The Doctor is alarmed to find Ben and Polly have boarded the TARDIS. He informs them that they have boarded the ship just as he has set off for a voyage through space and time - however, he cannot tell them where and when they have arrived. Expecting to find themselves in London 1966, Ben and Polly are surprised to find themselves on a beach. They wander off through a cave to another beach, enjoying themselves - the place resembles Cornwall - they need to find a bus stop or a train station. The Doctor follows them - to make sure they don't come to any harm. Investigating a nearby churchyard, the Doctor, Ben and Polly meet an old man. Assured that they have not come from the sea, the old man offers them food. They enjoy a short chat with the man, Longfoot - and decide to return to the TARDIS. As the tide is in though, he advises against going to the beach and instead they should go to the nearby inn. As they leave, Longfoot passes on a secret to the Doctor. Afterwards, a man emerges from hiding and enters the church. The man, Cherub, informs Longfoot that Captain Pike misses him and the treasure. Trying to find out more information, Cherub kills Longfoot but guesses that he has passed information to the old man he was talking to earlier (the Doctor).

The Doctor, Ben and Polly arrive at the inn as rain breaks. At first Kewper informs them there is no room at his inn, but when they tell him they are friends of Longfoot, he agrees to put the travellers up. Tom comes in and tells Kewper he has found Longfoot dead: he concludes that the Doctor is the culprit and so sends for the squire. Cherub and some other pirates arrive and grab the Doctor. Ben is knocked out and Polly is unable to stand up to the men. Squire Edwards arrives and questions Polly, wanting to know who they are and where they come from. Without that information he will do nothing to help the Doctor - in fact he believes them to be rogues and villains and has them arrested for the murder of Longfoot.

Snowcap, the South Pole tracking station is in contact with Zeus 4, a manned atmosphere testing probe in orbit around Earth. At first everything seems routine. A group of soldiers on watch spot the Doctor, Ben and Polly outside the base and apprehend them. General Cutler wants the travellers kept under close watch and has them brought to the tracking room. From that point the routine space mission becomes anything but. The probe is not where it should be, as if some external force is influencing it, pulling it away from Earth. One of the astronauts, takes a Mars reading to ascertain their position. However, he realises that its not Mars - its another planet, also between the orbits of Venus and Mars - a tenth planet. The Doctor asks to speak to the General: he has determined what they will see and he also predicts that they can expect visitors.

Zeus 4 is experiencing a mysterious power loss and is being pulled further away from Earth. All attempts to correct their orbit fail. Whatever the influence is its also affecting the astronauts - they can feel their strength ebbing. Meanwhile, Tito and the other guards of the watch are getting ready to go outside and check the TARDIS. The Doctor's prediction comes true - a ship lands near the Snowcap base and strange figures emerge. The new arrivals attack Tito and then proceed to take over the base - and reveal themselves to be Cybermen, from the Earth's sister planet, Mondas.

As the Doctor recovers from his regeneration, Ben and Polly try to come to terms with what has happened. Ben doesn't believe that this new man is the Doctor, Polly believes that he is. When the man seems to refer to the Doctor as if he was someone else it only serves to confuse his fellow travellers. Noting that the TARDIS has been landed for some time: they should go out and explore. Ben and Polly are alarmed when this man forgoes the usual landing checks, but he seems to know what the readings will say. The man goes outside, reading a diary, managing to avoid some of the pools and obstructions in his way. Another man tries to catch the Doctor's attention but someone shoots him. The Doctor dashes over but finds that the man is dead. Searching his pockets he finds ID on the body - the man was an examiner from Earth.

Someone knocks the Doctor out and drags away the body of the examiner. Ben and Polly have encountered the various mercury swmaps, and begin to sucumb to the fumes. All of them are found by some guards - finding the examiner's ID on the Doctor they assume that is who he is. The new arrivals advise that all Earth ships seem to overshoot on landing - they had seen the examiner's ship overshoot and so came out to find him. Presumably the examiner has come to check on Lesterton's experiments and the machine he has discovered. Its been in a mercury swamp for 200 years. Bits of metal show no signs of corrosion and nothing seems to affect it. Lesterton is faced with the possibility that the examiner is not there to help him but to stop him from opening the machine.

Ben and Polly emerge from the TARDIS and the surroundings suggest that they may have arrived home. Sounds of a battle nearby and the landing of a cannon ball don't fail to deter the travellers from having a look around.

The laird, Kirsty, Jamie and Alexander have taken refuge in a cottage. The laird is injured and they are hoping to hide from the English. Hearing someone approach, the highlanders prepare to defend themselves: the Doctor, Ben and Polly arrive - and are taken into the cottage as the sounds of battle get nearer. Alexander wants to kill them, believing them to be with the Redcoats. Ben manages to grab a gun and then disarms Alexander and Jamie. The Doctor tends to the laird and asks Jamie and Alexander to give their word they will not harm them. Then he tells Ben to put down the gun. Polly and Kirsty go out to look for water from a nearby stream, to help treat the laird.

Redcoats converge on the cottage, responding to the shot from the gun dropped by Ben. Alexander tries to draw the soldiers away but is shot. The soldiers accuse the Doctor and Ben of being spies, the Doctor tries to convince them that he is on their side. The soldiers will have none of this and intend to hang everyone, even the wounded laird.

Polly guesses that they have arrived on a beach in Cornwall, the Doctor cannot tell them accurately where they are. In the distance they can see what may be an extinct volcano. Ben, Polly and Jamie climb the slope of the volcano, leaving the Doctor on the beach. Ben and Jamie climb further up; tired, Polly pauses and enjoys the view. Hearing sounds she notices a cavern and enters it. Ben and Jamie hear her scream and dash back to investigate: they enter the cave too. They are captured and placed in a cage with Polly. The Doctor is soon captured and placed in the cage which then begins a long descent into a shaft. The Doctor determines that they are travelling to a depth well below sea level.

Initially the travellers suffer from their descent but recovery they are shown to a table and food is placed before them. Ramo arrives and declares that their appearance was foretold by the god, Amdo. They will be her special guests - which will entail them being sacrificed. The Doctor tries to buy time by informing Ramo that he knows Professor Zaroff (he recognises the food as resulting from a process developed by Zaroff).

Ben, Polly and Jamie are prepared for sacrifice - it appears that the Doctor has been unable to help them. When he does appear, it is in the company of Ramo - apparently he too will be sacrificed.

The Doctor appears to have fulfilled his promise to take them to Mars, but Ben recognises the lunar landscape from various images he has seen. Ben, Polly and Jamie inform the Doctor they would like to explore the Moon since they are there anyway. Once they have all put on spacesuits, the four travellers wander around the surface of the Moon. Polly wonders about the heavy boots they are wearing and tried out a jump in the lunar gravity. Ben and Jamie compete to see who can jump highest and/or furthest. Jamie jumps quite as distance and by the time the other travellers catch up with him, they see his limp body being carried inside a lunar base. The Doctor, Ben and Polly follow: they discover that the crew of the Moonbase are under a lot of pressure; some of them are collapsing from some strange viral infection; the gravitron which controls the tides and subsequently the weather on Earth is being affected by both the ailing crew and some other influence; and someone else on the Moon is monitoring communications between Earth and the Moon.

Hobson believes that the Doctor and his fellow travellers are the medical team they had requested from Earth and so is quite happy to let them tackle the mystery virus. Jamie, recovering from his injuries in the sickbay mutters something about the Piper coming to get him. Examining the other patients the Doctor is concerned that the infection has all the hallmarks of a deliberate infection rather than an ordinary virus. Ben is dispatched to help Ralph in the foodstore. Ralph is slightly on edge: before Ben arrived he had thought someone was in the store with him; he has also noted that some of the foodbags have been tampered with. Ralph asks Ben to got and fetch some of the other items from another part of the foodstore. Again, Ralph hears a noise, and finds that Ben is not the only other person in the store. Something is invading the Moonbase...

As happy tunes blare out and people look happy, the pilot is pleased. Then Medok bursts through: he has been resisting treatment and something concerns him. He tries to hide, eluding the pursuing guards. Witnessing the arrival of the TARDIS, he continues to hide. When the Doctor, Ben, Polly and Jamie emerge, Medok tries too attack them. The guards arrive and restrain him: Ola thanks them for their help, remarking that he is the last of a group of troublesome patients. Ola will take them to meet the pilot of the colony, he will also want to thank them.

The travellers are offered a variety of treatments - beauty, cleaning and so forth. Polly chooses hairstyling, the Doctor is offered the chance to have his appearance and clothing tidied up.

Medok tries to convince Ola and Questa that there is something wrong, with the colony - that there are horrible creatures infesting the camp, creatures which emerge at night. Ola explains that Medok just sees things. The Doctor decides that he wants to find out more, from Medok in person and so breaks into the cells where the man is being kept. The Doctor asks Medok if the creatures he sees crawl over the ground (earlier, the Doctor and his fellow travellers had seen an image of some crawling creature on the scanner). Medok takes the chance to escape, much to the anger of Ola who wants the Doctor sent to the mines in punishment. The pilot dismisses this, hearing the Doctor and Polly remark that they had helped capture Medok to begin with, besides which its hardly the way to treat newcomers.

One part of the colony oversees the mining of a gas which is vital to the colony. However, the gas is poisonous - why would the colonists mine deadly gas? Why does everyone else deny the existence of the giant crawling creatures with claws? And why is the camp such a happy place in spite of all this?

The TARDIS materialises on an airport runway: the travellers dash across the runway to get away from the various aircraft taking off and landing; a policeman spots them and the Doctor shouts "scatter!". Polly takes refuge in a hangar assigned to Chameleon Tours: there she witnesses a man being shot by another man wearing a pilot's uniform; Polly runs out of the hangar after being spotted by the pilot; she finds the Doctor and Jamie and tells them what she saw. The three travellers return to the hangar. The Doctor discovers that the man was shot with a gun not yet in use on Earth: Polly confirms that she would recognise the shooter again. Their conversation is being monitored by the pilot and Blade: on hearing Polly's words they realise something must be done about her. As the Doctor, Jamie and Polly wander around in search of Ben, Polly is grabbed by the pilot and stunned. When the Doctor and Jamie realise that Polly is missing they try to find her but conclude they must contact the authorities. Overhearing this, Blade decides that something needs to be done to stop the Doctor from worrying about Polly...

In trying to report to the authorities, the Doctor and Jamie are referred to the airport commandant. After speaking to him, the Doctor is asked to show the commandant and the police to the hangar in question. No body is to be found and the Doctor's talk of marks left by a ray gun only convince the commandant that his time is being wasted and that he should be investigating the presence of the Doctor and his friends in the airport. The mystery deepens further when the Doctor and Jamie spot Polly at the check-in desk, but she denies knowing them. Furthermore, her name is not Polly...

Just as the Doctor and Jamie reach the hangar where they left the TARDIS, a truck drives away with the TARDIS strapped in the back. Losing sight of the truck, the Doctor and Jamie look around for someone who might help them find out what has happened to the TARDIS. They find a mechanic, Bob Hall, and he informs them about the name of the firm that collected the TARDIS. Suspecting that there was more to the mechanic than met the eye, Doctor and the Jamie watch as Bob sets off. They follow him and find their way to a warehouse.

A string of clues point to The Tricolour, a coffee bar. Finding their way to the cafe, the Doctor and Jamie discuss the probability that this is all part of some kind of trap. Their attempts to find out about someone called "Ken" or "Kenneth" meet with little success. Mr Perry arrives at the cafe and seems to know who they are, and he invites them to meet with Mr Edward Waterfield at ten o'clock at his shop.

Mr Waterfield has an anxious meeting with another party: Kennedy is watching him, curious as to what is going on; his interest though is more with regard to what valuables he can steal. Instead, Kennedy's search brings him into contact with a Dalek...