Putting the last six years of playing sweet and innocent Anna Bates in her rear view mirror, Downton Abbey‘s Joanne Froggatt has already begun filming on her next project, Dark Angel, for ITV. The ‘did-she-or-didn’t-she-kill-Mr-Green’ downstairs ladies maid has stepped back in time to play real-life Victorian serial murderer, bigamist and adulteress Mary Ann Cotton. Dubbed Britain’s first female serial killer, Cotton racked up an extraordinarily high body count in a short period of time by secretly poisoning her victims in order to better herself socially and financially while, surprisingly, remaining unsuspected by Victorian society.

Alun Armstrong (New Tricks, Penny Dreadful), Jonas Armstrong (Robin Hood), Laura Morgan (Torchwood, The Suspicions of Mr. Wicher), Emma Fielding (Arthur & George, DCI Banks) and Penny Layden (Call the Midwife) also star in the two-part drama, which is currently filming in North Yorkshire and County Durham.

Cotton, who starts out as a loving wife and mother who is soon faced with extreme poverty and an ailing husband. Dark Angel will follow her as she takes ruthless measures, integrating “herself into unsuspecting families, marrying and creating new families of her own – before killing them, taking their money and moving on.” Loving wife and mother aside, Cotton is also a serial killer, a poisoner whose methods leave no visible scars, allowing her tally of victims to mount unsuspected by a Victorian society unable to conceive a woman capable of such terrible crimes. Traveling around the North East, she insinuates herself into unsuspecting families, marrying and creating new families of her own – before killing them, taking their money and moving on.

Nothing like adultery, bigamy, fraud and murder to be able to better yourself socially and financially. The obvious downside, however, is that the more she kills, the greater the risk her crimes will finally be exposed.

Fortunately, we all know how the story of Mary Ann Cotton turns out. Unfortunately, we are going to have to wait until at least September when the final series of Downton Abbey returns to find out how things turn out for Anna Bates.