Before the Earth's evacuation in the far future, the Monoids visited the planet. Like Earth, their own planet was dying, and they offered the humans their services in exchange for passage on the humans' giant ship that was taking the planet's population to a new home. The humans agreed, and for hundreds of years, the Monoids served as their servants aboard the vessel. Most of the Monoids were, like the humans, preserved in the form of microcells and stored in trays of one thousand Monoids apiece.
Physically, Monoids are about the same height as humans, but their skin is dark and scaly. They all have mop-top hair, although some have lighter hair than others. Their feet are like flippers, which probably contributes to their slow movement. They have no visible mouths, relying on sign language for communication, although they apparently eat and drink through a hole in the side of their necks. The most striking feature of the Monoids, though, is the single eye in the center of their heads. One presumes that their name was given to them by the humans, and after hundreds of years living with the name, the Monoids picked it up themselves.
The Monoids are more knowledgeable than the human Guardians give them credit, but they are either lacking somewhat in intelligence or very gullible, as they can be fooled easily. Monoids are fairly strong, a helpful trait in their subservient role. The voice boxes that they acquire sometimes require the Monoids to place a hand on it if they want to speak. However, this does not always seem to be the case. Monoids also emit a gurgling cry when they are killed.
When the Doctor, Steven and Dodo visited the ship (christened "the Ark" by Dodo), they brought with them a strain of the common cold virus, to which neither the Guardians nor the Monoids had any resistance. The virus spread rapidly and killed several Guardians and Monoids before the Doctor was able to devise a cure. Thinking all was well, the TARDIS crew left - but a mutation of the virus appeared, and apparently was more virulent to the human population than the Monoids. The fever sapped the humans' will, and the Monoids saw their opportunity to seize control of the Ark and end hundreds of years of servitude. The Guardians had encouraged research into voice boxes for the Monoids, and had also developed heat prods which the Monoids could use as weapons. With these advantages, and the fact that the Guardians were unprepared for conflict, the Monoids managed to kill many of them and make prisoners of the rest.
Following the revolution, the Monoids became the masters, and the humans their servants. They required complete obedience of the humans. In the new Monoid culture, a hierarchy was established - the leading Monoid was known as One, his direct subordinate as Two, and so on. The leaders apparently met in a Grand Council. Their intention was to create a Monoid world on Refusis once the Ark arrived there, and destroy the humans as revenge for their long-term secondary status. To this end, One had a fissure device installed in the head of a giant statue that had been intended to represent a human, but was changed to bear Monoid features.
Not all of the Monoids were convinced that this was the best plan. Once on Refusis, some of them rebelled against One, and a civil war broke out briefly. Most of the Monoids were killed. Those who survived were ordered, by the Refusians, to begin living in peace with the humans. Presumably the rest of the humans and Monoids were taken out of their microscopic stasis and began living together in harmony on Refusis II.
