The TARDIS arrives on a planet that appears to be utterly lifeless: and the Doctor concludes they should remain for a while and make use of the opportunity to take a break. Outside, a squat robot approaches the TARDIS and begins to circuit it, exploring it. The Doctor, Steven and Vicki watch its progress and note that it is transmitting information - but to whom? Deciding that it warrants investigation, the Doctor goes outside. The travellers note that the planet has three suns. The robot re-appears and after demonstrating its firepower it guides the travellers across the planet. En route, two women appear and throw a mesh over the robot, immobilising it. The women, introducing themselves as the Drahvin, from Galaxy 4, inform the travellers that they have been instructed to take them to meet their leader, Maaga. The robot, which Vicki has nicknamed a chumbley, had been programmed to take the travellers to the Rill. The Drahvins assure the travellers the Rill would kill them. As more chumbleys appear, the travellers and the Drahvin make for the Drahvin ship.

Aboard the ship, the Doctor listens to what Maaga has to say - that the Drahvin and the Rill are at war, a war which the travellers are now caught up in. Maaga also explains that the planet is due to explode: information that was given to them by the Rill who are repairing their spaceship in order to escape. The Drahvin ship is too badly damaged so they must capture the Rill ship. Maaga explains more about her civilisation - she is a living creature, the other Drahvin are products grown for fighting and killing. The Rill attacked their ship when it was surveying this planet. When the chumbley arrives, to broadcast a message, Maaga fires upon it - determined not to let its message be heard. The Doctor begins to wonder about the Rills - what was the message about? Why tell the Drahvin that the planet is doomed?

Garvey is lying on the ground, barely conscious. When he finally sits up he begins to feel better. Getting up he takes out his gun and begins to mutter "I must kill."

Cory and Lowery are trying to repair their ship: Lowery wonders why Cory wanted to land on Kembel in the first place; if they cannot get the ship repaired in time they will miss their rendezvous with the freighter; they also ponder on what has happened to Garvey - he has been gone some time. When Garvey does appear, Cory shoots him dead. Lowery is angry but Cory explains that it was either Garvey or Lowery. Checking the body he removes a thorn from Garvey's body. Cory reveals that he is from the Space Security Service: a recent report of a Dalek spaceship would appear to suggest that the Daleks are about to launch some kind of offensive...

Achilles and Hector are fighting: watched by the Doctor, Vicki and Steven on the TARDIS scanner. The Doctor decides that as the fighters are doing more talking than fighting he should go outside and have a word with them. As the Doctor emerges from the TARDIS, Achilles kills Hector, believing that the gods are answering Hector's blasphemy. Achilles then persuades the Doctor, or Zeus as he believes him to be, to go with him to the Greek army which has lain seige to the city of Troy for many years. Odysseus is sceptical that the Doctor is Zeus and orders his soldiers to escort Zeus. Steven changes into appropriate clothing and sets off to help the Doctor, instructing Vicki to remain in the TARDIS as her ankle is still injured.

Agamemnon and Menelaus are talking over drinks, about rescuing Helen - again. When Achilles arrives and informs them that he has killed Hector and that Zeus is being held by Odysseus, the two men decide they should speak with him. In his attempt to prove himself (with a view to returning to the TARDIS), the Doctor manages to upset Agamemnon. Placed under arrest until he can prove himself, the Doctor joins them for food and drink.

When Cyclops, a spy for the Greeks, spots Steven leave the TARDIS and head towards the Greek campy, gets there first and informs Odysseus who then captures Steven. Confronting Zeus with a man he claims is a Trojan spy, Odysseus suggests killing Steven. The Doctor denies any knowledge of Steven and states that he will take him as a sacrifice and offer him up at his temple (the TARDIS) tomorrow. Odysseus delights in advising Zeus that he will have his sword ready for tomorrow just in case...and then informs him that his temple has disappeared.

An injured Gantry watches as Vyon tries to contact Central Communications Control. They have a message vital to the safety of Earth, but are unable to get through. The monitoring staff are not paying attention: they do not expect a signal from Vyon for a while yet, instead they choose to listen to a broadcast by Mavic Chen, guardian of the Solar System prior to his deparure for a well-deserved holiday. By the time the broadcast finishes the team have missed Vyon's signal. Vyon decides that they need to move on, get away from their pursuers. Kert decides to remain, his injuries will only slow them down. Pulling a gun on Vyon, Kert waits for their pursuers to get closer. Reluctantly Vyon moves on, taking the transmitter with him. A Dalek appears and exterminates Kert, more arrive and they set off after Vyon.

As he attempts to escape, Vyon falls and the transmitter is damaged. He then witnesses the arrival of the TARDIS. The Doctor and Katarina emerge: they intend to find help to treat Steven - Katarina will remain in the TARDIS while the Doctor sets off. Vyon follows the Doctor as he reaches a city beyond the jungle. Intercepting the Doctor, Vyon demands the key from him. Vyon heads back to the TARDIS and enters it - where he meets Steven and Katarina. Katarina explains that only the Doctor can operate the ship, Steven recovers and knocks out Vyon.

After restraining Vyon, the Doctor continues on his journey to the city and witnesses a ship arriving. En route finds a body and nearby a recording which he puts in his pocket. At the city the Doctor discovers various aliens including the Daleks. They have assembled to form an alliance to conquer the Earth solar system. The new ship belongs to their newest ally - Mavic Chen. Everyone has contributed to the construction of a powerful weapon which will be used in the conquest. Somehow the Daleks must be stopped and the solar system warned...

16th century Paris: a group of Hugenots are drinking together in a tavern. Simon, aide to the Abbot, advises the landlord to report further on what the group talk about. The Doctor and Steven arrive and plan what they will do: the Doctor wants to meet Preslin; Steven is confident he can enjoy Paris by himself; so the Doctor sets off to find Preslin, leaving Steven with some spending money. Someone appears to follow the Doctor so Steven tries to follow but the landlord reminds him that he needs to pay for his drink first. Concerned, Steven tried to find out where he can find Preslin - just to make sure the Doctor is okay.

A terrified woman comes into the tavern and hides. The Hugenots obstruct the searching guards. The woman is feeling from the Abbot of Amboise. The Doctor, speaking to Preslin discovers that he is keeping low because of the Abbot of Amboise. The woman, Anne Chaplet, informs the group of Hugenots that she fled because she heard talk of a massacre similar to one which took place some years before - a massacre of the Hugenots.

The guard captain reports that he could not find the girl and that there were too many people in the tavern for them to search further. It would appear that Navarre, a Hugenot prince is being plotted against and that the Catholics intend to wipe out the Hugenots.

A single-eyed creature keeps watch over the animals in a forest. As it moves through the forest the TARDIS materialises nearby. Dodo emerges for fresh air, much to the consternation of Steven who warns her that the Doctor had not completed his checks of the air etc. Dodo believes that it is Whipsnade Zoo - noting the various animals in the forest. The Doctor concurs with Dodo that it is Earth. However, the jungle appears to contain animals from all parts of Earth. Above them, the Doctor observes a steel sky - no sun. There is also a vibration in the soil - leading the Doctor to conclude that its some kind of indoor nature park. The travellers decide to investigate further.

Elsewhere, a hearing is being finalised - a man has been found guilty of negligence: his failure in his duty could have resulted in the deaths of humans and monoids; his sentence is to be miniaturised for 700 years - the end of the voyage. A monoid joins the meeting and informs the humans that there are intruders in the jungle. As all humans have been accounted for these new arrivals can only be intruders. The commander doubts that these intruders can have any connection to the monoids. He also suggests to Zentos that the intruders be invited to join them, not arrested.

Zentos remains wary of the travellers, while the commander is prepared to welcome them with open arms. He explains that the monoids' own world was destroyed and that they came to Earth. Now, the Earth is on the brink of destruction - the humans and the monoids are beginning a voyage to Refusis 2, a new world where they will live together. Most of the human and animal population have been miniaturised and placed in hibernation for the voyage: those humans and monoids still awake are the guardians who will oversee the voyage.

However, a threat manifests itself when first some of the monoids and then the humans are stricken by a mysterious fever. It would appear that Dodo's cold has spread among the people: and since it is a virus virtually unknown in this time it may prove to be fatal. The success of the mission may be doomed. As people begin to die, Zentos has the travellers arrested.

Some force from outside the TARDIS has rendered the Doctor invisible and intangible. It must be powerful indeed to penetrate the ship's defences. The travellers leave the TARDIS - the Doctor is restored to visibility. There is something familiar about their surroundings...the Doctor soon realises that they are in the domain of the Celestial Toymaker. He is a powerful force for evil who captures and manipulates people into becoming his playthings. The Celestial Toymaker hides the TARDIS and then disappears, taking the Doctor with him. Steven and Dodo encounter two clowns - the games are to begin. The challenge is to win the games and gain their freedom. They must win all their games before the game between the Doctor and the Toymaker is finished. If they lose...they will remain in the Toyroom - as toys of the Toymaker, forever. Each of the games seem innocent enough, but there are dangerous elements to those games - one slip could be fatal. And their challengers are toys, once people, equally determined to win in the hope of gaining their freedom.

The Clanton brothers arrive in Tombstone: Billy takes a few shots at the sign above the OK Corral. He is warned to save his bullets, they also have a meeting with Seth Harper at the Last Chance Saloon. In a nearby livery barn, the TARDIS materialises. The Doctor, in agony with toothache concludes that he desperately needs to see a dentist. Spotting the sign for the OK Corral the travellers determine where and when they are. The Doctor doubts they will find a dentist nearby. Steven and Dodo get dressed up in what they feel is appropriate gear. Steven tries some fancy gun work only to have it shot out of his hand. Marshall Wyatt Earp asks them to come with him - the Clantons are in town and loose talk such as who is the fastest gun in the west could have terrible consequences.

After overhearing the conversation between Harper and the Clantons, Kate visits Doc Holliday to warn him. Her advice is for him to leave town. Holliday has other ideas though - he won't be run out of town. The sheriff, Bat Masterson also warns him to keep out of trouble. Earp introduces Masterton to the travelling troupe - the Doctor and his two companions. After commenting about his need for a dentist, the Doctor is told to visit Doc Holliday. Reluctantly the Doctor stops off with Holliday to have the bad tooth extracted. Steven and Dodo go to the saloon to book a room for the night and to wait for the Doctor. There they encounter Harper and the Clantons who force them to put on a performance while they wait for the Doctor. It seems they think he is Doc Holliday and they plan a special reception for him...with their guns.

Despite the look of the landscape outside, the Doctor announces they have arrived in the future, in an age of peace and prosperity. He wanders off to take some readings - which appear to confirm his conclusions. Two men, dressed in animal skins watch and follow the Doctor. Two guards meet up with the Doctor: their scientists have been expecting him. One of the guards, Exorse is sent to fetch Steven and Dodo, while the other, Captain Edal, escorts the Doctor back to the city.

Steven and Dodo stay near the TARDIS. While Steven tries to spot the Doctor, Dodo glimpses a primitive looking man. Steven wonders if they are in some primitive, past era rather than the wondrous future the Doctor thinks. A group of primitives appear and begin to stalk the travellers but the arrival of Exorse scares them off. Exorse escorts Steven and Dodo to the city of the Elders.

Meeting the Elders the Doctor is informed that they have been charting his travels through space and time; they make him a high Elder. The Doctor returns the compliment: he knew that there was a highly advanced civilisation in this region of the galaxy. It is not long though before the Doctor discovers that this highly advanced civilisation has a terrible secret...

The TARDIS materialises in present day London: the Doctor and Dodo note that the post office tower is complete; the Doctor has a funny sensation on his skin when looking at the tower so decides that it warrants investigation. Inside the tower, the Doctor surmises that the computer is the source of the sensation. Professor Brett reveals that the computer, named WOTAN (standing for will operating thought analogue) will become operational tonight. Deciding to challenge the abilities of WOTAN, the Doctor and Dodo ask it some questions - inlcuding what TARDIS stands for. WOTAN correctly defines TARDIS - much to the amazement of the travellers. Dodo feels a bit strange and so she sits down with Brett's secretary, Polly. They discuss great night spots in London and Polly suggests the Inferno nightclub. There, they meet a fed-up sailor named Ben Jackson.

At a press conference it is announced that in 4 days, on 16 July, all the world's computers would be linked through WOTAN. One journalist enquires if it is safe to put all such power in the "hands" of one computer? Sir Charles laughs off the idea: WOTAN is just a machine, it has no ambitions etc and certainly would not deem the human race as unnecessary. The Doctor, in attendance at the press conference, studies the model of WOTAN. Sir Charles is annoyed that Professor Brett was not at the press launch. Professor Brett is working late with WOTAN: he cannot shake the feeling that someone is watching him. As he prepares to leave, a strange force overcomes him and he asks what it wants. Brett then goes to the press conference but ignores everyone there and focuses his attention on Professor Krimpton, taking him away with little explanation.

After answering a telephone call, Dodo leaves the Inferno. She joins Professors Brett and Krimpton and Major Green in the tower control room. WOTAN then gives them their instructions...