Silence hangs heavily in the air of a planet which appears deserted. The ground is smooth and hard-packed, looking sun-baked and dried out. Yet even on a world such as this, plant life flourishes. But there are no other signs of life, no other sounds at all as the TARDIS materializes.

Vicki finishes up Steven's haircut as they and the Doctor survey the landscape for themselves on the scanner. The Doctor is immediately struck by the eerie silence. The readings are all quite normal. The planet seems ideally conducive to life, but appears deserted. They prepare to go outside.

Nearby, a strange little robot moves out from behind a rock outcrop. It observes the strange shape of the TARDIS and chatters mechanically to itself. Shortly, it moves off toward the ship.

Inside, Steven seems to be taking their new landing site a bit too casually, hoping in vain for a lake or a river to go swimming. The Doctor is quick to remind him that this is a serious scientific expedition. However, their conversation is interrupted by a bumping sound which comes from outside. The strange-looking robot has bumped into the TARDIS and is now moving slowly around it, keeping in constant contact with it, feeling its way around. The Doctor is quick to realize that whatever it is must be blind. Soon the bumping and sliding noises cease as the thing completes its circuit.

The robot moves off away from the ship and into view of the scanner. All three are amazed and amused by its appearance. Vicki immediately nicknames the robot a "Chumblie" for its odd wobbly locomotion. It extends an antenna array from its top section and Steven is sure it is sending a message, probably to its controller. When it finishes, the Chumblie departs. Steven is worried at the possible danger, Vicki thinks the Chumblie cute and unthreatening, and the Doctor is, of course, intrigued at the prospect of intelligent life on this planet. They all head outside to investigate

They notice the flora, including some sweet-smelling flowers not unlike roses, and they also notice the three suns in the sky. The Doctor thinks the silence reminds him of Xeros, but from a different cause. Their explorations are cut short when another Chumblie approaches them from behind the TARDIS. This one holds a rod-like appendage in front of it that looks suspiciously like a weapon. It is obvious now that the robot is indeed blind, but it clearly "sees" by means of heat sensors and has very acute hearing.

The Chumblie wobbles over to the Doctor and nudges him. It obviously wants them to go with it. When they decide to stay where they are, it makes a show of force by firing its weapon - some form of light ray - and burning up a nearby tree. The Doctor is impressed, but he also realizes the danger and agrees to go with the robot.

They have not gotten very far, when a metal mesh drops down from a rock ledge above and covers the Chumblie. It stops dead and falls silent. Two uniformed women climb down from the rocks and address them. They are Drahvins, from the planet Drahva in Galaxy Four. They say they have come to rescue the travellers from the machine. The metal mesh stops the robot's control waves from reaching it. However, when the Doctor and his friends start to ask questions, the two women cannot answer as they have had no instructions to do so. They will take them to their leader, Maaga, and she will answer the questions.

Vicki is sceptical and refuses. The Drahvins tell them that more robots will come after this one. They will be captured and taken to the evil Rills. The Rills are described as crawling, murderous creatures. Even though sceptical himself, the Doctor agrees that they should go with the Drahvins. At least these people have answered some questions, which is more than the Chumblies have done. In the distance, more Chumblies appear and come after the group. The two Drahvins try to remove the metal mesh from around the robot they have stopped, but the machine has been magnetized and the mesh will not come off. The Doctor and his friend beat a hasty retreat, with the Drahvins reluctantly following. They seem overly distressed at leaving the mesh behind.

The Chumblies reach their disabled mate. Working together using their mechanical arms, they remove the metal mesh. The disabled robot comes back to life immediately and the entire group heads off after the Doctor and the others.

The group reaches the Drahvin ship at a dead run. Entering, Vicki keeps her eyes on the landscape outside while the Doctor and Steven examine the inside. Both are less than impressed with the ship, the Doctor going so far as to call it "fossilized". They are interrupted by the arrival of Maaga, the Drahvin leader. The Doctor and company watch as she accepts the report from her subordinates. They are clearly frightened of her, especially when they must report that they could not retrieve the metal mesh. She orders them to sit while she addresses "the prisoners".

Maaga explains that the Drahvins are at war with the Rills, and that the travellers are now caught in between and may be obliterated as well. She tells them that this planet is in its last stages of life and will disintegrate in 14 dawns time. This information, she says, was given to them by the Rills themselves. Now the Rills have repaired their spaceship and are planning to leave. The Drahvin ship is irreparable, and so they plan to capture the Rill ship and use it to leave the planet.

Neither the Drahvins nor the Rills belong to this planet. She explains that they came here 400 dawns ago, surveying the planet as a possible colony. Maaga is a true Drahvin, a race of female warriors, and the others are clones, trained only to obey orders. She explains that as they were orbiting this planet, the Rills appeared and attacked them. They fired back, and both ships crashlanded here. The Doctor clearly sees the situation as she has explained it. A state of war exists and now both groups will soon be at the mercy of an exploding planet.

Discussion is halted as a Chumblie appears outside the ship. Maaga says it is trying to communicate with them, but she turns off the external speakers and will not listen, despite the Doctor's wishes to hear it. Maaga fires at the robot. It does no damage, but the robot does eventually wobble off, apparently sending a message to the Rills as it goes.

The Doctor is amused by Maaga's unsuccessful attempt at destroying the Chumblie, but she insists that she was only trying to scare it away. The Doctor wonders why the Rills, supposedly so evil, would bother to tell the Drahvins that the planet was going to disintegrate. It sounds to him as if they were trying to help, but Maaga insists it is simply to tempt them onboard the Rill ship to destroy them.

The Doctor thinks of another possibility. Perhaps the Rills' information was a pack of lies...or wrong completely. He offers to double-check the Rills' calculations back in the TARDIS. Maaga is quick to accept, but she wants to keep Vicki as a...guest. Only Steven will be allowed to go with the Doctor. Despite Maaga's words to the contrary, everyone knows Vicki is a prisoner to ensure their return.

The Doctor and Steven head immediately for the ship. However, they find two Chumblies outside it, trying to break in. They use several different methods, but do not appear to be having much success.

Meanwhile, Vicki overhears while Maaga mercilessly berates her warriors for losing the metal mesh in their struggle with the Chumblie earlier. She uses their fear of the Rills to instill obedience and remorse at their failure.

The Chumblies try their light rays on the TARDIS, but again they are unsuccessful. The robots communicate a mechanical message to their controllers and then move off. The Doctor takes a moment to congratulate himself on the strength and precision of his force barrier before entering the ship.

The Doctor and Steven realize they must rescue Vicki and get off the planet as soon as possible. However, before they can leave the TARDIS, they spot another Chumblie on the scanner. It carries a black vial of some sort and two long wires. It moves right up to the ship, bumping into it before stopping. The robot drops the vial, then wobbles off to a safe distance. It touches the two leads together and a mighty explosion rocks the TARDIS. Steven and the Doctor are OK, but knocked down again as a second explosion follows.

When the smoke clears, all is well. The robot has failed again to gain entry. The Doctor and Steven watch gleefully as the Chumblie retreats in defeat. The Doctor and Steven hasten to the Drahvin ship while the coast is clear, fully aware that time is against them.

Vicki sits by as Maaga eats. She herself has refused, thinking the leaf-like food disgusting. Vicki is worried about her friends and how long they have been gone. She tries to convince Maaga to let her go and look for them. But Maaga believes they would just get in their ship and leave. If the others have not returned soon, a Drahvin search party will be organized.

The Doctor and Steven shortly arrive outside the ship. Steven takes note of the inferior engineering and construction of the Drahvin ship. The Doctor, too, notices the weakness of the ship's hull material, concluding that the Drahvins really aren't very intelligent after all.

Entering the ship, the Doctor actually repeats his opinion of the ship's inferiority to Maaga. She is clearly bothered by this, but is more interested in the Doctor's findings. He tells her that the planet will explode, but tells her that the Rills were right about the timing of it: 14 dawns from now. Maaga absorbs this information, then asks the Doctor point blank if he will help them to capture the Rill spaceship so they can leave the planet. The Rills will be left here to die. The Doctor refuses, not believing Maaga's word that the Rills are evil, and refusing to involve himself in this affair.

Maaga tries to force him, drawing her gun and pointing it at the Doctor's head. Steven attacks Maaga and tries to knock the gun away. They become locked in a struggle for the gun, Steven facing a far tougher opponent than he had first thought. The contest comes to an end when three Drahvins enter the ship from outside, returning from their patrol. They all level their guns at Steven and the struggle is over.

Back in command, Maaga demands the Doctor's help. With no other choice, he agrees. Maaga reminds him he has only 14 dawns in which to succeed. However, the Doctor and Steven exchange a look which Maaga immediately notices. She is now certain the Doctor is lying about the planet's destruction. She aims her gun at Vicki and threatens to kill her unless he admits the truth. He tells them the planet has only 2 dawns left.

Maaga tells the Doctor that he must hurry and capture the Rill ship and that she will keep Vicki hostage to ensure that he fulfils his side of the bargain. Steven steps in and offers himself instead. At first Maaga refuses, but the Doctor says he will do nothing unless she accepts. Reluctantly, she agrees, and the Doctor and Vicki set out for the Rill ship. Steven wonders aloud why Maaga believes they can accomplish this task all by themselves. She knows that either they will succeed or they will die with all the others.

Outside the ship, the Doctor looks up, ruminating on the length of night and day on this planet. With three suns, night is likely to be very short. He also takes a moment to examine the metal comprising the Drahvin ship. He is certain that the Chumblies' light rays could easily penetrate the ship's hull. If the Rills mean the Drahvins harm, why haven't the robots broken into the ship. They set out on their journey, Vicki still a bit worried by the Drahvins' "nasty" description of the Rills.

Inside the Drahvin ship, Maaga is elsewhere and Steven makes an attempt to survey the armaments of the clone Drahvins. To his surprise, he finds that both of their guns are currently non-functional. But he still cannot escape. They offer him some food, but he refuses when he sees the twigs and leaves they present him. He learns that Maaga has different and better food, as well as the most powerful weapon since she is leader. Thinking quickly, Steven uses this information to try and drive a wedge between Maaga and her soldiers, telling them that they could probably destroy the Chumblies with Maaga's gun and tempting them to try and take it from her.

However, Maaga enters and exposes the trick, ending the conversation. She then turns the tables, explaining that all this ordeal would be finished if he would take the Drahvins with him in the TARDIS. But Steven sees through this. Besides, even if he wanted to take them, he couldn't as only the Doctor knows how to operate the ship. He retires to a bunk in one corner of the room and lays down, listening as Maaga plans his death...when she is ready.

The Doctor and Vicki have come across an area of high Chumblie activity. They are obviously sentries and the Rill ship is nearby. They hide on top of a rock outcrop to observe the robots. Vicki suddenly gets an idea. She grabs a sizeable rock and tosses it down behind the next Chumblie to pass by. The Doctor is angry at her rashness, but she has proven her theory: the robots can only pick up sounds directly in front of them. If they follow close behind one, they can reach the Rill ship undetected. They head off to follow a Chumblie.

Steven, feigning sleep in the Drahvin ship, watches as Maaga charges up the power units on her soldiers' guns. She wants them to be at full power, as they will soon use them to break into the Rill ship and destroy every one of the hideous creatures.

The Doctor and Vicki are successful in reaching the Rills' crash site. Around their ship, they have also constructed a protective stone enclosure and outside appears to be some kind of drill rig. They move in closer, nearly getting caught by an approaching Chumblie. The Doctor is most impressed by everything that he sees. The drill rig, the enclosure, and what appears to be an oxygen converter all earn his approval. He is sure that the Rills are a very advanced species. With no other Chumblies about, they decide to explore the passageway which leads into the stone enclosure alongside the Rills' ship.

They do not get far when Vicki detects a familiar chemical odour, but neither of them can place it. It gets stronger as they proceed. Up ahead, they hear a Chumblie moving, but it turns aside before it reaches them. The passageway ends in a large, high chamber, one side of which is comprised of the hull and doorway of the Rill ship. The Doctor examines the metal and pronounces it of much higher quality than the Drahvin ship. Equipment and spare parts fill the space and the Doctor speculates that it is a robot repair area.

Two large eyes survey the Doctor and Vicki from within a scaly non-humanoid face. But they do not have long to regard the face as the sound of an approaching Chumblie galvanizes them into action. The Doctor and Vicki race down the passageway, but are forced to turn off when another Chumblie approaches. After several twists and turns, they end up in the exit tunnel. The Doctor makes it out, but Vicki slows down and turns to see if they've lost the Chumblies following them. A metal grille closes and Vicki is trapped in the passageway, with Chumblies heading toward her.

The Doctor tries the grille but cannot move it. He then hits on an idea, turning his attention to the oxygen converter built into the stone enclosure. It is used to convert oxygen into ammonia, which is the substance they smelled inside the passageway and is obviously what the Rills need to breathe. If the Doctor can sabotage this, he may be able to force the creatures to release Vicki.

However, the Chumblies reach her and gesture for her to follow them. With no other choice, she must return with them. The Doctor tells her to be brave and he will continue with his work on the converter.

Back at the Drahvin spaceship, one of the clone soldiers announces it is time for patrol. However, Maaga decides not to patrol due to the change in circumstances with the Doctor and his friends. She must then struggle to make the change in her orders understood, her soldiers being only very simple killing machines and unable to cope with the strategies required to escape their present situation. No surveillance of the Doctor and Vicki is necessary, as they will do what they are told in order to save Steven. The warriors do not understand this friendship response, but Maaga is aware of it and will use it to achieve their escape from this planet.

Maaga is clearly contemptuous of these unthinking clones and resentful that she has been saddled with them on this mission. Speaking slowly and clearly, she sends one of the clone warriors to rest, one on patrol and the third on guard over Steven. She then heads off to perform other tasks. Steven, feigning sleep, listens intently, hoping for a way out as his guard sits nearby, staring stonily ahead.

Vicki is led back to the central chamber at the end of the passageway. She nervously watches the viewport, which is now closed. Two Chumblies chatter mechanically with each other, then one begins nudging her leg. She shouts at it to stop, which it does. Then the robot begins to repeat her words back in a speeded-up metallic voice. Shortly, the voice settles down and repeats her words in a more cultured voice. Then it begins speaking in its own words, which Vicki can understand. While it does so, the viewport reopens, exposing again the strange bulbous eyes.

The robot asks for information. Vicki admits that the Drahvins have sent her and the Doctor to capture their ship, but that they were forced to do so. She is told that the Rills have offered to take the Drahvins away with them but have refused as they prefer killing, especially the Rills. She protests that the Rills killed a Drahvin, but the voice refutes that story.

Disconcerted by this method of conversation, Vicki asks whether she is speaking to a Rill or a Chumblie. The speaker seems amused at Vicki's name for the wobbly robots and adopts it to ease her fear. The voice tells her that it is a Rill speaking to her through the machine. They do not have vocal cords and communicate with one another through means of thought waves. The Chumblie is translating those thought waves into a language Vicki can understand. Vicki asks why she is not allowed to see the entirety of a Rill, and the response is clear: their appearance would shock Vicki, just as it did the Drahvins.

Unaware of Vicki's breakthrough with the Rills, the Doctor continues with his sabotage.

The Rill tells Vicki a much different story of how they came to crash on this planet. They encountered the Drahvin ship in orbit around the planet, but they would not respond to communication. Afraid of being attacked, the two ships hung in orbit for 4 dawns until the Rills decided to attempt to leave. When the Rills turned to go, the Drahvins attacked them. The Rills fired back and both ships crashed. Using portable ammonia supplies, they went out to offer aid to the Drahvins shortly after the crash. They came across a wounded Drahvin and tried to help, but Maaga appeared and forced them off with gunfire. The Rills could have easily killed her, but they are peaceful creatures and would not do so. Watching as they retreated, the Rills witnessed Maaga kill the injured warrior herself.

The Rill tells Vicki that they are still willing to help the Drahvins if an agreement can be reached. Vicki is convinced of the Rills' sincerity, but wishes she could get a good look at one of them. They cannot come out of the ship as they need ammonia to breathe. This comment reminds Vicki of the Doctor's plan and she races off to stop the Doctor from sabotaging their atmosphere converter.

At the Drahvin ship, Steven watches as his guard dozes off. He waits a moment longer and then reaches for her gun. He grabs the gun and her, but the force of his move causes them both to topple over. She strikes her head on the floor and is knocked unconscious. Steven takes the gun and heads for the door.

Maaga enters just as he reaches the outer door. He holds his gun on them and closes the inner door. Opening the outer door, he exits quickly, only to find a Chumblie waiting for him. Faced with a choice, he returns to the ship. Through the porthole in the inner door, he sees Maaga watching him.

Vicki reaches the end of the passageway and stops the Doctor just in time. Vicki tells him that the Rills are friendly. When two Chumblies arrive behind her and speak, the Doctor agrees to follow them back to the Rill spaceship. One of the robots stays behind to repair the damage the Doctor has done. As the Doctor and Vicki head back down the passageway, he wonders about the Rills' appearance, but they tell him through the Chumblie that they are afraid their appearance will be unpleasant to them so they prefer not to show themselves.

They reach the central chamber and the Doctor is quick to begin asking questions. He learns that the drill rig outside is to help the Rills find power to launch their spaceship. The suns here are too weak to power their solar batteries. The Doctor informs them that they have only 2 dawns time until the planet explodes, and the Rills are despondent. They have found only a natural gas supply in the ground and they cannot convert it to usable fuel in that short space of time.

The Doctor seems convinced of their sincerity, and offers to help them escape by providing power from the TARDIS. The Rills set about providing a cable and conversion specifications as the Doctor directs.

However, they are interrupted when the Chumblie stops relaying the Rill voice and starts its mechanical chittering again. The Rills inform the Doctor that the Chumblie placed at the Drahvin ship has detected Steven's attempted escape. But before it could contacts Steven, he went back inside the ship. But the Doctor is convinced that Steven can take care of himself and gets back to work.

Back at the ship, Maaga seals the doors and orders Steven to surrender. Trapped between the Drahvins and the Chumblie, he decides to stay where he is, refusing to surrender. However, Maaga informs him that he is in the ship's airlock and that she will suffocate him if he does not give up. When he does not respond, she begins depressurizing the airlock.

The Doctor checks over the cable provided by the Rills. It is long enough and of the right strength to handle the energy transfer. He and Vicki start back to the TARDIS when another message comes in from the Chumblie watching the Drahvin ship. It has detected what seem to be Steven's cries of distress, trapped inside the airlock. The Doctor and Vicki, along with a Chumblie, race to the Drahvin ship.

On the way, they encounter a Drahvin warrior on patrol. Losing precious time, they try to convince her that they have captured the Chumblie and it is now under their command. When she expresses distrust, Vicki simply grabs her gun and they all head back to the ship together.

The warrior Drahvins are pleased with Steven's near-collapse. They will be glad to see him dead. Maaga realizes that if he should die, they would lose their bargaining power over the Doctor. But before she can act, the Chumblie outside the ship launches a small projectile weapon into the Drahvin ship. It pierces the hull and lands inside, filling the cabin with a choking cloud of ammonia.

The Doctor and Vicki arrive with their Chumblie, which uses its powerful light ray to cut through the outer airlock door. With a hiss of air, Steven tumbles out. He recoils at the sight of the Chumblie but is too weak to move. The Doctor tends to him.

Maaga leads her warriors out of the ship, guns at the ready. However, a Chumblie keeps its light ray trained on them and they hold back. The Rills urge the Doctor to return to their ship, telling Maaga that they will not hesitate to fire if their friends are threatened in any way. As the Doctor and his companions hasten off, the Chumblie forces the Drahvins to return to the ship. When they do so, it remains outside on guard.

Maaga severely chastises the patrol warrior, but does not kill her as she needs all the help she can get. She quickly hatches a plan. It is evening now and soon it will be night, the last night of this planet's existence. She orders one of her warriors to sneak out of the ship via the forward hatch as soon as night falls and sneak up behind the Chumblie to try and destroy it.

Back at the Rill ship, Steven catches his breath whilst the Doctor gets on with the work of transferring power. He speculates they have only 6 hours left before the planet explodes. The Rills have finished the conversion calculation and the Doctor takes one end of the power cable. He asks Steven to stay behind, both to rest and to monitor the connections on this end.

Steven watches warily as the Chumblies go about the business of packing up to take off. The Rills sense immediately that he does not entirely trust them, even though they rescued him from the Drahvins. He is particularly worried that the power transfer might take more time than the planet has left. He asks them point blank if they would allow him and his friends to leave if they could not help in time. The Rill explains that the Doctor's life is valuable to them, even more so than their own. They would do all they could to see that the TARDIS left the planet before it was too late. Steven questions them as to their sincerity in offering to help the Drahvins, and the answer is the same. Soon he is convinced of their kindness and wisdom, and he sets to work helping the Chumblies make the power connection to the ship.

Some time later, night has fallen. A warrior Drahvin sneaks out of the ship armed with a heavy metal bar. As Maaga and the others wait, a tremor shakes the planet. It is the first sign of the coming disintegration. The warrior circles round behind the robot and brings the metal bar crashing down on it and putting it permanently out of action.

The Doctor and Vicki reach the TARDIS, trailing the power cable behind them. The Doctor takes a moment to muse on the death of a planet before beginning to hook up the cable. When he is finished, he returns to the Rill ship. Power begins flowing, but it will still take 3 to 4 hours for full power to be achieved. Hopefully it will be completed before the planet explodes.

Their work is interrupted by an alarm. The Rills have detected the destruction of the Chumblie outside the Drahvin ship. The Doctor is initially worried about the TARDIS, but the Rills assure him that the Chumblie on guard there cannot be taken by surprise. Besides that, it is the Rill ship that they really want.

Maaga leads the way to the Rill ship, her warriors close behind. They reach it shortly and engage in a gunfight with the Chumblies. One of the Drahvins manages to get through the robot perimeter and into the passageway. She reaches the central chamber and draws her gun on the Doctor, Steven, and Vicki. The Doctor, standing near the doorway to the Rill ship, uses it as an escape route. A Chumblie comes up behind the Drahvin and paralyses her. Steven and Vicki locate the Doctor and he beckons them to join him inside.

Outside, Maaga and the two remaining warriors hide out after retreating from the robots. They are safe for the moment, but they were unable to destroy a single Chumblie. Worse, more are on the way.

The Doctor, Vicki and Steven after finally get a full look at the "hideous" Rills. Although they are not humanoid and quite shocking at first, there still seems to be no reason why the Drahvins should hate them at first sight. They are glad of the opportunity to see the Rills fully, and to enjoy the intelligence and friendship of the race. Even though they are contained behind a glass partition, the ammonia atmosphere still seeps out and quickly affects Vicki. They exchange fond goodbyes and leave the interior of the ship.

The Doctor takes a moment to check the power transfer. Only an hour and half left before the planet explodes and slightly less than that for the Rills to gather all the power they need.

Outside, the Chumblies have the Drahvins pinned down. Their guns are proving useless on the robots. The two warriors, faced with the failure of their weapons, abandon Maaga and plan to resort to brute force. The terror of their impending demise spurs their actions beyond reason.

In the central chamber, a signal sounds, announcing that the Rill ship has achieved full power for takeoff. Once in space, they can recharge using solar power. The Rills say another fond goodbye to the travellers. Despite the physical differences between them, they are very much alike. This brief encounter, they say, has been an honour. The travellers hurry off for the TARDIS with a Chumblie for protection. They have less than half and hour left.

Maaga finds her warriors at the end of a battle. They have destroyed several of the Chumblie robots using metal bars, but the others have fled...into the Rill ship. They are too late.

The Drahvins and the time travellers watch as the Rill ship blasts off with a roar. The Doctor and his friends run for the TARDIS. However, Maaga spots them and the Drahvins hurry after.

Along the way, Vicki stumbles on the rocky terrain and hurts her ankle, but they all reach the ship safely. The Chumblie left to aid them engages the Drahvins and holds them off long enough for the TARDIS to dematerialize.

Then the planet explodes, and the Drahvins with it.

The Doctor tends to Vicki's injury, wishing they could all just stop somewhere for a time and have a good rest, but the whole universe still lays before them. Vicki sees a planet on the scanner and wonders what is happening there.