We know Joanne Froggatt as Anna Bates, the loving, resilient lady's maid of Downton Abbey. Meet Joanne Froggatt as Mary Ann Cotton, Britain's first female serial killer. Based on a gruesome true story, seen on Masterpiece, this two-part drama tells the tale of the notorious poisoner, Victorian wife, mother of 13, Sunday-school teacher, and nurse who murdered at least 21 people, dispensing death from the spout of a warm teapot. With Alun Armstrong (New Tricks) as Mary's stepdad, George. 3 hrs, 1 DVD, SDH.