The Doctor with Tegan and Adric

The TARDIS materialises inside a very large, advanced alien spacehip. As The Doctor, Adric, Nyssa and Tegan Jovanka, leave the TARDIS to investigate, putting on space helmets with air supplies, their progress is monitored by hovering monopticons - mobile surveillance device which conveys the arrival of The Doctor and his three travelling companions to an observing being that is the commander of the spaceship.

Adric and Nyssa soon become separated from The Doctor and Tegan, as the latter meet up with their hosts - the green-skinned, frog-like Urbankan Monarch, and his ministers Persuasion and Enlightenment. Monarch explains that they are all headed for Earth - their first visit in 2,500 years. Monarch offers The Doctor his hospitality for the remaining four days of their journey.

The Doctor discovers that the Urbankans have made many periodic visits to Earth, each time getting speedier in their journeys. This time though they have left their homeworld after erratic solar activity, storing three billion of their species on slides aboard their craft, and it seems the current journey is their last and they now wish to settle on Earth.

Exploring Their Surroundings

After exploring Adric and Nyssa eventually meet up with The Doctor and Tegan in their rest room. There all of them meet Bigon, an Athenian; Villagra, a Mayan of the Americas; Lin Futu, a Chinese; and Kurkutji, an Australian Aborigine. All these people it seems are travelling back to Earth.

The Doctor though becomes suspicious of Monarch and soon learns that the Urbankan does not plan on having a peaceful co-existence on Earth. The time travellers become split up: Nyssa and Adric find themselves in areas of the spaceship 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. While watching the entertainment 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.

The Monarch

Adric and Nyssa 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 then hypnotised by Enlightenment and taken to the Mobiliary so that her personal history can be placed on a microchip.

Bigon tells The Doctor that Monarch caused the destruction of Urbanka and is now, aided by his ministers Persuasion and Enlightenment, engaged in a complex scheme to plunder from Earth the raw materials needed to enable him to travel back in time. Monarch 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 so as to subdue humanity.

Tegan decides to get away and enters the TARDIS, inadvertently moving it into a position just outside the spaceship. 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

The Doctor manages to warn Adric and the pair convince Lin Futu to help them and Bigon, who is about to be re-programmed, is also rescued. The Doctor manages to get to the TARDIS by space-walking between the Monarch’s spaceship and his time machine - and with the aid of a carefully thrown cricket ball when he becomes stranded in space - and returns the TARDIS to the recreational hall. Persuasion and Enlightenment - both of them also revealed as 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 spaceship 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, Adric, Nyssa and Tegan then bid farewell to the humanoid androids, Bigon, Villagra, Kurkutji and Lin Futu, who have decide to pilot the spaceship to a new home on a new world.

Back in the Console Room The Doctor dematerialises the TARDIS. However, once on their way, Nyssa suddenly collapses to the floor…