The Slitheen are a criminal family of aliens from the planet Raxacoricofallapatorius, primarily concerned with crooked business dealings and the attainment of profit. Like other Raxacoricofallapatorians, they are around eight feet tall, with green skin, large bellies, and long arms ending in vicious claws. Their long necks lead to a babylike face with fat cheeks, a small but big-lipped mouth and wide black eyes with lids that fold in from the sides. Their bodies are made of living calcium phosphate, and they hatch from frond-bearing eggs. When threatened, they hiss. Female Slitheen produce a poison which they can expel in the form of a dart from their finger, if threatened, or - as a last resort - in the form of a gas exhaled from the lungs. The Slitheen seem to have no particular taboo on nudity; in fact, they encourage and revel in it.
However, when necessary, they can take on the form of other beings. Because they are large aliens, they need large bodies to inhabit; among humans, they tend to choose overweight specimens. When they find a suitable person, the Slitheen kill them and turn them into a skin suit (how they dispose of the bones and organs is unclear). A zip is installed around the forehead, and the Slitheen wear a compression field around their neck which shrinks them down so that they can fit inside the skin suit. Once zipped inside, if the zip is hidden well enough, they are indistinguishable from an actual human, except for one thing: flatulence. The compression field has the unfortunate side effect of weakening the calcium in the Slitheen's bodies, making them susceptible to acetic acid; if enough vinegar is poured onto them, they explode. The compression field also necessitates a big gas exchange, which periodically needs to be expelled (and has an odor akin to halitosis, owing to the calcium decay caused by the field); the Slitheen blame it on a nervous stomach, if anything. To emerge from their skin suit, the Slitheen simply have to reach up to the left side of their forehead, draw the zipper across the forehead and around the back of their head, then pull the opening down over their body and step out of it. The compression field seems to create a great deal of electrical energy, which crackles and flashes blue as the Slitheen unzip and shed their skin suit. This energy apparently even affects the ambient lighting, which dims until the Slitheen step out of the suit, during which time the only light comes from the crackling blue energy.
Slitheen culture places a strong emphasis on the ritual hunt, and are encouraged (or perhaps forced) to commit their first kill at an early age, around 13 years old. Slitheen have an excellent sense of smell, and they can run very fast, meaning that they find it easy to track and pursue their prey. One swipe of their powerful claws is enough to kill. Slitheen have an apparent telepathic bond with one another, such that they are immediately aware when one of their number is killed; this may also be related to the apparent interconnectedness of their compression fields, where tampering with one causes a similar effect on them all. They also seem to enjoy indulging in life's little vices while in their human suits, including food and sex. They seem to dislike the contents of a discharging fire extinguisher. Raxacoricofallapatorians are a spacefaring race, and their ships are powered by slipstream engines.
The Slitheen, however, are so notorious for their crimes on Raxacoricofallapatorius, that their entire family was tried in its absence and found guilty, with no chance of appeal. This basically means that if any Slitheen return to the planet, they are to be immediately put to death. This involves a particularly lovely form of public execution in which the Slitheen is lowered into a cauldron filled with a thin solution of boiling acetic acid, which strips away the skin and causes the internal organs to fall out into the acid solution, thus turning the Slitheen into soup while it is still alive and screaming.
Approximately ten Slitheen travel to Earth and park their ship 100 fathoms deep in the North Sea, with the intention of causing a nuclear war that wipes out all life on Earth and reduces the planet to slag; they will then be able to divide it up into radioactive chunks and sell it off piece by piece to power starships and cargo ships. While they set their plan into motion, the Slitheen beam an advertisement out into space in a repeated loop. Meanwhile, they begin to murder their way to the top of the government, taking over useful bodies along the way: Joseph Green (MP for Hartleydale and Chairman of the Parliamentary Commission on the Monitoring of Sugar Standards in Exported Confectionery, taken over by Jocrassa Fel Fotch Pasameer-Day Slitheen), Margaret Blaine (MI5, taken over by Blon Fel Fotch Pasameer-Day Slitheen), Oliver Charles (Transport Liaison; this suit is later swapped for General Asquith of the British military), Police Commissioner Strickland (taken over by Slip Fel Fotch Pasameer-Day Slitheen), Group Captain Tennant James (RAF), Ewan McAllister (Deputy Secretary for the Scottish Parliament) and Sylvia Dillane (Chairman of the North Sea Boating Club). When the government notices the blip of radiation left by the Slitheen ship in the North Sea, they launch a smaller ship into space, which makes a slingshot orbit of the Earth and touches down again in London, "crashing" into the Thames (but actually merely parked). The ship is crewed by a pig, which the Slitheen have augmented and put into a spacesuit to pose as an alien. With the government investigation now focused on the clear alien threat, they assemble a team of alien experts (including UNIT) which the Slitheen plan (successfully) to assassinate, using electrified ID cards. They then claim to the world that the experts were killed by aliens whose mothership, hovering directly over the United Nations, is armed with massive weapons of destruction capable of being deployed in 45 seconds. By creating worldwide panic, the Slitheen convince the human race that unless the UK is able to obtain the nuclear-missile launch codes from the UN and strike first, they face total extinction. Secretly, the Slitheen plan to use the codes to attack other countries with the nuclear missiles, who will then retaliate, causing World War Three and nuking the planet while the Slitheen ride out the holocaust in their spaceship under the Thames. The Doctor, with Mickey's help, is able to remotely activate a missile and aim it at 10 Downing Street, destroying the building before the Slitheen are able to launch their missiles. Once the Slitheen signal has been dispersed, they pose no more threat to the Earth.
However, one member of the family survives: Blon Slitheen, aka Margaret Blaine, who is able to teleport herself out of Downing Street before the missile hits, ending up in a skip on the Isle of Dogs. Blaine makes a rather public life for herself as the Mayor of Cardiff (albeit one who does not allow personal photos to be taken) and institutes a plan, called the Blaidd Drwg project, to build a nuclear power plant in the heart of the city, on the site of Cardiff Castle. In order to facilitate her project, she makes sure that the European safety inspector, the entire Cardiff Heritage committee, the architects and her nuclear advisor meet with untimely "accidental" ends. Otherwise, she fears, they will let slip the news that she has designed the power plant to explode and open the temporal rift that runs through Cardiff. Blaine then plans to use her (stolen) tribophysical waveform macrokinetic extrapolator to shroud her in a forcefield; she can then feed it co-ordinates and let it ride the wave of the rift like a pan-dimensional surfboard, right out of the solar system. As a backup plan, if her scheme is discovered by someone from a high-technological culture who would be interested in the extrapolator, she has programmed it to lock on to the nearest alien power source (presumably belonging to the person who wants the extrapolator) and use it to open the rift instead.
Blon Slitheen possesses a pair of earrings that double as a teleportation device when they are pressed together, although it can only convey one person. It is implied that she used to be married. Blon suffers the occasional attack of conscience that keeps her from killing everyone who threatens her, but it is nevertheless a rare occasion on which she actually spares a life. However, deep down, she does seem to regret the murderous, criminal path of her life, and longs for a second chance. Her wish is granted when she looks into the heart of the TARDIS, which regresses her to an egg - an opportunity for which she seems truly grateful. The TARDIS crew take baby Blon to a hatchery on Raxacoricofallapatorius where she can start over with a new family.
