In stature, the Sensorites are somewhat shorter than the average human; but in appearance, they are very different. Their hearts, for example, are in the center of their bodies. Their egg-shaped bald heads are covered with tufts of cottony hair, their ears are pointed and their eyes are lidless. They wear coveralls and seem to have disc-shaped feet. Additionally, they seem to be able to survive the vacuum of space unaided, although they do prefer to travel in spacecrafts which make a high-pitched whine as they approach.
The Sensorites communicate verbally, but can also communicate telepathically if they want to have a private conversation or if they wish to converse with someone a great distance away, in which case they do so by pressing a flat communication disc to their forehead. They can receive quite well without the disc, but require it to send telepathic messages - and, in fact, the disc can even enable humans to project telepathically. The Sensorites can also use their telepathy to control crafts, control brains or blanket out the minds of other beings. The number of telepathic frequencies covering the surface of the Sensesphere are numerous. However, their ability to communicate telepathically is not entirely advantageous - they can be disoriented by a concerted mental attack. It also means that they do not entirely trust non-telepathic races. Anyway, they find human names absurd.
Their remarkable telepathic abilities are countered by their more delicate aural and visual reception - more simply put, the Sensorites hate loud noises and darkness. Their eyes are completely dilated, making them unable to see in even semi-darkness. Sound can be used as a weapon against them, even yelling. Extreme sound stuns their brain and paralyzes their nerves.
The Sensorites live on a planet known as the Sensesphere, where the society is structured in both family groups and a caste system. The elders think and rule; the warriors fight; and the other Sensorites work and play. There also seems to be a scientist caste. Each caste is led by a First, who is aided and advised by a Second (personally chosen by the First), as well as a Third and so on. These are distinguished by markings on their coveralls - for example, the First Elder wears a twin sash, while the Second Elder wears only one sash. While these distinctive markings set the elite apart from the rest, the ordinary people are satisfied with their similarities. There is no disgrace in belonging to any particular caste; it is simply what one is best fitted for, implying that birth has nothing to do with caste selection. Even the City Administrator hails from the lower caste. However, the assertion that all Sensorites are content is a fallacy - some are assuredly not.
In some ways, the Sensorites are very technologically advanced. Among their inventions is a hand-ray weapon - shaped like a metal circle, with crosshairs in the center, on a handle. This can paralyze a being for up to one hour from a distance of as much as thirty yards, burn wood, and even remove the lock from the TARDIS. They have created a disintegrator which can be aimed precisely at almost any point; this is enabled with a firing key, which only the First Warrior and the First Elder possess. They use portable radio-electric lights and have a system equipped with electrothermal cuttles in their palace which enables them to tell where people are. And yet, despite these wonders, their grasp of some basic scientific principles is astonishingly poor - they are not very skilled in scientific method, for example.
Ten years prior to the Doctor's arrival on the Sensesphere, five humans visited the planet. The Sensorites welcomed them, although their minds were closed...but it was a mistake, for the men disovered that the Sensesphere was a very rich planet. They quarrelled, and two of them took off from the planet, but the ship exploded a mile up in the atmosphere. The Sensorites assumed that the others had hidden themselves aboard and fought for control of the ship. Following the visitors' arrival, the Sensorites began dying in greater numbers every year, and soon became severely xenophobic. When a ship approached the planet, the Sensorites would put its occupants into a deep, death-like sleep and provide them with food, but they would not allow them to leave orbit.
When Maitland and his crew arrived at the Sensesphere, one of the crewmembers - John - discovered that the planet contained high levels of molybdenum. The Sensorites detected his excitement and had to silence the ship's crew, lest news of the planet's riches spread. The Doctor was able to ingratiate himself with both the ship's crew and the Sensorites, and persuaded the planet's inhabitants to tell him about the disease that had been destroying their population.
As Ian soon discovered, the first symptom of the disease was a burning throat - and after the appearance of the first symptoms, no-one had lived longer than three days. Oddly, they had noticed, the disease did not affect the Elders. The Doctor discovered why when he was told that the Elders only drank a special kind of "crystal water" from a rare spring in the Yellow Mountains that were around the city. The other Sensorites drank water from an aqueduct beneath the city, though it was well-refined before consumption. The Doctor investigated and narrowed down the cause of the "disease" to atrophine poisoning, a diagnosis borne out when he discovered deadly nightshade in the aqueducts. It had been introduced into the water system by a group of humans who had been living in the aqueducts ever since their crewmates had attempted to leave the planet. Thinking they were at war with the Sensorites, they had tried to kill them all by poisoning the water supply. The Doctor was able to cure the Sensorites and free Maitland's crew.
