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The Half-Face Man was a Clockwork Droid assigned to the SS Marie Antoinette sometime in the 51st century with the mission of finding a fabled realm of paradise. At one point, the ship crashed on Earth in prehistoric times and remained stuck on the planet for millions of years. For eons afterward, lacking the proper technology to keep itself pristine, it stole replacement parts from the environment around it, some of which were taken forcefully from living creatures.
Eventually, it had added such a large mixture of biological parts into itself with its machine components that its appearance was half-man, half-machine. Consequently, it gained vestiges of emotion and personality, letting it recognise that it was committing murder and working towards fulfilling an empty goal.
The Half-Face Man was originally the control node aboard the time-travelling ship, SS Marie Antoinette, sister ship of the SS Madame de Pompadour. At one point, the ship crashed on Earth in the planet’s distant past. With the ship damaged, the clockwork droids aboard the ship remained stuck on the planet for millions of years, replacing their parts using both organic and mechanical parts. They did this time and time again as they got older, until there was virtually nothing left of their original selves at all. (Deep Breath)
By Victorian times, the Half-Face Man had begun harvesting organs from humans in the city of London. This included the eyes from a man named Alf and the hands from two separate humans. The droids incinerated their targets to conceal what had been taken from them, ensuring their presence in London was invisible. The Half-Face Man at one point also acquired Mancini’s Family Restaurant and turned it into an organ-harvesting factory, whereby guests unlucky enough to enter it would be taken down below into the droids’ crashed spaceship and their organs used to repair the ship.
When a dinosaur mysteriously appeared in the Thames, the Half-Face Man extracted tissue from its optic nerve to use as part of his ship. He then incinerated the creature so as to leave no trace of his presence. This did, however, alert the Doctor, newly regenerated into his twelfth incarnation, to their nefarious activities throughout London.
The Doctor and his companion, Clara, were led to the restaurant by a mysterious coded message left in a newspaper. They were then taken down to the spaceship and accosted by the Half-Face Man, who revealed to them that his sole objective was to reach the “Promised Land”. While Clara and the Paternoster Gang fought the clockwork droids, the Half-Face Man escaped to the ship’s escape capsule, a hot air balloon made of human skin. There, the Doctor managed to convince the droid that the Promised Land did not exist, and so the robot either fell — or was pushed — to his death on top of Big Ben. His death deactivated the rest of the clockwork droids, saving the Doctor’s companions. (Deep Breath)
THE PROMISED LAND
The Half-Face Man’s mind was uploaded to the Nethersphere. (Dark Water) He found himself in some sort of garden, where Missy explained to him that he had reached the “Promised Land” — he was now in Heaven, at last. (Deep Breath)
As one of the people she had met during her travels with the Doctor, the Half-Face Man was included in a series of notes written by Clara when she was planning to confess to Danny Pink via phone call about her adventures with the Doctor. (Dark Water)
The Half-Face Man was equipped with a blowtorch hidden underneath one of his human hands. He used this both as a torture device and to look threatening as well as, presumably, to incinerate his victims so as to remain undiscovered. While he was immensely clever in some capacities, his programming was also rather limited in other respects, and was, like his fellow clockwork droids, fooled into thinking an organic was a droid like him as long as they were not breathing. (Deep Breath)