Mister Sweet was a Red Leech that had somehow survived extinction from the Cretacious Period.
It was discovered by a river in 1893 by Winifred Gillyflower an insane, sadistic yet incredibly rich elderly lady, she allowed it to use her as a host feeding her his "Nectar". Using Gillyflower's money the pair open Sweetville a some town were only the most perfect and beautiful could live, however using the Red Leech Venom they were brainwashed into mindless drones, however many also died during the operation. He planned to place a vial of the venom in a rocket and have it explode over Yorkshire letting only the strongest survive. However the Doctor and Clara Oswald with the help of Strax, a Sontaran, Madam Vastra, a Silurian and Jenny Vastras lesbian lover and house maid, manage to remove the vial from the rocket, Gillyflower tries to shoot them but is shot at by Strax knocking her from a great height, as she dies Sweet abandons her but before it can get away it is smashed to death with a cane by Ida Gillyflower the much abused daughter of Winifred who had been blinded by the duo experiments on her.