The Usurians are a spacefaring race of poisonous fungoids from the planet Usurius which are known to operate in a huge interstellar Company and dominate other races by economic means. They are described by the Doctor as having the appearance of "a lump of seaweed with eyes". They do not breathe air and possess technology which allows them to assume a humanoid form.
Usurian culture is highly based on economic dominance, which they consider more effective than military tactics. They acquire enormous profit by exploitive taxing of their subjugated races. They are also known to sell their products to more advanced races in the galaxy. They are aware of the existence of the Time Lords of Gallifrey, but do not consider them a viable market for their consumer products.
At some point in the distant future, during a period of severe crisis in which the Earth was becoming increasingly uninhabitable for humanity, the Usurians assisted Human colonists in settling on Mars, exploiting them with oppressive taxes in return. When Mars too became uninhabitable, they relocated the Human populace to Pluto, setting up six artificial suns on that world to make it inhabitable, and claiming this as an excuse to increase their taxes even more to pay for their costs of terraforming and maintaining the Plutonian colonies. After several generations, the Humans on Pluto had been tamed by the use of a chemical agent in the air called PCM, which provokes instinctive fear and inhibits the willpower, preventing rebellion. The resulting culture became completely submissive to their oppressors, only vaguely aware of their origins, and no longer aware of the real nature of those ruling the Company, as well as the purposes of it. As the Plutonian colonies were becoming less profitable, the Usurians planned to eventually retire that branch of the Company and leave the Humans to die. Before that could happen, a resistant movement managed to gain extensive support, assisted by the arrival of the Doctor and Leela, leading to a full scale rebellion which allowed Humans to regain their freedom and eventually re-colonize the Earth.
- Doctor Who season 15 (Fourth Doctor) – The Sun Makers (1977)