With the Doctor's ability to travel in time and space restored, he decides to take Jo on a trial journey, hoping to land on the famous blue planet of Metebelis Three.

However, they appear to arrive in a rather smelly cargo hold with only some chickens for company. Jo notices a crate marked 'Singapore' and leaves with the Doctor, who wants to take a look around.

Elsewhere, on the planet Inter Minor, more arrivals are taking place, this time through the cargo thruster. Two brightly clad people are busy recovering their Scope (a miniaturised peepshow of Galaxy life forms) from the thruster. The older one is a flamboyant con man and side show artiste called Vorg, while his pretty female assistant is known as Shirna.

While they are busy with their Scope, two of the planet's leading officials are preparing to meet the new arrivals. Kalik, brother of the President, is bemoaning the fact that the planet is allowing aliens to visit. His companion agrees with him. Suddenly, there is a distraction. One of the slave workers, known as Functionaries is making a speech inciting his fellows to rebel. Quickly Kalik shoots him down, his blaster set on stun, so that a physical and mental examination of the rebel will be possible. This done, Kalik and Orum turn their attentions to Vorg and Shirna.

Meanwhile, Jo and the Doctor have found their way to the deck of the ship. Jo is now convinced they are on Earth, but the Doctor is not so sure - he senses there is something very wrong here. Entering a passage, they hear voices coming from a little saloon and it is soon evident from what they overhear that this is a 1920s ship en route to Singapore from England. The occupants of the room appear to include an old Major, his daughter Claire and a young officer called Andrews. After some small talk, Andrews asks Claire to take a turn around the deck with him. The Major declines to join them, saying he wants to finish his book before they arrive in Bombay.

The Doctor tells Jo that he still doesn't think this is Earth and he suggests returning to the TARDIS. Then, they hear Claire scream - she rushes into the little saloon with Andrews, who tells the Major that they are under attack from some sort of sea monster. The Doctor and Jo look out and see the monster, which the Doctor identifies as a plesiosaurus. If this is the 1920s, then the plesiosaurus ought to have been extinct for millions of years.

Andrews goes off to fetch a rifle, while the Major consoles Claire. Jo and the Doctor try to creep away unnoticed, but the Major spots them. Jo tries to convince the surprised father and daughter that they are passengers too, and that the Doctor is her uncle. At this point, Andrews returns and says that the Major and Claire are the only passengers on board - therefore the Doctor and Jo are stowaways.

On Inter Minor, Vorg and Shirna have problems of their own. The Functionaries are fascinated by their Scope and gather round it. Immediately putting on his best showman act, Vorg starts to promote the machine. He says the Scope is a carnival of monsters, a kind of technological goldfish bowl where different life-forms can be observed in their own natural habitats. The Functionaries are frightened away by this act, however, and it is then that Shirna notices a systems defect flashing on the machine. Vorg gets his tool-box out.

All this is watched by the scornful figures of Orum and Kalik. Another of their rank, Pletrac, joins them, bringing further news of unrest among the Functionaries. While they discuss the situation, Vorg locates the fault - he says it is a small blue box, and that he will have to remove it from the circuit.

Back on the ship, Andrews has decided to lock the Doctor and Jo up until the Captain is ready to see them. On the way to the Major's cabin, where they are to be held the Doctor notices an octagonal plate in a passage floor. Not only can Andrews not explain it - he can't see it either!

Once locked in, the Doctor finally realises what has happened, because of the name of the ship, the S.S. Bernice. In its day, it was as famous as the Marie Celeste - it was a ship which simply disappeared. Escaping from the cell using Jo's skeleton keys, the Doctor decides to open the metal floor plate. To do so, he needs a tool from the TARDIS.

The Doctor says that to retrieve the TARDIS, they must find a way off the ship. Vorg has got the miniature TARDIS, which he says he must return if it is to remain small.

Having checked their credentials, the officials now wish to know about Vorg and Shirna's Scope. The Doctor and Jo are making their cautious way back to the metal plate outside the Major's cabin. Passing through the saloon, they are caught by the Major, who does not recognise them.

Meanwhile, Vorg is shattered to learn that the officials do not regard their status as entertainers sufficient to warrant them a visa. He is told that he, Shirna and the Scope must all leave as soon as possible. Vorg appeals, claiming that they must see his Scope in action. The officials agree to this but remain unimpressed at glimpses of Ogrons, ferocious creatures called Drashigs and finally the humans on the ship. Sensing he is failing to impress, Vorg adjusts the aggrometer which makes the specimens behave in a much more aggressive manner. The Doctor is now being confronted with a markedly more threatening Andrews. A fight ensues in which the Doctor knocks Andrews down. He and Jo escape, hotly pursued, and are caught at the metal plate. They are about to be shot when Vorg turns the aggrometer down as he doesn't want his specimens damaging each other. Their memories wiped, Andrews and the Major leave the amazed Jo and the Doctor.

Shirna has noticed that the Doctor and Jo are new to the Scope. Inside the machine, they have broken through the plate and are walking inside the Scope's innards, trying to find a way out. As they are doing so, Pletrac orders the Scope destroyed, fearing infection from the life forms within.

The Eradication is a failure. But at first the officials believe that all life inside has been disposed of, until they see that this is not the case. Kalik speculates that Vorg and Shirna are alien spies, saying that the Eradicator is now their only defence from the strangers. They search the machine for a transmitter, and find the tiny TARDIS.

All of a sudden it grows to normal height - because, Vorg explains, it was left out of the Scope's compression field for too long. Shirna now suspects that the Doctor and Jo have escaped from their circuit.

She is quite correct; the Doctor and Jo now find themselves in a swampy marshland. Vorg notices and says they've had it - they will be devoured by the Drashigs, who live in this circuit. Kalik wishes to watch this, and despite Shirna's protests, Vorg is too scared of having his hand bitten off by a Drashig to intervene and help them.

The Drashig seems to be blind, hunting by scent alone. The Doctor says they must try to reach the cave before it gets there. The viewers on Inter Minor listen fascinated as Vorg tells how the Drashigs never give up a scent, and since they are omnivores, they even eat spaceships.

The Doctor and Jo have other ideas, however, and using distractions caused by the Doctor's sonic screwdriver are just about to reach the cave mouth when another Drashig appears to block the way. At this Vorg intervenes, giving the Doctor and Jo just enough time to get back into the cave and the Scope's workings beyond.

The Doctor now explains to Jo exactly where they are - inside a MiniScope, and that these were a kind of galactic peepshow, banned by the Time Lords because of the cruelty involved. This one must have avoided being impounded. The Doctor and Jo are now aware that the Drashigs haven't yet given up the hunt and are trying to follow them into the Scope. They rush off pursued by the giant creatures, which simply eat their way through the metal of the Scope.

After an exhausting flight through what seem to be miles of pipelines and small channels, the Doctor and Jo reach a junction with a deep chasm ahead. The Doctor says that logically there should be an extractor at the bottom of this shaft, the only way of escaping the Scope altogether.

Jo suggests going back to the ship where there is plenty of rope, and using this to get down to the exit point. Luckily for them; they come out in the ship's hold, and they begin to collect some rope, accompanied by the distant roars of the Drashigs. Then they hear the Major and Andrews approaching - both having heard the screams.

The Doctor and Jo hide, but Jo is discovered and taken up to the saloon, while the Doctor gets on with the escape plan. Andrews is telling Jo that the Captain will see her, when a much louder Drashig scream is heard from the hold. Leaving Jo with Claire, Andrews and the Major rush down. Down in the hold, the Drashig bursts through and confronts the Doctor. Just then, Andrews arrives with some armed men.

The Drashig reacts to the gunfire by crashing through the hold of the ship on to the deck where it is finally shot to pieces. In the hold, Andrews blows up another Drashig, giving the Doctor a chance to escape back into the Scope's interior. In the saloon, Jo is horrified to see that the Major, Andrews and Claire have all slipped back into their original behaviour patterns. She rushes off to find the Doctor, watched by a bemused Major.

On Inter Minor, Shirna confirms to Kalik that the Drashigs have broken out of their circuit, and might break out of the Scope. Taking Orum to one side, Kalik says that this could serve his purpose by providing an opportunity to take over from his brother to lead a new, stronger, more traditional form of government - with Kalik as its president. The Eradicator would be used to destroy the Drashigs and the Scope.

The Doctor grows to full height and Pletrac, fearing alien infection, orders his destruction at once. Kalik prevents this, having sabotaged the Eradicator until his purposes suited him. At this point the Doctor intervenes and says they will not harm him. Finding out his location, he sees the TARDIS and assumes an air of authority. He accuses the officials of operating an illegal Scope and says they must help him rescue Jo from inside.

The officials appear to swallow this, while Vorg and Shirna decide he must be a showman like themselves. The Doctor goes over to examine the Scope. Vorg decides to help him and is very surprised when the Doctor rounds on him for calmly accepting that the Scope is going to fail, killing all its occupants. On the ship, Jo has made her way back to the hold, only to be caught yet again by Andrews. Locked in the Major's cabin, she waits a short time before using her skeleton keys on the door again.

Back on Inter Minor, Vorg and Shirna realise that the Drashigs will soon break out of the Scope. They try to leave but are stopped - Kalik wants Vorg on hand to take the blame if his rebellion goes wrong.

The Doctor has managed to rig up a system whereby he will be transported back into the Scope, and by switching a linked button (fed through the TARDIS time travel facility), it will return the Doctor and Jo to the planet and all the occupants of the Scope to their rightful places in the Universe.

Just as he is going to be interrogated, the Doctor is transported back into the Scope by Vorg's intervention.

The enraged Pletrac fires at the Doctor's link-up, damaging it badly. The Doctor finds Jo in the hold and they return to the Scope's innards to await Vorg's rescue. Urged on by Shirna, Vorg is trying as hard as he can to repair the damage inflicted by Pletrac's blaster. Inside the Scope, the life support system is beginning to fail and the Doctor and Jo find it increasingly hard to breathe.

On the ship, the Major, Andrews and Claire sink into unconsciousness as does Jo. Finally the Doctor succumbs too, unaware that Vorg is about to make the last vital re-connection. Orum meanwhile informs Kalik that his dream of rebellion will have to wait now, as Pletrac has arrived to dispose of the Scope. Kalik draws a gun forcing Orum to delay Pletrac. In the time available, Kalik forces open the extractor on the Scope's side, freeing a Drashig.

The Drashig grows to an enormous height and immediately consumes Kalik, while the others in the area flee for their lives. Acting very quickly, Vorg snatches the abandoned Eradicator and fires at the massive creature. He then destroys all the other omnivores now breaking out from the Scope. This done, he preens himself only to be reminded about the Doctor's escape switch by Shirna's hurried voice.

At first it seems as if all the power has drained away and then there seems to be a slight hum as the machine revives for one last time. Pulling the switch home, Vorg and Shirna wait.

Inside the Scope, slowly but surely, all the specimens vanish and re-appear in their rightful times and places. The Doctor and Jo rematerialise next to the TARDIS and awaken to find a joyful Vorg greeting them. The S.S. Bernice returns to 1926, at last able to finish its voyage, while the Major is finally allowed to finish his book. Claire struggles to remember the events buried deep in her memory but gives up.

Vorg is now surrounded by an admiring band of Functionaries and officials, busily recounting, in highly exaggerated and mock-heroic style, how he saved the day for Inter Minor. The Doctor and Jo announce their departure, but Pletrac says they must wait until the Court of Inquiry has finished. Humouring him, the time travellers begin to edge towards the TARDIS. Unwittingly, Vorg provides a diversion with one of his old money-making side shows.