"Weng-Chiang" was originally going to be the Master, following on from The Deadly Assassin, but Philip Hinchcliffe had this changed as he didn't want to have the Master revealed as the secret villain again. Robert Holmes agreed that using him once that season was enough. There are several clues in the story that still point to the original plan: the time cabinet ("read 'TARDIS'") and Greel referring to Leela as "the first morsel to feed my regeneration".
Mr Sin is presented as the magician's wooden assistant, but turns out he's a murderous creature created by the villainous Magnus Greel from the 50th century and is called the Peking Homunculus. This is not the first time that Tom Baker faced one of these creatures. In The Golden Voyage of Sinbad (1973), his character, Kouras, is the villain that creates the homunculus to further his evil schemes and become all powerful.
This episode was watched by 9.3 million viewers on its original transmission.
This episode takes place in London in 1889.