Amateur detective Dixon Druce becomes involved in a household where the eldest surviving sibling will inherit a fortune, and where the female siblings are under the sway of the mysterious title character.
This one takes a bit to get going, but once the weak supporting actors are out of the way, and Fraser and Benet are allowed to work, it gets good. Murcell and Delgado give their usual best, and the murder method is ingenious. Animal lovers and odontophobes might best steer away, though.
Based on the 1902 story "Madame Sara" by L.T. Meade and Robert Eustace, and noteworthy as the first real example of a female supervillain (oops, spoilers, I guess) in detective literature.