The date is 28 February 1981 when the TARDIS materialises on board a massive spaceship en route for Earth. As the travellers leave the TARDIS to investigate, putting on space helmets with air supplies, their progress is monitored by hovering monopticons – mobile cameras. 

Adric and Nyssa are separated from the Doctor and Tegan, and the latter meet up with their hosts – the frog-like Urbankan Monarch (Stratford Johns), and his ministers Persuasion (Paul Shelley) and Enlightenment (Annie Lambert). Monarch explains that they are all headed for Earth – their first visit in 2500 years – and offers the Doctor his hospitality for the journey. Tegan, meanwhile, sketches the latest fashions for Enlightenment to see. 

Adric and Nyssa have been exploring and meet up with the Doctor and Tegan in their rest room. There all of them meet Bigon (Philip Locke), an Athenian; Villagra (Nadia Hammam), a Mayan of the Americas; Lin Futu (Bert Kwouk), a Chinese; and Kurkutji (Illarrio Bisi Pedro), an Australian Aborigine. All these people are travelling back to Earth. 

Monarch’s ministers also enter the rest room but their appearance has changed to that of Tegan’s sketches. Enlightenment explains that Urbanka’s sun, Inokshi, collapsed into a black hole a thousand years ago and that the population – all three billion of them – are on board the space ship awaiting a new start on Earth. The Doctor decides to explore and deactivates the monopticon in their quarters. The time travellers become split up: Nyssa and Adric find themselves in areas of the ship tended by zombie-like Aborigines, Chinese and Greeks while the Doctor and Tegan attend a Recreational – dance and music from the different ethnic groups on board. Bigon quietly tells the Doctor that he must speak with them and when they have returned to their room, he reveals himself to be an android. 

Nyssa and Adric are brought before Monarch who explains that he intends to bring the benefits of an android existence to Earth. Adric seems convinced but Nyssa is horrified. She is hypnotised by Enlightenment and then taken to the Mobiliary where her personal history will be placed on a microchip. 

Bigon tells the Doctor that Monarch caused the destruction of Urbanka and he now intends to strip the Earth of its silicon, carbon and other minerals in order to achieve his great plan which is to travel faster than light and thus return to the creation of the Universe where he believes he will meet himself as he thinks he is God. He intends to use a poison produced by the Urbankans to shrink matter to subdue humanity. 

Tegan decides to get away and enters the TARDIS, inadvertently moving it into a position just outside the ship. Meanwhile the Doctor and Bigon manage to rescue Nyssa. Monarch is about to have the Doctor executed when Adric intervenes on his behalf. Monarch agrees to stay the execution as he hopes to learn more about the TARDIS from the boy. Bigon is scheduled to be re-programmed. 

The Doctor manages to warn Adric and the pair convince Lin Futu to help them and Bigon is also rescued. The Doctor manages to get to the TARDIS by space-walking between the ships – and with the aid of a carefully thrown cricket ball when he becomes stranded in space – and returns his ship to the recreational hall. Persuasion and Enlightenment – both of them androids – are disabled and their circuits thrown into space. 

Monarch orders that the life support systems be shut down to stop the time travellers and Bigon sets the ship on a return course to Urbanka. The Doctor takes the Urbankan poison to analyse but returning to the TARDIS, they are confronted by Monarch. The Doctor throws the poison at the Urbankan who shrinks away to nothing. 

The Doctor, Tegan, Nyssa and Adric bid farewell to Bigon, Villagra, Kurkutji and Lin Futu and leave in the TARDIS. Once on their way, Nyssa suddenly collapses …

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}