The Nestene Consciousness was a disembodied life force which could inhabit plastic.
The Consciousness had the ability to possess plastic, shunting some of its consciousness into it and making it quasi-organic. While they were part of the group mind, Autons seemed to have some individuality, such as when Channing could directly control another Auton and was unaware when the Scobie Replica was destroyed. A Nestene creature served as the central brain/nervous system for Autons, but required the intelligence of all energy units to be created.
Over the course of its invasions it possessed the Autons, inflatable chairs, dolls, telephone cords, plastic daffodils designed by the Master, and rubbish bins. It could also create a body out of the plastic itself, allowing it to manipulate its surroundings. After the stresses of a war, it manifested as a more liquid form, which had a humanoid face.
The Nestene Consciousness was a telepathic gestalt entity. It travelled through space in hollow spheres known as energy units. These units made planetfall in formation, appearing like normal meteorites to indigenous lifeforms. When they became active, the spheres would pulse with light and ring with electronic beeps. Once the consciousness within those spheres was recombined and placed in an environment tank, the Nestene Consciousness would take on a form that it believes to be perfectly adapted to the planet on which it has landed.
On some worlds, its takeover strategy was as follows: The first spheres to arrive would attempt to place a local being under some form of mental control. It would then take over a plastics company and fashion plastic automatons known as Autons. These Autons might include replicas of key authority figures as well as simpler automatons. The Nestene would then use the Autons as an army to conquer the planet. In some cases, the Nestene required a relay station or transmitter to control Autons. This only seems to be necessary to control large numbers at once.
In 102, the Nestene were among the factions who joined the Alliance. They came to Stonehenge and helped imprison the Eleventh Doctor in the Pandorica in order to save the universe. As well as the obvious spaceships, they also created a legion of Autons disguised as Roman soldiers. These Autons, believing themselves to be genuine Roman soldiers, tricked the Doctor, who realised too late what they were, and imprisoned him in the Pandorica. When the plan failed, the Autons and the Nestene were turned to dust and removed from the universe along with the rest of creation, except for an Auton copy of Rory Williams, who rebelled against his nature and tried to help the Doctor. They were returned when the Doctor managed to "reset" the universe.
The Nestene's first attempt to invade the Earth in the 20th century, using human-looking Autons to replace key government figures, was thwarted by UNIT with the help of the Third Doctor. The remains from this attempt were stored by UNIT.
In their next attempt, the Master obtained one of the Nestene meteors and took over a plastics company. Using the company to create fake daffodils, disguised Autons would hand out these flowers for free. Eventually, once millions had been distributed, the Nestene would activate them by remote radio signals, causing them to suffocate and kill millions of people. The Master also created dolls, inflatable chairs and telephone cords to kill difficult individuals. The Third Doctor was able to figure out this plan. He convinced the Master that the Nestenes were too dangerous to be reliable allies, and they reversed the radio beam the invasion force was coming in on, sending it back into space.
In 2005, the Nestenes infiltrated Earth once more, using warp shunt technology. The Nestenes had lost their food planets in a war, so their intention was to overthrow and destroy the human race, as Earth was ideal for their consumption needs, being filled with smoke, oil and various pollutants. Rose Tyler eventually defeated them by knocking a vial of anti-plastic solution that one of the Autons had confiscated from the Ninth Doctor into the vat of molten plastic which housed the main bulk of the Consciousness, causing it to explode.