The scene is a busy, noisy jungle full of exotic birds, lizards, mammals, and insects. Amid these familiar creatures there is a decidedly unfamiliar figure - an upright biped with shiny scale-like skin, a mop of shiny gold hair, and a single eye. The creature patrols the jungle like an overlord.

Nearby, the TARDIS materializes. Dodo is the first out, wearing a strange black/white harlequin costume. She lets out a mighty sneeze and then takes a good look around. Steven follows her out quickly, angry at her impetuousness. She bucks his authority and he tries to appeal to reason, alerting her to the dangers she might have found outside: no gravity, poison air. But she will have none of it and moves off to explore further.

Steven persists, following her for fear that she might get lost. But Dodo says if she gets lost, she'll simply take a bus home. She is sure that she has been here before: this is Whipsnade Zoo just outside London! Aghast at this clueless youngster, Steven tells her they are probably not even on Earth. Amused, Dodo mocks Steven, then shows him a chameleon, a monitor lizard, and a locust. She tells him there's nothing she doesn't know about nature, having learnt all about it at school, and these are all very definitely Earth creatures.

The Doctor steps out of the TARDIS and confirms that Dodo is right. This place is more likely Earth than anywhere else, given the readings, but there are also some very strange things registering on the instruments. He then returns to the ship to continue checking, watched at a distance by the strange one-eyed creature.

Elsewhere, in a large central chamber, a trial is being completed. A group of humans observe, along with a large group of the one-eyed bipeds. One of them acts as prosecution counsel as well, with a human counterpart. The Commander, sitting in judgement, finds the prisoner guilty of carelessness. His negligence could have caused an explosion which would have been fatal to humans and Monoids alike (these are the one-eyed bipeds). The Commander imposes the lightest sentence he can - miniaturization. The man will be reduced to microcell size for approximately 700 years, and returned to normal only after he can no longer be a danger. The mute Monoid prosecutor, through a series of hand signals, accepts this. His human counterpart conveys the acceptance to the Commander.

The Commander's own daughter, Mellium, pleads for clemency, arguing that it was simply a mistake that anyone could have made. Surely he won't do it again. But the Commander is pledged to protect the human race and must punish the man. His defense counsel accepts the punishment, thanking the Commander for his leniency. The prisoner is miniaturized and placed in a tray where he will be stored until he is returned to normal. Through hand signals and interpretation, the Monoid prosecutor thanks the Commander for the care that he takes of all of them.

Back in the jungle, the Doctor and company spot an elephant grazing nearby. The Doctor is disturbed. This is all most strange. The elephant was from India, but other animals here are from very different parts of the world. The geographical range is too diverse. Also, the Doctor points out, this world has no sky, just a metal roof radiating light. And the ground vibrates with a mechanical precision. The Doctor can only conclude that this is an indoor nature park, on the grandest scale he has ever seen.

Dodo sneezes again and the Doctor is quick to take her to task for not using her handkerchief and for the ridiculous clothes she has chosen. Dodo quickly becomes defensive again. Perturbed, he tells them they will all take a last look round then head back to the TARDIS. Nervously, Dodo asks if she is going to be sent home. The Doctor laughs, saying he couldn't take her home if he wanted to. At that, Dodo is happy again, really beginning to enjoy her travels. The entire group wanders off, again watched by a Monoid.

In the central chamber, another Monoid signals to Deputy Commander Zentos. He in turn relays to the Commander that three intruders have been spotted. The Commander is more curious than upset, although Zentos reacts angrily. On a scanner screen, they see the Doctor, Steven, and Dodo as well as the blue box that must be their spaceship. They appear to be human, but that seems impossible. Zentos wants them arrested immediately. The Commander, too, wants to question the strangers, but he tells Zentos that they are to be invited in, not arrested.

Back in the jungle, Dodo beckons Steven and the Doctor to see the "fab pictures" she has discovered: cave drawings of strange animals. The Doctor chides Dodo for her poor English, but he stops when he sees the drawings. One of them looks like a zebra with two heads. Steven thinks it a work of imagination, but the Doctor reasons that as this place is already illogical in its set up, why not a two-headed zebra? Steven points out ominously that the more they discover in this place, the less like Earth it becomes.

They cannot speculate further as a loud blaring noise fills the jungle, sounding like an alarm. The travellers decide to head for the TARDIS, but there is a group of Monoids surrounding the TARDIS. They spot a cave nearby and head for that, quickly realizing that if this is Earth, it is no longer inhabited by human beings.

They reach the cave and hide inside, taking care to be quiet. A sneeze comes upon Dodo as the Monoids near the cave and Steven barely manages to stifle it in time. A moment passes and so do the Monoids. Dodo sneezes mightily, drawing Steven's ire. The Doctor leads his companions out of the cave, hoping to sneak past the creatures and get to the TARDIS. However, one turn takes them to the edge of the jungle where they see the control area and get a good look at their surroundings. Triumphantly, the Doctor announces he knows where they are: on a spaceship. Steven and Dodo are incredulous at this, but they have no further time to speculate as a group of concealed Monoids rise up and surround them.

Steven is the first brought into the control centre, and he is bombarded with hostile questions from Zentos. Steven explains that they are travellers in time and space and that only the Doctor knows how the ship works. The Commander is sceptical, but more cordial than his deputy. He tells Steven that time travel experiments were undertaken in the 27th segment of time and were unsuccessful, and he asks how it is that they are able to travel in time. Again, Steven defers to the absent Doctor.

Steven then turns the questions on the Guardians and discovers that the Doctor was right: all of this -- the jungle and the city -- is indeed inside a spaceship. The Commander shows him readouts and indicators showing the immense size of the craft.

The Commander tells him that he and his people are humans, from Earth, and that the origins of the Monoids are obscure. They came to Earth many years ago from their own planet, which was dying. They have provided invaluable service to humanity and, in exchange, were allowed to come along on this voyage. They are heading for the distant planet Refusis II. The Earth, too, is dying, and they have left it for the last time. Soon it will be swallowed in the pull of the sun and burnt up.

Steven listens with incredulity, realizing that they must have travelled forward millions of years in the TARDIS. Zentos is sceptical of this, theorizing that they may have been sent by the Refusians to intercept them and to sabotage their mission. The Commander demands an explanation of this allegation. Zentos says that they only know the Refusians as intelligences. No one knows what they look like or how they are. Perhaps they have the ability to assume human form and plan to use this ability to mingle amongst them and cause havoc.

But the Doctor arrives and puts paid to the paranoid assertions, maintaining that he and his friends are the same as the humans in the ship, with all their shared imperfections. He points to Dodo as she sniffles as proof of this, but the humans on the ship do not recognize "the chills". The Doctor tells them it was a virus fever that used to be quite common. This draws a laugh from the Commander, who explains that the cold was eradicated so long ago, they had forgotten about it. To him, it is like history come to life. He leads the Doctor and his friends away, chatting about time travel. Zentos, however, is still not convinced. The Commander hears the Doctor's story and is amazed, realizing that the travellers come from the "first segment" of time. This is now the 57th segment, at least 10 million years in Earth's future.

Steven asks about the ship's own travels. The Commander says it will be nearly 700 years before they reach Refusis. It is the nearest planet that has the same features as Earth, but admits they know very little else of the planet and its inhabitants from their audio space research. The Doctor and his friends are amazed. No wonder the Guardians had to bring everything. Dodo likens it to Noah's Ark, but the humans are not familiar with that allusion. The Commander tells them that they have brought the entire population of Earth, both human and animal. These few humans are the Guardians, who run the ship and tend the plants and animals. The rest have been reduced to microcell size and are stored in trays to be returned to normal at the end of the journey. The Commander is thrilled with this chance to talk to the Doctor. He offers to show him the control deck while Steven and Dodo are shown to "the statue" by Mellium.

"The statue" is an incomplete representation of a human being. It was started at the beginning of the voyage and will be finished just before the landing on Refusis. It will take 700 years to sculpt by hand using "the ancient methods". Mellium says they and their descendents will watch it grow gradually. Mellium starts to explain its composition to Dodo, but is interrupted when an alarm blares all over the ship.

A Monoid is borne into central control, lying very still. Humans and Monoids gather round as Zentos relays their report. A strange disease-fever is spreading among the Monoids. Everyone looks to the Commander, but he too is looking very ill. He collapses, complaining that it is very hot in the control centre. The Doctor, Steven, and Dodo surround the Commander as Zentos and Mellium tend to him. The Commander has the same fever as the Monoids, and it was brought by the strangers. Zentos pushes them away from the Commander as the crowd murmurs.

Dodo doesn't understand the fuss. It's obvious that the Commander simply has a fever. He has caught her cold. But the Doctor knows it is more than that. This generation of humans have never experienced the common cold. It was wiped out long ago, and now they have no resistance to it. The Doctor says he should have foreseen this. It may be fatal, and he blames himself for letting it happen.

Zentos, standing nearby, overhears this and repeats it to the increasingly hostile crowd. It is the strangers' fault. When the ill Monoid dies before them, Zentos repeats his earlier accusations. These strangers are evil and have deliberately inflicted this disease. They are to be arrested and stand trial for their crimes...

The travellers are led away amid the taunts of Zentos and the crowd. Zentos tells the worried Mellium that her father - that all of them - may die. If so, their great undertaking in leaving the doomed Earth might have been pointless after all.

In the travellers' cell, Dodo is furious at herself, even as her cold begins to go away. The Doctor tells her she is not to blame. He blames himself for not realizing the potential severity of the problem. Steven wonders if such a thing has happened before in their travels, but the Doctor doesn't want to even think about it. Even the extent of this outbreak depends on the strength of the virus. Dodo is despondent, realizing how bad her cold was. The Doctor concentrates on action, telling Dodo to get some rest and recover completely. If only they could get out of this cell, he is sure he could help them find a cure.

Zentos discusses the situation with another Guardian, a man called Baccu. They monitor incoming reports all over the ship, watching as several other Monoids fall ill, including one in the jungle right in front of the TARDIS. The two men are thankful that no humans have yet died, but they know it is only a matter of time. If one does, it may be disastrous for the Guardians. Each person has a specific task to do, and no one had reckoned on a premature death. Zentos tells Baccu that their microvirologist is doing all he can, with little success. He is with the Commander now.

But the microvirologist is stumped. He examines the Commander and tells Mellium that he does not know how to proceed. There was a cure for the cold, but all the data on it was lost in the 10th segment of time. As he leaves, the Commander regains consciousness, asking to speak to his daughter. He tells Mellium that she must promise - if something should happen to him - that she and the others will do everything in their power to make sure the voyage continues. Nothing matters but the voyage and the eventual landing of their descendents on Refusis. She promises him this, but still hopes for his recovery somehow.

Later, Dodo awakes from her rest to the sound of primal drumming. She and the Doctor watch outside the cell window as a Monoid funeral procession passes, bearing the dead creature wrapped in a shroud. Humans are gathered respectfully on the control desk to watch. Zentos gives the ceremonial permission for space burial of the Monoid. The creature is borne to a launching bay and ejected into space.

The ceremony galvanizes Zentos into action. Before it is too late, the strangers must be made to pay for their crimes. The hearing will begin immediately, with Zentos presiding. Baccu will act as prosecutor, and Zentos calls for anyone who will speak for the travellers. A young man called Manyak comes forward, as does Mellium. Both believe their story, and Mellium knows that her father wants to see justice done.

The hearing begins without the prisoners in attendance. They watch on a screen as Zentos reads his mandate to try anyone who jeopardizes their journey. Dodo realizes that this is a trial and that things look bad already. Mellium appears over the screen, explaining that she and Manyak will represent them. However, one of them must appear on their behalf. Steven is quick to volunteer, saying that he is stifling in the cell. He hopes to convince them they are friendly and that they must free the Doctor to develop a cure for the disease. Mellium agrees and Steven is quickly fetched from their cell to the control centre.

Steven is placed in a cage on the control desk, where Baccu presents the assertion that he and his friends are agents of the Refusians who have deliberately inflicted this disease to stop their journey. Steven reacts angrily, saying they are human too and have no reason to harm the Guardians. Zentos dismisses their story of time travel as ridiculous and so they must have come from elsewhere. Steven attacks their closed-mindedness, insulting their lack of medical records and their so-called "advancements".

The hearing degenerates into a shouting match between Zentos and Steven. The Deputy Commander sticks with his assertion that they are Refusian agents, and Steven tells them they are only reacting in fear of the unknown and will likely end by ejecting them into space. Observing from his quarters, the Commander agrees, but he is too weak to act. Baccu asks Steven to prove the good faith he asserts. He asks if the Doctor would know how to cure the disease, and Steven is sure that he would given a chance to experiment. But Zentos, stirred up by his fear, says that the Doctor would only use the opportunity to spread the disease further and faster. He turns to the crowd and whips them into a frenzy against the travellers.

Dodo and the Doctor watch in concern as Steven sags weakly against the bars of his cage. The Doctor fears he, too, may have caught the fever.

Manyak barely manages to quiet the crowd and takes his one opportunity to speak. He tells the crowd that the Doctor and his friends have never denied bringing the disease, but they say it was an accident. If they were agents of Refusis, they certainly could have found some other way to kill the Guardians which would not have risked their exposure. But neither Zentos nor the crowd buy this argument. He then argues that the disease is here and they have no answer for it, no matter how hard they try. The Doctor may be the only one able to come up with a cure and that he and his friends should be spared. The crowd is then quiet, and this sees to be working... until Baccu relays the news that the first human has died from the fever. In his cell, the Doctor hangs his head in sadness.

Zentos incites the crowd again, and the travellers are swiftly sentenced to be ejected into space. At this, Steven collapses in his cage.

Steven is taken back to the cell and dumped on a bench. The Doctor demands fair play, saying that Steven is as ill as anyone else. How could he possibly defend himself. But Zentos appears on the screen and confirms the verdict, despite the Doctor's pleas that he can find a cure for the fever.

Zentos looks to Baccu and Manyak for support. Both agree that a verdict had been reached, but Manyak still believes that the Doctor is their only hope. Zentos prepares to give the expulsion order, but is stopped when the Commander's voice booms out over the speakers. Prompted by Mellium, he orders his deputy to release the travellers and give them full research facilities. Zentos protests, but the Commander issues the orders quickly before his consciousness slips again. The Doctor will be given a chance to find a cure, but he must use Steven as a guinea pig. The Commander then slips back into unconsciousness and Zentos resigns himself to their fate.

Quickly, the Doctor takes charge. He orders Mellium to warmly wrap first Steven then all of the other sick people on the ship. He gives Dodo a list of compounds and equipment to fetch from the TARDIS. Then he begins instructing Manyak and the Guardian scientists on the nature of the disease...

The humans are astonished at the Doctor's knowledge as he recreates the old vaccine which was lost so long ago. It is a natural compound derived from animal membranes. Samples are taken from all of the animals in the jungle to provide the right material. Baccu supervises the collection.

Later, the Doctor has completed his researches and believes he has found the cure. With a Monoid assistant, he mixes the final batch to try on Steven. He is very pleased with the help of the clever Monoid. Together, they create a medicated patch which the Doctor applies to Steven's arm. Dodo marvels at the absence of hypodermic needles, but the Doctor knows those were superceded long ago. Now they must wait and watch. The Doctor heads off to administer the medicine to the Commander, knowing that despite his agreement, he must act now before anyone else dies. This he does, but the waiting is the hardest part on all of the Guardians. Zentos simply waits for the Doctor to fail.

However, the cure has worked. After an hour or so, Steven's fever peaks and breaks. The Doctor is jubilant, and Dodo rushes into the control centre to announce the news. A large crowd of humans and Monoids hear this and rejoice.

A short while later, the Doctor enters the control centre and tells Zentos that all those who were ill have received the medication, and the rest have been immunized. Zentos admits his wrong as Dodo returns and announces the Commander is on the mend as well.

However, the happiness in the ship is crushed when a technician turns all eyes to the long-range scanner. All gathered must watch helplessly as the Earth plunges into the sun and burns up. There is no going back now; the journey to Refusis must continue.

When Steven is up and around, the travellers say their farewells. The Doctor points out the statue, saying they can now get back to work on it, finishing it and the journey in 700 years time. Zentos extends sincere apologies for having misjudged the travellers. The Doctor tells him that he and his people must travel in understanding as well as hope. Then he is off.

The Doctor, Steven, and Dodo head further into the ship, arriving at the control centre. Along the way, they find the place dirty, unkempt, and seemingly disused. There is no sign of the Guardians. The Doctor reaches the control panel and sees that some form of automatic guidance has been introduced to the controls since their last visit, but even so surely someone should be standing by to make sure nothing goes wrong.

Via the internal scanner, the Doctor finally locates a couple of Guardians elsewhere in the ship. However, they all appear to be servants. In one room, a human serves a drink to someone unseen. In another room, which looks like a kitchen, a human makes a mistake and is shot at... by a Monoid!

The Doctor is shocked - the Monoids have become overlords and the humans slaves. But they cannot act as they are shortly arrested by a party of Monoids and their human servants. The Doctor protests that they have been here before and are friends, but the Monoid does not believe him, and speaks out loud to tell him so! The Doctor points out that much has changed since their last visit, including the Monoids' ability to speak. The Monoid, known as "2", tells him that they are now the masters of this ship following the recent revolution, getting confirmation of this from one of the cowed humans. The Doctor and his friends will obey just as they do. They are then taken to the Monoid leader.

In another part of the ship, the Monoid leader, known as "1", sits on a throne attended by humans. As the travellers are brought in, he is watching a recording of their departure in the TARDIS 700 years ago. He scoffs at the Guardians' disbelief of time travel and asks why the Doctor has returned. Steven explains that they cannot control the ship, at which "1" derides them. He explains that history says they brought a strange fever which killed many Monoids. Dodo says that they also cured it, but "1" says that it was only a temporary cessation of the fever. Another strain of it broke out later, which sapped the will of the humans. But that was only a part of the reason for the takeover. The Guardians, a "simple people", encouraged the research which led to the development of Monoid voice boxes and heat prod weapons, leading to the revolution for which the Guardians were unprepared. Many humans were killed, and the rest were enslaved.

"1" tells "2" to take the travellers to the security kitchen, then summon a Grand Council. As they leave, he taunts his servant, Maharis, warning him not to get any ideas of freedom or rebellion just because the travellers have returned. Maharis bows subserviently - he is content to obey.

In the security kitchen, rumours of the travellers' return has already reached the humans there. Two people, Manissa and Dassuk, talk about them. Manissa is excited about the possibility, believing that it is the Doctor and his friends as their legend says, but Dassuk is pessimistic, saying that even if it is them they still can't regain control of the Ark.

The two hurry back to work as the doors open and the Doctor and company are ushered in. "2" tells them that they will stay here and prepare food for the Monoids, as well as sleep here like all the others. And they must always obey. As "2" leaves, Manissa and Dassuk approach the newcomers, confirming that they are indeed the travellers of legend. They are full of questions, but Steven urgently cuts them off. It is far more important that they find a way to get out of here.

Back at the control centre, "1" convenes his Grand Council to announce that the landing on Refusis is imminent. There, they will create a new Monoid world, one which will have no reference to the time when Monoids were secondary beings. "1" announces he has a simple plan to destroy them, for which he rallies the support of the others. "2" is concerned about the landing on Refusis, saying that they cannot be sure of the planet or its inhabitants. "1" says he has devised a forward landing party which will deceive both the humans and the Refusians...

In the kitchen, the slaves continue their work while the Doctor and company try to rally them to rebel. Dodo finishes explaining that she was the first one to call the ship the Ark, and Dassuk confirms that they cured the strange fever they themselves brought to the Ark. Shortly, the last doubter has been convinced. The Doctor stresses they must act quickly, but Dassuk fears that there are too few humans to do anything. Dodo suggests rising up against them as the Monoids are rather slow creatures, but Dassuk argues that they are armed and the humans are not. Steven says they must take a chance, try to wrest a heat prod from one of them first so they have a weapon too. The Doctor again stresses the urgency of action.

In the control centre, "1" has finished outlining his plan for the landing party. He sends a group of Monoids off to prepare the launcher, and he sends "2" to fetch Dodo and the Doctor.

Before "2" can reach the kitchen, however, another Monoid enters to harass the humans. But they have been so cowed that they don't react fast enough. One is nearly shot until another person causes a distraction. Then Steven and others attack the Monoid, nearly wresting the heat prod from him. But "2" arrives and kills one of them. He calls the attempt reckless and stupid, then fetches Dodo and the Doctor. Steven tries to go with them but is stopped. The other two will be making the first landing on Refusis, but he will be kept her as security for their conduct.

The Doctor and Dodo join "2" and Yendom, another human, on board the launcher as it is set on course for Refusis. The trip is short and the landing soft. The Doctor and Dodo are the first to emerge onto the surface. It appears to be a sparse jungle world. "2" and Yendom join them. There is no sign of the Refusians and "2" reasons they are hidden somewhere. He sets the others to search for them and warns them not to run off as he is armed.

As they move off, curious things occur. The plants are moved aside by an unseen force, then something enters the launcher. The seat cushion depresses as if something has sat upon it. The controls operate themselves and the doors open and close. A deep-throated but disembodied voice expresses curiosity as the unseen force leaves the launcher and returns to the jungle.

"2" is convinced that their radio research was wrong and that there are no Refusians. The Doctor encourages him to relay that message to the Ark and begin the landing, then wanders off. Dodo suggests that they should get started soon as it will likely take a long time to get everyone down, but "2", agitated by the banter, suggests that it will not take as long as she thinks. Dodo realizes that the Monoids don't intend to bring the humans down to the planet at all. "2" tries to obfuscate, but Dodo is not put off.

The Doctor interrupts this exchange when he returns, saying that the planet is inhabited after all. He leads them to a nearby residence, which Dodo thinks looks like a castle. Still, there is no sign of the Refusians. Inside, they find the place quite comfortable and almost luxurious. It, too, appears deserted. "2" becomes quite agitated, insisting that the Refusians are simply hiding from them as they are frightened. He decides to challenge them by destroying one of the beautiful vases on the table. There is no response.

The Doctor tries to stop "2" as he picks up another vase, saying this is no way to establish friendship. But it is the disembodied voice which booms out through the room which really forces him to stop. It orders "2" to put down the vase.

The Doctor, Dodo, and Yendom figure out that the voice is coming from inside the room, but no one can be seen. "2" tries to threaten the being, but he is suddenly grabbed by the unseen being and the vase is forced from his hand. The vase returns to the table and the flowers inside are returned, as if by magic. The Doctor laughs at "2"'s agitation and posturing, saying he won't be needing his gun now.

Back on the Ark, "1" awaits "2"'s report. While waiting, he outlines his plan for destroying the humans, telling "3" that he has had a fission device secretly made and hidden on the Ark. It will be detonated shortly after all the Monoids have left, destroying the ship and the humans on board. He points to its location, inside the head of the giant statue. A human in service to the Monoids overhears this but does not see where "1" pointed. He hurries off to warn the other humans. In the security kitchen, the other humans do not believe the man's story, as he is a "subject Guardian" -- a collaborator. But Steven does believe him, to his horror.

Back on the planet, the Doctor has made friends with the invisible being, a Refusian native. He explains the humans' original purpose in coming here in the Ark. The Refusians have known of the coming of the Ark and have welcomed it. They themselves lost their physical forms during an intense solar flare long ago. They cannot even see each other. It would be good to have life on the planet again, but the Refusian warns them that whoever shares this planet with them must be peaceful.

Nearby, "2" and Yendom head for the launcher to report their findings. But on the way, Yendom realizes that the Monoids will not allow the humans to come to Refusis and vows not to let "2" contact the Ark. He tries to get "2"'s gun, but does not succeed and is killed by the weapon. The Doctor, Dodo, and the Refusian hear Yendom's scream and head outside. They come upon Yendom's twisted body and realize "2" must be at the launcher.

"1" does not seem to be unduly alarmed at the loss of communication from the surface, especially as "2"'s report began favourably. He decides to proceed with the main landings and instructs "3" to see to it. Nearby, "4" speaks with another Monoid, questioning "1"'s wisdom in proceeding, saying that they still do not know what Refusis is like. "3" overhears this and warns "1", but the leader is not worried. If it is necessary, "4" can be gotten rid of as easily as this spaceship will be.

The Doctor chats with the Refusian, who is worried about the arrival of the Ark and its effects on the planet. They have only known peace here, and already he has had to destroy the launcher and the Monoid. Dodo says that the humans also are peaceful, but the Doctor reminds her that they can also be intolerant and selfish. That's what led to their subjugation by the Monoids. The Refusian is still unsure of all of this and tells the Doctor that his people will give just one day for the differences to be resolved between the humans and the Monoids. If this is not done, his people will have to take defensive measures against both species. Now, Dodo says, it is up to Steven and the other humans on the Ark to prevail.

Looking out the security kitchen window, Steven is worried when he sees increased activity among the Monoids. They must be preparing to leave.

The only way out is through the door, and only the Monoids open it. Manissa tells him, though, that sometimes the subject Guardians open the doors as well. As they could not overpower the Monoids, they will have to trick a human into helping them. Meanwhile, the Monoid population trays are loaded into the launchers, under "3"'s direction.

A short while later, Maharis enters the kitchen bearing a tray. Steven's preparations are barely completed, so Manissa engages him in conversation about the impending departure of the Monoids. He believes they will take the humans with them, as they will still need slaves on the planet, but Manissa derides him. Maharis, sensing trouble, looks for Dassuk, but the others tell him Dassuk is sleeping. They point to the bed. However, Dassuk is sneaking out through the open door at that very moment, Steven having made the bed look occupied. Maharis leaves quickly.

In the hallway outside, Dassuk is nearly caught first by Maharis and then by a group of passing Monoids. But he evades them all and opens the door. Steven and the others leave the kitchen quickly. The first thing they must do is find that bomb!

In the control centre, landing preparations continue. "4" continues his talk against "1", saying that he and his followers will wait until the landing and then decide if "1" should remain in command. But they must decide quickly as they must have enough time to get back to the Ark before the bomb explodes. Nearby, "3" worries that there is still no word from "2", but "1" will wait no longer. The landing will commence now. The launchers head toward the planet, leaving just 12 hours before detonation of the fission device.

The humans loose on the Ark hear the signal and realize what's happening. Time is running out for them to find the bomb and defuse it.

The launchers arrive on Refusis and "1" is the first out. The planet is quiet and peaceful. He quickly claims it for the Monoids. As he and the others spread out, they find the remains of "2"'s launcher. The Refusians must be responsible for this. They will be found and destroyed. The Doctor and Dodo watch as "1"'s party spread out from the landing site. They also watch as "4" and his allies find the destroyed launcher. "4" decides the place is too dangerous and that the landing was a mistake. There is still time to return to the Ark before the bomb goes off. They hurry after "1". The Doctor and Dodo head for the nearest launcher.

On the Ark, the humans are no closer to finding the bomb. Steven is becoming daunted by the enormity of the task. They receive a call from the Doctor on the communications system, who tries to warn them that there is a bomb on the ship. Steven is way ahead of him, but none of them knows where the bomb is. The Doctor says he will find that out, and that he will send back one of the launchers to evacuate the humans if necessary. Steven renews his search for the bomb.

The Doctor suggests that the Refusian pilots the launcher back to the Ark to help the others. That way the Monoids will not know who took the ship. The Refusian agrees and the Doctor and Dodo exit the launcher.

They are greeted by the sight of "3" and some other Monoids, whom the Doctor gleefully welcomes to Refusis. He goes happily with them to meet "1". However, the launcher takes off seemingly on its own before they get too far. "3" cannot figure who is piloting the ship as he saw no one inside. The Doctor says he too saw no one!

On the Ark, the search gets more frantic, but not for Maharis, who stands staring into space. Steven tries to stir him up to search, but he feels betrayed by the Monoids and cannot function. They are interrupted by Dassuk, who tells them that a launcher has returned.

Everyone heads for the landing bay, where an empty launcher sits. They are startled by the disembodied laugh of the Refusian.

On the planet, "1" questions the Doctor about the location of the Refusians, but the Doctor flippantly maintains he hasn't seen any. He refuses to say more, no matter how much "1" questions him. "4" takes this opportunity to speak up, saying that all the Monoids are in danger on this planet and they are wasting time questioning the Doctor. "1" says that he may return to the Ark if he wishes, but there they will have to deal with the fission device. It is inside the statue and the statue is too heavy to lift. The bomb cannot be removed. They must stay here. "4" decides to take his chances with the bomb and convinces several others to join him in the return to the Ark. "1" appears to let them go, but takes action once they are gone. He and his men will ambush them out in the open and destroy them. The Doctor will be dealt with later.

Meanwhile, Steven decides that some of the Guardians should go down to Refusis in the launcher to help the Doctor and Dodo. The rest should stay and try to find the bomb. Maharis, nearly out of his mind with worry, says that staying would be suicide and that all of them should go down to the planet. But Steven and Manissa argues that all Earth life is here on the Ark and cannot be sacrificed that easily. Its preservation was the goal of their forefathers. Maharis, however, does not want to die for his forefathers' dreams. Steven agrees to let him go with the landing party. Dassuk will lead them and make contact with the Doctor as quickly as possible. Manissa agrees to stay with Steven and the others to help find the bomb. Back on the planet, "4" and his followers near the launchers, but they are ambushed by "1" and his men and a gunfight breaks out. Many of "4"'s followers are killed, "4" and the others scatter into the jungle. At the landing site, Dassuk's launcher arrives. Dassuk tries to stop him, but Maharis is the first one out.

He sees "1" and his men approaching the landing site and calls out to his master... who calmly shoots him dead without thinking. "4" and his men arrive and the gunfight continues. Dassuk and the others manage to escape during the fighting.

They reach the dwelling where they find a Monoid guarding Dodo and the Doctor. Dassuk calls the Monoid "master" and gets him to leave by telling him that "1" needs his help urgently. The Doctor tells Dassuk that he knows where the bomb is and they must relay that information to the Ark immediately. Dodo worries that they may run into Monoids on the way to the launcher, but Dassuk assures her they are busy killing each other.

They reach the landing site, passing many Monoid corpses on the way. Outside the launcher, they see a lone Monoid standing over his dead kin. The creature, disgusted by the slaughter, tosses down his weapon and wanders off into the jungle.

The Doctor calls the Ark and tells them the bomb is inside the statue, but no one on the Ark can figure how to lift the massive object without lifting equipment. All they have to do is get it into the launching bay and they can eject it into space. Just as they are about to give up, the invisible Refusian speaks up, offering its strength. He clears everyone off the deck and then easily lifts up the statue and puts it into the launch bay. All of the humans are amazed, but Manissa snaps out of it long enough to send the statue off into space, where it explodes harmlessly.

Later, the Doctor and Dodo are back on board the Ark as preparations for the full landing proceed. Dassuk says the process will be easy if the Refusians help them. The Refusian offers all his people's support, providing the humans make peace with the remaining Monoids. The Doctor echoes this, reinforcing the responsibility the ancient Guardians undertook for the Monoids. Those humans treated the Monoids like slaves, and reaped a bitter reward. The Doctor tells Dassuk, as he told Zentos 700 years ago, that the humans must travel with understanding as well as hope.

After a few short goodbyes, the Doctor, Steven, and Dodo board a cart into the jungle and the waiting TARDIS. Manissa watches them go, wondering if they will see the travellers again. Dassuk is not sure whether it will be them or their children's children, but he is sure the Doctor will return. Their memory must be preserved, not just as a legend this time.

On board the TARDIS, Steven and Dodo have changed clothes and Steven indicates that they are beginning to land. The Doctor sneezes and then appears to vanish. Steven and Dodo, alarmed, watch him fade in and out. Dodo thinks it could have something to do with the invisible Refusians. At first the Doctor is not aware of it, but as he disappears completely, his disembodied voice warns them that they are in grave danger. This is some form of attack...