Millions of years ago, Earth had a twin planet called Mondas. Mondas was the mirror image of Earth, but it drifted away on a journey to the edge of space. The scientists of Mondas realized that their race was getting weak, and their lifespans were becoming shorter, so they began to develop spare parts for their bodies. Eventually, their technology reached a level where practically the entire body could be replaced with cybernetic parts. One of the human parts that remains is the brain - but the Cybermen have neurosurgically altered their brains to remove all their emotions and all sense of pain, leaving them more machine than human. (In fact, as emotion is alien to the Cybermen's nervous systems, even having mild levels of emotion induced in their bodies can send them mad.) Not just logical and calculating, the Cybermen are also emotionless, ruthless and vicious. They desire to eliminate all dangers to their existence, and are very single-minded in their determination to do so, often setting up complex plans that are carried out in discrete phases. As a result of their machine parts, the Cybermen are very, very strong. They can knock people unconscious simply by clutching the person's head with one hand, and their grip is easily strong enough to break bone.
The Cybermen are constantly making modifications and improvements to their design. The earliest Cybermen still retain vestiges of humanity. Cloth covers their faces, and they still have names (like Krail, Talon, Shav, Krang, Jarl and Gern). They carry around a lamp-like device, which seems to be a heat weapon, set into a two-handled shield. This weapon gradually changes location, later being set within the Cybermen's chest units, then in the head lamps; these head lamps can also emit a mesmerizing beam, which hypnotizes their victim into becoming a servant of the Cybermen. Eventually the lamp weapon is altogether replaced by stubby guns on slings. Earlier Cybermen may also augment their lamp-weapons with long, thin guns that fire like flamethrowers, and they can emit an electronic flash from their fingertips that renders their victim unconscious. Later modifications make the Cybermen less and less human. Their heads eventually become almost entirely metal, and the design of their bodies becomes more sleek and silver.
Cybermen create more of their kind by converting humanoids. This process replaces human limbs and organs with cybernetic parts, and utilizes a drug that irreversibly affects the brain (presumably to eliminate emotional reactions). The Cybermen then establish control of the human body through a metal skullcap. At the final step, humans are put into a capsule that converts them, bit by bit, into full Cybermen. The Cyber-conditioning is not always successful, and rejects are used in work parties.
Cybermen also make use of devices that control people's minds, apparently through sonic control, making them dedicated servants of the Cybermen (a function that is also sometimes a feature of their head lamps, as mentioned above). Fortunately, this control signal can be blocked by affixing a metal plate and a transistor to the back of the neck. They can weaken the humans they wish to control by first infecting them with a neurotropic virus, which follows the course of the nerves under the skin; infected humans are distinguished by the black lines on their skin that the virus creates. As an alternative to human labor, the Cybermen can build matte-black featureless androids with weapons set into their palms that turn people into puddles of steaming ichor. They use these androids as sentries to guard their bombs.
Cybermen are fond of using a variety of Cyberbombs, including small charges that can blast through bulkheads, the Cyber-Megatron bomb which is fired at planets from space, and a bomb that can be hidden behind a guarded hatch on the target planet. At one point they mine the surface of Telos with high explosives, with the intention of studying the effect of the explosion on the planet's atmosphere. Among their other devices is a thermal lance, which can burn through metal, and small silver capsules which can convert oxygen into pure ozone.
Early Cybermen also make use of Cybermats. These small metallic creatures look like a cross between a rodent and a worm, and can move about fairly inconspicuously. They detect humanoids by tuning into their brain-waves. Cybermats are usually armed with a poison (with similar effects as the Cybermen's neurotropic virus, only fatal) that is transmitted through their "bite", and presumably their bite can be modified to corrode metal. Cybermats are quick-moving and able to jump long distances, which is usually how they attack their prey. They can also emit a beam from their eyes, which range from painful at best to apparently fatal at worst. They reportedly have a range of at least ten feet, although it is unclear whether this applies to the range of their beam weapons or the range of their jumping. Cybermats can be disabled by a variable audio-frequency with high-current phase contrast, which seems to overload their systems, causing them to lose motor control and explode.
Cybermen have few real weaknesses - guns certainly do not harm them (except, of course, for when they do) - but the one thing that will consistently cause them problems is gold (dust, arrows, coins, et cetera). Gold dust clogs up their breathing apparatus, quickly killing them. This weakness was thought to have destroyed them during the Cyber War, when humans invented the glitter gun as a means of combatting them, but their offensive was not as successful as they had thought. Their other most exploitable weakness is that they always respond to the distress of their own kind, making them more predictable and vulnerable. Early Cybermen are sensitive to bright lights and highly susceptible to radiation. Aside from gold, they can also be destroyed by Cyber-lamps, grenades, bazookas, being caught in an electrical forcefield, liquid plastic fired at their chest unit, laser cannons, devices which cause strong emotions, having their heads lopped off, sonic lances jammed in their chest units, exploding vastial (a mineral found in the cold regions of Telos), fire from shuttle rockets, and other Cyberguns. They can also be killed with a powerful solvent made from acetone, benzene, ether and alcohol that dissolves their chest units. Puncturing their fluid lines injures them, although it does not seem to destroy them, and when this happens they leak green fluid.
Cybermen follow a Leader, who is often marked by a design feature, such as a black head lamp. The Cyber Controller, with its domed head unit, is a higher rank yet. When working with humans, a Cyber Director - which looks like a large machine made of glass and tubing - may be employed as a liaison to organize and direct events. Patrolling is done by Cyberman scouts, some of whom are solid black (possibly for easy concealment). Cybermen tend to travel in large fleets containing thousands of warships which can be cloaked, and they can also pack themselves for long storage so that a single room can hold hundreds of Cybermen waiting to be revived. Sometimes they are encased in cocoons, or plastic sheets. They can also put themselves into hibernation for long periods of time, provided they have a sufficiently cold environment for it. If the Cybermen wish to transport themselves even more surreptitiously, they can do so by apparently enclosing themselves - or Cybermats - inside shrinkable silver eggs that can be absorbed through matter (apparently this procedure causes a temporary breach, but no permanent damage). At a preordained time, these eggs can then revert to their full size and the Cybermen or Cybermats within can break out.
There have apparently been occasions on which the Cybermen have encountered the Doctor in untelevised adventures. These include an instance where the second Doctor, accompanied by at least Jamie, defeated them on Planet 14.
A brief summary of their televised exploits:
DD - The planet Mondas, equipped with a propulsion unit, approaches Earth from deep space. With its energy nearly exhausted, it needs to draw all the energy from the Earth. The Cyberman plan to transfer the Earth's population to Mondas, as the planet will be destroyed. The Doctor predicts that, if they can only wait long enough, Mondas will eventually absorb too much energy and blow itself out, shrivelling up. Either way, one planet must be destroyed for the safety of the other. And indeed, the Doctor's prediction is correct; after holding the Cybermen at bay for a sufficiently long period of time, Mondas seems to melt in space. With their planet destroyed, the Cybermen also disintegrate.
HH - The Cybermen infect the crew of a weather-control station on the moon, using sugar contaminated with a neurotropic virus. They convert some of these crewmembers into Cybermen, and intend to take over the Gravitron so that they can use aberrant weather patterns to destroy the surface of the Earth. The Doctor is able to point the Gravitron at the Cyber-fleet to deflect them back into space.
MM - An archaeological party breaches the Cybermen's tomb on Telos. One member of the group awakens the sleeping Cybermen, hoping they will serve him, but of course the Cybermen intend to do no such thing. The Doctor manages to find a way to put them back into cryogenic suspension, and then re-seals their tomb.
SS - The Cybermen commandeer a rocket and steer it toward space station W3. They manage to secretly infiltrate the station and disable its defensive laser, so that the crew will try to obtain replacement supplies from their rocket. The Cybermen then take control of the crewmembers, who smuggle a pair of Cybermen back onto the space station. The Cybermen then begin to sabotage the station, hoping to kill all of the humans so that they can take the station over and use it to attack the Earth. The Doctor and the station crew are able to destroy the Cybermen on board the station, repel the invading Cybermen with a neutron forcefield, and destroy the Cybership with the repaired laser, modified with the TARDIS' time vector generator.
VV - The Cybermen work with Tobias Vaughn, leader of electronics corporation International Electromatics, for five years planning an invasion of the Earth. They intend to transmit control waves through the micromonolithic circuits that IE have installed in their products, which will act as artificial nervous systems and produce a Cyber hypnotic force that will control everyone in the world. The Doctor and his associates are able to protect themselves by wearing depolarizers made of neuristors on the backs of their necks, and the Cyber fleet attacking Earth is completely destroyed with missiles.
PPP - Zorg's miniscope apparently contains at least one Cyberman, which is later sent back to its proper time and place.
4D - Voga, the famous planet of gold and cause of the Cybermen's defeat during the Cyber War, becomes the Cybermen's next target. Their attempt to detonate bombs in the planet's core fails, so they decide to crash Nerva Beacon into Voga. They depart before the collision, and the Doctor is able to avert it just in time.
5M - Romana and Skagra find a Cyberman among the cryogenically-frozen prisoners of Shada.
6B - the Cybermen plan to destroy the members of a peace conference by piloting a freighter into the Earth. When the freighter travels back through time, the Cybermen end up killing off only the dinosaurs. (well, and maybe one other person.)
6K - several Cybermen are brought to the Death Zone on Gallifrey, where they are either hacked to bits by a Raston Warrior Robot or fried on a booby-trapped chessboard in Rassilon's Tower.
6T - when a time vessel lands on the Cybermen's adopted planet Telos, they see an opportunity to stop the destruction of their home planet, Mondas. Capturing the time vessel, several Cyberman travel back to Earth, 1985, hoping to cause Halley's Comet to impact the planet. But one of the indigenous natives of Telos - a Cryon named Flast - is able to cause an explosion that completely destroys the Cybermen's base, assuring the safety of Earth.
7K - the Cybermen attempt to gain control of the Nemesis statue, hoping to use validium to turn Earth into a new Mondas. The Doctor instructs the statue to destroy the Cyber fleet instead of following the Cyber Leader's orders, while Ace and Lady Peinforte pick off the remaining Cybermen with gold coins and arrows.
