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.
The TARDIS topples and falls off the ledge...