The TARDIS materializes in a cave, quite dark and featureless. Elsewhere, on the same planet, its arrival has been marked by a sensor inside a wrecked spaceship. A steady bleeping noise indicates a landing nearby and finally draws the attention of a young girl who has been busy outside the ship. She reacts with delight, barely giving the locator grid a glance as she rushes to the rear of the ship, calling out for her companion, Bennett. She bursts into his cabin and breathlessly announces that the rescue ship has landed and they'll be taken home. Bennett is an older man, apparently a quasi-invaled, with a dark look and a dour demeanour. He promptly squashes the girl's enthusiasm by telling her it cannot be the rescue ship. It should still be 3 days away and cannot even reach this planet - Dido - without a homing signal sent from the surface.

But the girl, Vicki by name, refuses to believe him. The sensor contact is plain as day. She offers to help him up to go and see, but he will not go. He does relent in his negativity somewhat, asking her to radio the rescue ship and ascertain its location. Then she will see her mistake. Still not quite believing him, Vicki turns to go, but stops when Bennett tells her solemnly to "watch out for Koquillion". Vicki's face becomes a mask of fear at mention of the name. Bennett tells her that even though she hasn't seen him yet today, he'll be about and she mustn't say anything about the rescue ship. If he ever finds out, he'll kill them both. She nods gravely and goes off to the main cabin.

Vicki races to the radio and contacts the rescue ship, her hope still alive. But she is crestfallen to learn that they are still 69 flight hours away. They will not even call in for the homing beacon for another 17 hours. Yet the sensor still registers. The girl's hope turns to desperation, but the ship's captain breaks the connection in the midst of her question. She is alone again, and left to wonder who - or what - has landed on the mountain.

On board the TARDIS, a worried Barbara drags Ian over to the console. She's sure she has detected the familiar change in vibrations that signals materialisation. Sure enough, the time rotor darkens and slows to a stop. It appears that they have landed. In the background, the Doctor sleeps soundly in a chair. Ian is quite concerned - the Doctor has never done this before. They hurry to wake him. The Doctor seems refreshed for his nap but still a bit groggy. It takes some effort on the part of his companions to make him realize he's slept through the landing. The Doctor is not overly concerned but does hurry over to turn off the power. He does try to remind himself never to do such a thing again.

The Doctor goes about checking the instruments. The air and temperature are good, wherever they've arrived. The scanner shows only bare rock walls and endless gloom. It is a cave or perhaps a cavern underground. Still concerned by the Doctor's distractedness, Barbara is worried about whether the ship can take off again from such a place. The Doctor assures her they can, then suggests a quick look round outside. He calls to Susan to open the door, as is his custom, suddenly realizing that she is no longer with them. A look of sadness creeps onto the old man's face. Barbara asks if she can learn to open the doors. After a moment's distraction, the Doctor points her to the proper switch. The doors open and they all go outside, Ian and Barbara exchanging a look of concern.

They emerge into the cave, lit mainly by the intermittent police box lamp. Barbara detects a strange odour, which is unlike anything on Earth. The Doctor agrees, but says it smells quite familiar to him. Suddenly, he is off back into the ship, leaving Ian and Barbara to explore alone. He says he's going to have another nap. Ian and Barbara are stunned by this apparent lack of curiosity, as well as his sleeping through the landing. It is clear he will miss Susan for a long time to come. Barbara thinks they may see a new side to the Doctor, but Ian thinks he may be heading round the twist. Here the Doctor interrupts them, having heard all that they said. He asks Ian to hand him a nearby stone and then goes back inside without another word. Barbara thinks it's all very clear: the Doctor is clearly grieving Susan's loss. It has not been very long at all since she's been gone. He cannot be expected to move on so easily. Ian agrees, wondering what Susan's doing right now. Barbara surmises that her new beau David is teaching her how to milk cows; one step in the process of rebuilding Earth after the Daleks' invasion. Then they are off. Barbara has noticed some faint light coming in from a side gallery and they decide to follow it.

The two fail to see the strange creature which has come out of the darkness behind the TARDIS. It is bipedal, but its hand and feet are covered with spikes like a porcupine. Its head contains more of the spikes and two cylindrical eyes. It waits until Ian and Barbara are gone, then steps forward to examine the TARDIS.

Ian and Barbara emerge onto a ledge in full sunlight. They are on the side of a mountain overlooking the valley below. There they see the crashed spaceship, broken in two in the centre. They are excited to see a Union Jack painted on its side; this is a ship from Earth. Nearby the ship is a small cluster of buildings, burnt by the look of it, perhaps due to the crash. But there are no beings of any kind visible, human or otherwise. Ian debates whether to tell the Doctor about the spaceship, thinking they should look for any survivors, but Barbara cuts him off when she hears movement behind them. The creature from the cave has followed them out.

The creature speaks to them - imperious and commanding. It demands to know if the strangers are from Earth. When they tell him, the creature inquires as to the whereabouts of their ship. He seems hard pressed to believe that the box in the cave is a spaceship, but also seems somewhat relieved. Ian lets slip that there is one more crew member and then is forced to go and fetch him from the TARDIS. Once Ian has gone back into the cave, the creature blocks Barbara's escape, trapping her out here. The creatures brandishes a strange jewelled club at her and she backs away, frightened. He tells her she can trust him, then proceeds to force her off the cliff!

Meanwhile, the Doctor has been running tests on the rock sample from the cave. He is very pleased with the results - he knows this is the planet Dido, home to a friendly race of people that he visited many years ago. He wonders if he can fool Ian into thinking the return trip was deliberate, but then remembers he was asleep on landing. What a pity.

However, things have perhaps changed on Dido as one of the native creatures steps back from the ledge he has just pushed Barbara over and then brandishes his club again. A whine of power emanates from it and the exit to the cave is sealed with a gigantic explosion and rockfall. Ian, who had been trying to get back out to Barbara, barely managed to escape being buried in the fall, but is stunned when hit by a rock and collapses. The Doctor hears the explosion and hurries out of the ship, but he can see nothing in the dust and the now-absolute gloom. He hurries back inside the ship to fetch a torch.

Meanwhile, Barbara lies on the sandy ground at the foot of the cliff, a tree branch covering her. She does not even move as a long shadow falls over her.

The Doctor emerges again with a powerful torch and joins Ian just as he's struggling to stand. He is stunned and choking on the dust but is otherwise unharmed. Ian can only gasp out Barbara's name and a portion of what happened. They go toward the exit, only to find the entire passage blocked by immovable rock. Ian tells the Doctor this was no accident and starts to describe the creature they saw. But the Doctor is far ahead of him, explaining that he's been here before. He tells Ian the people here are friendly, but Ian certainly does not agree. Barbara's scream was proof enough for him, whatever's happened to her. Ian describes the jewelled club "weapon" the creature carried, but the Doctor describes it as a construction tool he saw on his last visit. And then they turn their attention to getting out of the cave, but they must find another route. The Doctor checks Ian over for any broken bones and finds he is OK, mentioning in passing he's sorry he "never got that degree". But his joke hides great concern. What can have happened here to turn such peaceful, friendly people into killers?

On board the spaceship, Vicki is agitated, working quickly to smooth out the covers on her bunk. She finishes, but then jumps when she hears a noise outside. She snatches up a pile of stones and sits down with them at the table. Just as she does so, the door bursts open and the creature from the mountain barges in. It is the dreaded Koquillion, and Vicki physically shrinks from his mere presence. Koquillion forces her to stand up and then interrogates her. Why was she outside? There are indications she was dragging something - what was it? Vicki explains that she was collecting stones in a bag. They are very beautiful, but quite heavy. Koquillion believes this, but knocks the stones from her hand to make a point - from now on she will go no further than 50 yards from the ship. He reminds her that she and Bennett owe their lives to him - only he can save them from his people.

But as he turns to go and talk to Bennett, Vicki plucks up her courage and asks him about the explosion she heard on the mountain. Koquillion explains that a ship landed, full of warlike people who wanted to pillage her ship. He saved her by killing all of them. Vicki actually has the nerve to accuse him of lying, until Koquillion threatens her with the jewelled club. He reminds her sternly that she is alive only because of him. She should be grateful for his protection. Then the creature stalks off to Bennett's room. Vicki hears Bennett's voice as he tries to keep Koquillion out, but the creature enters his room and closes the door. There are no more sounds after that.

Sure she is alone now, Vicki hurries back over to her bunk and pulls the covers back to reveal Barbara - alive, conscious, and unhurt. Vicki is still concerned that the evil Koquillion will return and so speaks in a nervous whisper. She and Barbara make swift introductions. Vicki tells her that Koquillion just keeps them here in the ship - perpetual prisoners. She explains about the impending rescue, but still fears that Koquillion will find out and stop them leaving. At Barbara's prompting, she goes on to talk about the ship's first arrival here; how the humans made contact with the natives. Everyone was invited to a grand meeting in the city, except she was ill and could not go. She woke up to hear a tremendous explosion which destroyed the city and everyone inside. Only Bennett survived, but he was unable to walk. He told her that the natives engineered the explosion. Vicki subsides into tears, but Barbara presses on gently. Why if the Didionians killed the others are she and Bennett kept alive? Vicki doesn't know the answer. All she knows is that she is alone now, her father having been killed in the explosion as well. She realizes too that Barbara is also alone - repeating Koquillion's assertion that he killed her companions on the mountain.

However, the Doctor and Ian are still very much alive. They have found a path leading away from the TARDIS that seems promising. As they stop to rest, the Doctor continues to muse about this seeming change in the ways of the natives. Ian thinks things could have changed quite easily with a new leader or some extraordinary circumstances, but the Doctor knows better. When he was here before, the entire population amounted to barely 100 people - all the more reason to renounce violence and embrace peaceful coexistence. No, something is very wrong here. With that, they continue on.

Later, Vicki is more relaxed, figuring that Koquillion has left the ship in secret as he usually does. She tends to Barbara's cuts and bruises - sustained when she caught hold of a tree branch to break her fall. Barbara continues to ask questions. Vicki explains that Koquillion is the only native she has ever seen, even though they "live nearby". Vicki shows Barbara the flare gun she plans to use when the rescue ship arrives. However, her mood darkens again when she realizes that Koquillion could still do something to prevent her rescue. Then she gets agitated when she sees what she thinks is pity in Barbara's eyes. She doesn't need sympathy - no matter what happens, she is strong enough to survive quite well, thank you very much. And then her mood changes again, this time to panic when she hears noises from the rear of the ship. She barely has time to hide Barbara again before the inner door opens. Thank goodness it's only Bennett.

Bennett holds himself up in the doorway, looking worn and haggard. Koquillion tortured him to get information, but he did not give away the impending arrival of the rescue ship. He tells Vicki that another ship did land on the mountain but that Koquillion killed everyone aboard. Vicki, however, is pleased to show him he's wrong. She pulls the covers back and reveals Barbara. Bennett looks as if he's seen a ghost.

In the cave, the Doctor and Ian continue to follow the path; however, it's become a ledge only, hugging one side of the cave wall. Below is a steep drop into the darkness. Worse yet, the ledge is beginning to narrow out. The Doctor stumbles and Ian catches him. They must move very carefully, keeping pressed against the rockface. Suddenly, a growling sound reaches them from below. Shining the torch down, they catch a glimpse of a ground-dwelling creature with luminous green eyes and large teeth. It waits just below them. While repulsed by it, Ian is also heartened by the presence of its eyes. This means it is used to light and must live outside. They should be near the exit at last. The Doctor agrees, but encourages him to stop musing and move along.

Looking up, Ian spots a series of ornately-carved handholds along the wall - the face of a creature with a ring in its mouth. They make traversing the narrow ledge much easier. However, the third one in the series is loose, the ring coming out in Ian's hand as he grabs it. He makes it to a larger section of the ledge, but an ominous grinding sound begins. The Doctor is horrified to find oil on the loose ring - it is a booby trap meant to be pulled out by an unwitting passerby.

The Doctor works at the trigger mechanism, trying to reverse the spikes, but he's moving too slowly. Changing tack, he instructs Ian to remove his coat and throw it over the nearest wall of spikes, using it to cover them and make them safe. Ian realizes what he's getting at and finishes the rest - holding onto the covered spikes and using them to swing himself back around. The Doctor helps him to safety, leaving the creature below sounding somewhat disappointed. The Doctor is even more worried, knowing that this sort of trap is regunant to the natives he knows, but he puts this aside to try and retract the spikes. Ian helps out. Both are still worried about finding Barbara.

On the spaceship, Barbara has helped Vicki care for Bennett. He passed out after he saw Barbara the first time. The women think that this was due to the exertion of dragging himself out of bed and down to the main cabin. When he comes to, Bennett acts as if nothing has happened, telling Barbara instead that Koquillion has killed her friends. However, Barbara refuses to believe this. Koquillion didn't manage to kill her, so she believes her friends are still alive. What's more, she is already thinking of a way to surprise and capture Koquillion the next time he shows up. Vicki supports this, but Bennett is dead set against it. The rescue ship is due to arrive any day. Fail in this plan and Koquillion will kill them all; succeed and his people will come looking for him. They must sit tight until the rescue ship arrives. Vicki reluctantly agrees and an agitated Bennett orders them to help him back to his room. He tires to exact a promise from Barbara to do as he says, but she doesn't exactly agree. Angry, Bennett finishes the trip to his room by himself. Barbara is concerned over Bennett's lack of fighting spirit but allows herself to be led back to the main cabin by Vicki. Together, they prepare supper.

Meanwhile, the Doctor has succeeded in releasing the trigger and the spikes slowly retract into the cave wall. He and Ian continue on, hoping there are no more surprises like that along the way. Going along with them is the green-eyed creature from below. Losing interest in the activity above it, the creature heads toward the cave opening at ground level. Nearby, Vicki collects water for dinner.

Ian, too, spots the sunlit exit to the cave and points it out to the Doctor. However, he is more interested in a metal door set into the rock wall. It has been put there purposely, but they do not have time to investigate now. Ian hurries him along, noting the narrow ledge in front of the door, although the Doctor is more concerned that someone may come out of the doorway to follow them.

Vicki returns from gathering water, carrying two heavy containers. She stops to rest next to the outlet to the cave, not seeing the hideous creature lurking just inside. Barbara, however, can see into the opening from a porthole in the ship. She sees the creature watching Vicki intently and spring into action. She grabs the flare gun, the only weapon she can find, and runs out the door. She calls to Vicki to look out and then fires, completely ignoring Vicki's call for her to stop. The flare hits the target and the creature is killed, writhing and screeching in pain.

Hearing the noise and activity close by, the Doctor and Ian hurry their pace.

Barbara drags a protesting Vicki back into the spaceship, certain she has saved the girl's life. But she learns instead that the creature whom Vicki had named "Sandy" was a docile plant-eater that she had treated as a pet. Sandy was only coming to Vicki for food. Barbara is abject in her regret, but she swears she could not have known the truth. All she saw was the creature's giant jaws as it approached Vicki. Vicki collapses in sobs at the death of her pet.

Barbara tries to comfort the girl but does not know what to say. She turns in panic at a noise from outside but is overjoyed to see it is Ian and the Doctor. All are pleased to see the others unharmed. Vicki raises her tear-stained face to watch the reunion, unsure what to make of the newcomers.

Out in the cave, the door that the Doctor noted opens up and Koquillion emerges. He sees the footprints left by Ian and the Doctor and follows them out. He knows they have gone to the spaceship.

Later, Vicki has calmed down and all 4 are seated around the table. All have been apprised of the situation. The Doctor and Ian do their best to cheer Vicki up, but Vicki gets agitated again when the Doctor wishes to talk to Bennett. She is tired of explaining that they must do what Koquillion says or risk being killed - or stuck here forever. Barbara tries again to get through to her but only succeeds in making the girl angrier. She lashes out, accusing them all of meddling in something they don't understand. Everything was under control until they arrived and she wishes they'd all go away!

In the awkward silence that follows this outburst, the Doctor works out what to do. He sends Ian and Barbara outside and beckons Vicki to him. She's much calmer now and agrees as he talks that she really doesn't want them to go away. The Doctor assures her that they are only here to help her, not jeopardize her rescue. Vicki goes off on a tangent, explaining that when they return to Earth, Bennett is going to make certain Koquillion and all his people are wiped out for what they did. The Doctor says he agrees that Koquillion should be punished, but thinks that there is another solution to the problem. Vicki agrees that he may be right and cheers up tremendously. The Doctor must go and speak with Bennett, promising the girl that he will listen carefully to everything Bennett has to say. She shows the Doctor the way back to Bennett's cabin but hesitates before returning to the main cabin. Still angry at the death of her pet, she doesn't want to be in the same room as Barbara. The Doctor patiently explains that Barbara was afraid for Vicki's life and did a very brave thing to try and save her. Vicki mustn't be angry with her for that. Reluctantly she agrees and goes back to the main cabin.

The Doctor then turns his attention to Bennett's cabin. He knocks and door falls open a crack. Bennett's voice tells him he can't come in but says nothing more. When he cannot open the door on his own, the Doctor turns to find the remains of a metal support on the floor. He picks it up and begins to batter the door.

In the main cabin, Ian and Barbara return. There is an uneasy silence, finally broken when Vicki apologizes to Barbara. She, too, apologizes for her actions and all is forgiven. The conversation turns to origins and Vicki is confused. She assumed Ian and Barbara were from Earth like she was. Vicki explains that the year is 2493. She and her father left Earth after her mother died, heading to the planet Astra where her father was to take a new job. They crashed here on the way. Ian tells her they travelled through time with the Doctor to get here, leaving Earth in 1963. Vicki is taken aback, pointing out that Barbara must then be 550 years old! Ian finds this funny, but Barbara does not. Vicki refuses to believe their story as she knows there was no time travel in 1963. Humans, she says, "didn't know anything then." They tell her that the time machine belongs to the Doctor, who comes from a different time and planet altogether. He visited 1963 - thinking it was all that backward - and picked them up. Vicki is still unsure.

The Doctor finally manages to break into Bennett's room. As expected, he finds it empty. A quick search reveals a tape recording activated by the door with Bennett's voice saying, "You can't come in." There is also a listening device to monitor conversations in the main cabin. The Doctor hears Vicki saying how much she trusts him, even though he dresses and acts so strangely. Amused but not distracted, the Doctor turns the speaker off and continues looking around. He finds a control which activates a trap door in the floor - an easy way to get out of a locked room.

Some time later, Ian is impatient at the Doctor's prolonged absence. He goes back with the other to knock on the cabin door. But there is no response whatsoever. They go in to find the room empty. The trap door is closed, so the Doctor's friends can find no trace of his escape.

The trap door has led the Doctor back to the cave, directly to the metal door in the rock face which he and Ian passed on the way out. It is unlocked this time and he enters. He finds himself in a high-ceilinged chamber carved out of the very rock. It is gloomy but not in total darkness - light filters in from high above, obscured by the dust into a haze. The room contains carved rock pillars and what appears to be a meeting table and an altar. There is also a chest filled with robes of different colours. The Doctor seems to know what it is he's seeing.

Ian and Barbara are very concerned over the Doctor's disappearance, not to mention that of Bennett. Vicki wants to stay put, but the others opt to head for the TARDIS. They know the Doctor will make for there in the end and they hustle Vicki out with them.

Meanwhile, the Doctor has made himself at home in the stone chamber, sitting calmly at the table. He is casual when he invites in the shadowy figure at the door. He has been waiting. Koquillion slowly approaches. The Doctor casually informs him that this chamber used to be the People's Hall of Judgement. Quite fitting under the circumstances. He stands, addressing the creature before him as Mr. Bennett and telling him of the sacred ceremonial occasions for which the costume he is wearing was intended. Knowing he has been discovered, Bennett removes the mask and gloves to reveal himself to the Doctor. He is very angry.

Bennett, proving he can not only walk but talk as well, prowls the chamber while telling the Doctor the reason behind his elaborate plan. He killed a crewmember on board the ship and was arrested, but the ship crashed before the crime could be radioed to Earth. If he could get rid of the crew, his crime could be hidden forever. He arranged the explosion in the Didonian village, killing the entire group assembled and blaming it on the natives. The Doctor denounces the man as insane, but he takes no notice. Vicki knew nothing of the crime and would support his story; he invented "Koquillion" to convince her of how terrible the natives were. He is still certain he can get away with it - "Koquillion" just has to kill 3 more people.

The Doctor knows he must stop this madman and so grabs the jewelled club that Bennett had earlier placed on the table. But Bennett is ready for him and fights back. The club activates and brings down part of the ceiling. The Doctor is forced to let go and try to escape. He makes a valiant effort but Bennett catches him, grabbing him by the throat and squeezing hard. But he stops, startled, when he sees before him 2 men - Didonians - watching him from the shadows. Again he looks as if he's seen a ghost. He was certain he'd killed all of the natives in the explosion. But some have survived and look to take their revenge.

The 2 men - ordinary humanoids dressed in white - move toward him in calm silence. Bennett backs away, too scared to turn away from them. He cannot stop them and they slowly force him up and out the door. Bennett barely notices he has reached the rock ledge until it is gone from under him. A dazed Doctor watches as Bennett falls with a scream to his death below. Then the Doctor passes out, his last sight the 2 natives looking worriedly over him.

He awakes some time later in the TARDIS, Ian and Barbara watching over him now. He had been left beside the ship by the natives, where his friends found him. The Doctor starts to explain about Bennett but is still a bit groggy. He goes outside for some air and to talk to young Vicki, who waits in the cave. As he goes, Barbara too is thinking of Vicki. She thinks they should take her with them.

The Doctor tells Vicki the whole story as gently as he can. She takes it as well as could be expected, especially when she realizes it was Bennett who killed her father. She is now completely alone. The Doctor asks her to join him in his travels, promising adventures throughout time and space. He appeals to her sense of adventure but also promises her that she'll be amongst friends as well. While she seems interested, he leaves to give her some time to think about it.

Back inside, the Doctor quickly realizes that Ian and Barbara have come to the same conclusion about Vicki. They call her inside to see what she has decided. The girl is amazed upon entering, the tattered shell outside having given no indication of the fantastic size and shape inside. Once her amazement subsides, she addresses the question at hand. Yes, she would love to come with them, if they'll have her. That settled, the Doctor sets about dematerializing.

Onboard the spaceship, a message crackles over the R/T. It is the rescue ship calling in for the homing beacon. It cannot land on Dido without it. However, the only people to hear the message are the 2 natives from the cave. They enter the ship and disable the communications equipment, then leave. The rescue ship will never arrive.

The Doctor, for one, is glad to be gone from Dido. Some of the natives survived Bennett's genocidal actions and they have their planet back to themselves again. They can rebuild without alien interference. Ian is sure this is not the only reason the Doctor brought Vicki along. He admits there was another reason, but does not care to elaborate on his coping with the loss of Susan. Instead, he indicates an imminent landing. Perhaps they can have a rest this time, wherever they are.

Soon the ship materializes on a hillside amidst trees and singing birds, but it is not completely on the ledge and immediately starts to rock forward and back. Inside, Ian is first to feel the gentle rocking motion. But it quickly becomes more pronounced. Ian begs the Doctor to take off again, but it is too late. All the Doctor can do is shout at them to hang on as they pitch backward onto the floor.