Powers and abilities
They lacked a proper sense of self-conscience, and possessed a limited intelligence, which kept their desires limited to pure survival. They were able to create physical duplicates of other living beings and, to a certain extent, copy their thoughts and memories. However, they were unable to recreate particular physical structures, as those of handicapped people and species only seemingly human, as Time Lords.
In order for the duplicates to work, the originals had to be alive, so the changelings knocked them unconscious and kept them imprisoned inside a sort of dark jelly, as in a coma. The psychic link between the original and the copy was so strong that the original could feel any pain or injury caused to the copy.
As they lacked personal indivualities, they all existed thanks to a knot-heart, whom they placed on the man or thing they used as a source of their power. If this source were a living being, in order to keep him active, they kept him active and conscious, although trapped in jelly.
They existed under the ocean for millennia, until Michael Newman, a Jamaican immigrant to England, who had latent psychic abilities, fished out their knot-heart, the central brain connecting them all, and brought it to London in the 1950s. His powers awoke them, and they instinctively possessed his mind and created a duplicate of him, which disembarked in London in his place. The changelings kept using him as a source of energy as they started copying other people he knew, with the purpose to live all on mainland.
When the TARDIS randomly passed near their time period, they sensed its massive psychic energy, and decided to use it as a source of energy. They then psychically connected to it and forced it to land in London. The attack activated the defence system of the ship, which locked itself and ejected its occupants, which by that time were the First Doctor, Steven Taylor and Sara Kingdom. The attack had also the effect of sending the Doctor into a coma. The crew were given shelter by Joseph Roberts, Michael Newman's uncle, in whose house also lived the fake Michael together with Audrey, the wife of the original one. That night, the changelings dragged away both the Doctor and the TARDIS to their centre of operation, a warehouse at the dock where Michael worked. They went on using the TARDIS as a source, but made the mistake of deeming the Doctor unimportant, and did not realize that he was still awake and conscious of his surroundings.
Two weeks later, Steven found their hideout by following the fake Michael and other duplicates to the warehouse. He was trapped and copied, but managed to awake the Doctor from his coma, and he in his turn went to find Sara. With the information he gathered, he instructed Sara to go to the warehouse, take away the knot-heart from Michael's chest and destroying it. Sara did just that by shooting it with her gun while throwing it in the air, and the duplicates vanished, as the pieces of the knot-heart fell into the water of the Thames.