There is a dark old nursery rhyme that goes, "Mary Ann Cotton, she's dead and forgotten / Lying in bed with her bones all rotten" – but after Dark Angel, no-one will be able to forget the life of the mother, wife and killer whose crimes scandalised Victorian society. 

The two-part drama is based on the the true story of poisoner Mary Ann Cotton, who is played by Downton Abbey's Joanne Froggatt. 

Here's more about her, and all the characters you need to meet...


Joanne Froggatt (Mary Ann Cotton)

Who does she play? Mary Ann Cotton is one of the least-known but most-prolific murderers in British history. Using arsenic, she secretly poisoned probably 16 or 17 people before she was caught and hanged at the age of 40. And they weren't just any people: they were her husbands, her mother, her children, and her stepchildren. Mary Ann was born into poverty in 1832 in a County Durham mining village and soon embarked on a life of births, marriages and gruesome deaths, moving around the country to avoid detection and also encouraging her victims to take out life insurance. This series will focus on nine of her murders. 

Where have I seen her? Froggatt played lovely maidservant Anna Bates in Downton Abbey, winning a Golden Globe in the process. However, this is not her first brush with the "true crime" genre — she previously starred in See No Evil: The Moors Murders (2006) and Joanne Lees: Murder in the Outback (2007) on ITV.

Alun Armstrong (George Stott)

Who does he play? Mary Ann's father died in a mining accident when she was just a kid, but her mother soon re-married, to publican George Stott. He adored his stepdaughter and her own children and tried to help them when he could. George looked for the good in Mary Ann, but struggled in the end to reconcile his idea of her with what the law says she did. 

Where have I seen him? If the sight of Armstrong and Froggatt together gives you deja vu, that is because he made a guest appearance in a Downton Abbey Christmas special as Stowell, the butler of the Sinderby family. The 70-year-old has also starred in New Tricks, Penny Dreadful, Little Dorrit, and The Hollow Crown. 

Jonas Armstrong (Joe Nattrass)

Who does he play? Joe Nattrass is described as "a hard working coal miner with a twinkle in his eye". He doesn't seem put off by the murderous glint in Mary Ann's eye, because the two have a long affair. They spend time apart and come back together, but the whole thing doesn't quite work out. 

Where have I seen him? Firstly, hold on to your hats because this is going to get confusing: Jonas Armstrong is no relation to Alun Armstrong. BUT Alun Armstrong does have a son called Joe Armstrong – and Joe and Jonas have previously worked together, on the drama Robin Hood, in which Jonas played the title role. AND, in Dark Angel, Jonas is playing a character called Joe. The casting director must have been having a laugh. 

Besides Robin Hood – where he also encountered Froggatt, who was playing a peasant named Kate – Armstrong has starred in Edge of Tomorrow, Ripper Street and Line of Duty. Where, incidentally, his character was also called Joe.