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The Peking Homunculus, also known as Mr Sin, was a murderous cyborg.
The Peking Homunculus was a 51st century cyborg made by Dr. Sa Yy Findecker for Ingrid Bjarnsdottir, the commissioner of the Icelandic Alliance, as a toy for her two children. It contained a series of magnetic fields on a printed circuit, a small computer, and a single organic component: the cerebral cortex of a pig.(The Talons of Weng-Chiang; The Butcher of Brisbane)
The Homunculus attacked Commissioner Bjarnsdottir’s son with a table fork. However, he survived and could not see what all the fuss was about. In fact, he believed that the cyborg was playing a game with him. (The Butcher of Brisbane)
It played a key role in history when the pig part took over and almost caused World War VI, then disappeared. It later showed up in the possession of Magnus Greel, a fugitive war criminal from the fallen Supreme Alliance. He took it with him to the 19th century. Armed with a knife, it did his bidding.
During the final battle between the Fourth Doctor and Greel, Mr Sin, whom Greel had unwisely allowed access to a laser rifle, became trigger-happy and fired at anyone who moved, including Greel’s men and Greel himself. The Doctor finally threw it to the ground and tore out its fuse. (The Talons of Weng-Chiang)
Sherlock Holmes later investigated Mr Sin, terming it The Affair of the Walking Ventriloquist’s Dummy, but was unable to solve the matter. (All-Consuming Fire)
Sin was later reactivated by Hsien-Ko Chang, the daughter of Greel’s former ally, Li H’sen Chang, using the prototype control fuse and brains from a slaughterhouse. Hsien-Ko believed that she could use Sin as a weapon the way Greel had failed to do so. Although she maintained a tighter hold on Sin’s more violent tendencies — disguising him as a child to avoid attracting attention — she lost control when he was damaged in a fight at her base. In the subsequent fight, the Doctor decapitated Sin. K9 later destroyed the cyborg’s head to prevent it ever being reactivated. (The Shadow of Weng-Chiang)
Jago and Litefoot discovered Mr Sin inside Greel’s time cabinet in 1968. (The Bloodchild Codex) It was later revealed that Mr Sin was reconstructed by Guinevere Godiva as an assistant for the recreated Magnus Greel. However, when Greel was killed, Jago and Litefoot destroyed Mr Sin again. (The Final Act)
Mr Sin was among one of the faces seen by the Eleventh Doctor when falling down, having flashbacks. (Hunters of the Burning Stone)