F is for... Flesh

The Flesh was the name given to a milky organic substance which could be programmed to mimic human beings in the 22nd Century. Workers engaged in hazardous tasks could connect themselves up to a vat of Flesh and create doppelgangers, known as Gangers, which could then carry out the dangerous work for them. These avatars held the same personality traits and memories of their originals for as long as the real people were connected. The Doctor decided to visit an acid mining operation off the coast of England in order to study the substance, for reasons he kept hidden from companions Amy and Rory.
Their arrival coincided with a solar storm which broke the link between the miners and their avatars - but left the Gangers intact as living individuals. Having all the memories of their originals, they insisted on the right to survive. Until they stabilised, the Gangers would revert to having blank white features. Conflict between the two groups soon broke out.

One Ganger - of a young woman named Jennifer - decided to lead a revolt against all humans for what they had done to her kind, discarding them to rot when they were of no further use. The Doctor touched the vat of Flesh, and it absorbed his likeness and personality, creating an avatar of him. Both Doctors then attempted to find a peaceful solution to the situation, but the Jennifer Ganger decided to sabotage the mining complex. She transformed into a hideous parody of the human form. The two Doctors swapped shoes, so that the real Doctor could see what his companions really thought of the Gangers. The Gangers decided that they too wanted peace, after a number of them had been killed, along with some of their originals. The Ganger Doctor and the Ganger Cleaves, chief of the mining operation, sacrificed themselves to allow the others to escape and to destroy the Jennifer creature.

The original Cleaves, and the Ganger of crewman Dicken, were dropped off by the Doctor at the HQ of the company, Morpeth Jensen, which employed the Gangers, so that they could tell of what had happened and to argue for Ganger rights. Another Ganger, of a man named Jimmy, had been dropped off at home so that he could celebrate his young son's birthday.
Back in the TARDIS, the Doctor informed his companions that Amy was not real. She was also a Flesh avatar, having been abducted and replaced some time before. He had guessed this, when the TARDIS sensors had failed to confirm or deny her pregnancy, and this is why he had wanted to know more about the substance.
He and Rory then set out to find her, discovering her on the asteroid of Demons Run, where she had given birth to the child who would one day grow up to be River Song. They rescued Amy and the baby - Melody - only to discover that the child had been swapped with a Flesh avatar.

Played by: Karen Gillan (Amy), Raquel Cassidy (Cleaves), Mark Bonnar (Jimmy), Sarah Smart (Jennifer), Marshall Lancaster (Buzzer) and Leon Vickers (Dicken). Appearances: The Rebel Flesh / The Almost People, A Good Man Goes to War (2011).

• Technically, Amy is a Flesh avatar from some point during The Day of the Moon right through to The Almost People.