The actress – who plays Anna Bates in Downton - will take on a very different kind of role in the sinister sounding two-part drama which is based on a true Victorian story.
Mary Ann Cotton lived in the 1800s and is infamous for having killed three husbands and possibly slaughtering as many as 21 others including her own children and two former lovers.
Her chief method was poison which left no visible signs on the bodies. She travelled around the North East and integrated herself into families, where she would marry, have children, then kill them all, take the money and move on to her next victim.
She becomes wealthier and higher up the social chain with every murder…
ITV exec Victoria Fea said: "Dark Angel is an extraordinary and chilling true story. We're delighted to have an actress of Joanne Froggatt's calibre in the lead role. The combination of a tautly written script, an outstanding cast and great producers in World Productions make this a really exciting addition to the ITV drama slate."
Filming begins in August in North Yorkshire and County Durham.
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