Barbara Wright, history teacher, and Ian Chesterton, science teacher, both from Coal Hill School, are worried about 16 year old student Susan Foreman. Her knowledge of science and history is second to none. However, her ignorance of common place matters such as British currency, is appalling. After Susan refuses an offer of home study on the grounds that her grandfather, a Doctor, does not like strangers, Barbara looks further into her background. Susan's home address appears to be a junk yard and it is there that the two teachers wait one night in the hope of finding out what she is keeping hidden.

Following Susan into the junkyard, the two teachers are puzzled to find that she has disappeared. Even more intriguing is the police box sitting in the centre of the junkyard, humming with power yet it appears that it is not connected to any power source. When an old man arrives and then tries to divert their attention away from the police box, the teachers are determined to solve the mystery of Susan Foreman. What they find inside the police box is beyond their wildest imaginings. As Ian and Barbara struggle to make sense of the TARDIS, which both the Doctor and Susan claim is a ship capable of traversing time and space, the Doctor shows his determination to keep their secret. At first, the Doctor refuses to allow Ian and Barbara to leave the ship. When he appears to relent, even to allowing Susan to remain with the two teachers, the Doctor instead sends the ship hurtling back through space and time.

Emerging from the TARDIS, Ian and Barbara come to terms with what has happened to them. When the Doctor wanders off and is captured, Ian, Barbara and Susan set off to find him as he is their only means of returning to London 1963. However, they too are captured. Trapped in the stone age by a tribe that has lost the secret of fire, Ian and Barbara join forces with the mysterious Doctor and Susan...

The TARDIS has materialised in a petrified forest: the ground is cold and covered in ash and dust; the trees are like stone and the branches brittle; a small metallic animal found deeper in the forest is quite dead. Everything points to the aftermath of some horrific war - and the planet appears to be dead. At the edge of the forest, the Doctor, Ian, Barbara and Susan look down upon a huge city of metal. Again there are no signs of life but the Doctor is keen to explore the city. Feeling weary, the other travellers vote on a return to the TARDIS. Reluctantly, the Doctor agrees and, on the journey back, Susan is separated from the group and is sure something is stalking her. Re-united, the four travellers return to the TARDIS: the majority favour leaving the dead planet while the Doctor is determined to explore further. When something knocks against the outside of the TARDIS it would appear that the decision is made: they should leave.

An attempt to leave fails - a leak in the fluid links means that the TARDIS is grounded until the travellers can replenish the lost mercury. According to the Doctor there are no further supplies of mercury on board. It would appear that their only hope is to brave whatever terrors are hiding in the petrified forest and venture into the mysterious metal city beyond. If the forest is not as deserted as they thought it was then what could be lurking in the city?

A sudden explosion within the TARDIS stuns the four travellers and the ship is plunged into near darkness. As the travellers revive they begin to re-orientate themselves. At first they have little recall of who they are and where they are but as their memories begin to creep back they also become aware that not all is as it seems.

The Doctor suspects that something may have invaded the TARDIS - perhaps Ian and Barbara are agents for some sinister force? Even Susan sense that there is some force working against them. Barbara is convinced that the more immediate threat is outside the ship and that their time to counter that threat may be running out...

A fault temporarily grounds the TARDIS and so the crew investigate their latest landing site. An encounter with a Mongol warrior by the name of Tegana brings the travellers to the attention of the famed Venetian explorer, Marco Polo. He invites the time travellers to join his own group of travellers on their journey to meet Kublai Khan. In addition to Tegana, who explains that he wishes to sign a treaty with the Khan, Polo's party also includes a young woman by the name of Ping Cho. She has been betrothed to an elderly gentleman - and as the journey unfolds it becomes clear that it is not an arrangment that she desires. It also becomes evident that Polo sees the TARDIS as a means by which he can win the favour of the Khan and finally return home. As trouble dogs their sojourn, the travellers must convince Polo to return the TARDIS to the Doctor. They must also find out what Tegana's true intentions are...

There is nothing quite like a sandy beach and a dip in the sea to ease the stresses of time travel. Except that the TARDIS crew find that they have arrived on an island with a beach comprised of sharp glass surrounded by a sea of acid. The discovery of an empty wet suit with a tear in one side hints at the fate of anyone exposed to the sea. Further inland there is a huge building which warrants investigation. Susan, eager as ever, explores the building first but disappear. It is not long before the other travellers find a way inside the building where they encounter Arbitan, keeper of the Conscience of Marinus - a computer which can be used to control the emotions of the people of Marinus. In order for the computer to do so, four micro-circuits, known as keys, must be located and inserted into the computer.

With the threat from a sect known as the Voord, Arbitan wants to find the keys - but he must remain with the Conscience. Previous expeditions to locate the keys have failed to return, so Arbitan asks and then forces the time travellers to continue the quest. Using travel dials, the four time travellers set off to the various locations around Marinus where each of the keys are hidden. As they search for each key, the travellers learn why other expeditions have failed...and the various hazards which protect the keys.

The TARDIS has materialised inside some chamber. As they look around, Susan and Barbara note that they are in a burial chamber. Barbara finds a bracelet and tries it on and then she notes what appears to be a door. Going through, Barbara is horrified when the door closes behind her. As she tries to find a way back, a small group enter the outer chamber...

The Doctor and Ian join Susan - the latter takes them over to the panel Barbara had been looking at earlier. Discovering the doorway, all three go through to the outer chamber - and all three are horrified when the door closes. It appears to be a one-way door to prevent grave robbers. A man appears, introducing himself as Autloc, high priest of knowledge. He takes them to Yetaxa (Barbara) - the bracelet on her wrist belonged to the high priest interred within the burial chamber. In Aztec lore, a high priest is reincarnated as a god. The Doctor warns Barbara to be careful not to abuse her new status - there are dangers in trying to rewrite history.

One problem Barbara must overcome is that as a god she would know how to get back inside the burial chamber. Her servants ie the Doctor, Ian and Susan must find an way in by other means. Susan is sent to school to learn the ways of the Aztecs; Ian is to become a warrior but he is regarded as a threat by the lead warrior, Ixta; and the Doctor joins the other elders in and around the city, enjoying retirement. Watching them all is Tlotoxol, the high priest of sacrifice who is convinced that Yetaxa and her servants are impostors who will destroy the Aztec way of life. By various means he sets out to destroy them all...

The TARDIS materialises inside an Earth ship orbiting a planet. Most of the human crew are in a kind of coma, but one man is roaming the ship in a state of madness. Once the travellers suceed in reviving the other crew they learn that the planet is the Sense-Sphere, home world of a race known as the Sensorites. Ever since the Earth ship arrived at the Sense-Sphere its crew have come under mental attack from the Sensorites.

The Doctor sets out to learn why the Sensorites have launced such an attack - could part of the cause be a previous Earth expedition which nearly ended in disaster for the people of the Sense-Sphere? Or could it be the terrible plague which is ravaging the Sensorites...a plague which strikes Ian Chesterton? In a race against time to find a cure, the Doctor must also deal with a faction among the Sensorites who will not tolerate any kind of dialogue or action involving humans...

The TARDIS materialises in a forest: as far as the Doctor is concerned he has brought Ian and Barbara home. As soon as they have left the TARDIS, Susan and the Doctor will depart immediately and continue to explore the universe. Susan is alarmed at this abrupt move and asks Barbara if she really wants to go. Barbara assures Susan that she and Ian had always intended to return home at the earliest opportunity. Ian also assures her that it is for the best - the longer they leave the decision to leave, the harder it will become. Of course, it all depends on whether not they have arrived on Earth in the 1960s or not. Ian and Barbara manage to persuade the Doctor to come with them, just to explore the area and to make their parting more friendly.

In the fields outside, the travellers encounter a frightened boy who informs them that they are in France, just 12 kilometres from Paris. The next question they must find an answer to is what period in time they have arrived, but the terrified boy escapes. The travellers walk to a nearby house but the mounting evidence suggests that it is a time before the 20th century. An initial search of the house provides the travellers with local clothing. They also find food, drink, weapons, maps and documents - which suggests that the house is part of an escape chain. Some of the documents are passes which bear the name of Robespierre - they have arrived at the time of the French Revolution! When Ian decides that they should find the Doctor (he had gone to search upstairs and has been gone some time now), the travellers encounter two men - obviously refugees there to make use of the material left in the house. A group of soldiers arrive and it appears there is no escape. In 18th century France you are either for the Revolution or against it...and there is no neutral ground.