gravitron back in operation and get the Earth's weather back under control. The men are jubilant as Hobson begins assigning tasks. He hopes they'll be operational within 2 hours.

The Doctor sees that his usefulness if over here and leads his companions quietly out of the control room, unnoticed by the crew. Hobson turns to find them all gone and laughs. He was quick enough to want the strange little man out, and now that he's gone he misses the Doctor. Perhaps it's just as well, he muses, they've "got enough madmen here already".

The Doctor and his friends are shortly suited up in their spacesuits and helmets and moonwalking back to the TARDIS. As they reach the ship and the Doctor is unlocking it, Polly notices what looks like a shooting star in the sky. She wonders if it could be the Cybermen. It could be, and the Doctor hopes it is the last they see of the evil creatures.

As the flash of light disappears, they all pile into the TARDIS. The doors close and a moment later the engines come to life. The TARDIS disappears from the surface of the moon. It is once again desolate, lifeless, and infinitely quiet outside the moonbase.

The Doctor operates some controls and the ship is in perfect flight. He seems happy to be back in command of his ship, although his companions are dubious. They remember very well what happened last time. The Doctor dismisses it as a bumpy landing.

A thought strikes him and the Doctor activates another control, one they've never seen him use before. He calls it the time scanner. Instead of showing the normal picture outside the ship, the scanner will show them a glimpse of the future. Jamie likens it to the second sight, a very dangerous practice in his day, but watches nonetheless.