In Professor Thripsted's Flora and Fauna of the Universe, Usurians are listed under "poisonous fungi". But such a simple classification is a great injustice to the Usurians, who are considerably more than just poisonous fungi. In their natural form, they look like sea kale with eyes, but they can shapeshift by means of particle irradiation, which will hold them in the desired shape so long as they remain in a special mobile chair.
The Usurians are fiscal geniuses, very talented at facts and figures, finances and predicting trends, market surveys and commercial developments. They are the founders of a vast Company, whose purpose and/or products are unknown. Certainly no-one who works for it really knows its function. The head office of the Company is on Usurius, but it likes to broaden its scope. The Usurians feel that the use of economic power is much more effective than war at conquering planets. At one point the Company was looking for property in Earth's solar system. As the Earth was running down and its people dying, the Company made a deal with the inhabitants. Usurian engineers made Mars into a habitable planet, then moved the population of Earth to Mars. Then, naturally, the Usurians taxed them to recover their capital costs. When the resources of Mars were eventually exhausted, the Usurians created a new environment on Pluto (the intervening four planets being considered unusable), with six suns (in-station fusion satellites), one for each large Megropolis on the planet.
The Company on Pluto is overseen by a Usurian called the Collector, whose assumed shape is short and bald, with pasty grey skin, spiky eyebrows, green-tinged eyes and lips, painted fingernails and a very nasal voice. The Collector is quite the sadist - he enjoys torture (and even has his Palace built over the Correction Centre because he likes to hear the screams) and broadcasts executions live on special occasions (although people generally prefer to attend them in person, admission by ticket, proceeds to benefit the Company). In his office is a voice-activated information retrieval system, capable of intoning facts on most any subject if the Collector asks, a vault with unknown contents (possibly information, possibly money) and a computer which feeds and analyzes data from each Megropolis. The Citizens of the Megropolises are forced to work long hours for very little pay, and increasingly greater taxes. They are kept apathetic by a high pressure system which sends the gas PCM - pentocyeleinicmethylhydrane, an anxiety inducer - into the atmosphere. The Company claims that it eliminates airborne infections, when in fact it eliminates freedom.
Six suns being fuelled and serviced means that the costs of operation on Pluto are very high. Once the planet has been exhausted like Mars and Earth, there is no other economic place to move the Usurians' slave labor force, so the Company branch on Pluto will close. Within a few years, the suns will run down, and the humans will be left to die. As the Usurians have other operations which produce higher returns with less labor, they do not see this as a problem.
What the Collector does not bargain on is that the Citizens of Pluto decide to start a rebellion instead. The Collector believes it will be quelled, and decides to turn on sprinklers armed with the deadly gas dionane. Usurians, being fungi, do not breathe air; however, the Collector's human guard is none too happy about this decision and helps the Doctor and Leela to stop him. Megropolis 6 is taken over by the Citizens, and the Doctor feeds a 2% growth tax into the Usurian's computer, index-linked. This information tells the Collector that the economy has been ruined, and, unable to take it, he shrinks down in his chair to a green blob, which disappears down a hole in the seat while advising the computer that immediate liquidation is necessary. The hole is plugged to prevent the Collector from re-emerging, and presumably he is sent back to Usurius. The settlers, meanwhile, intend to return to their revitalized Earth.