Arsenic. In the Victorian era, it was everywhere: in make-up, candles, pots and pans, curtains, dresses – even the very wallpaper. A by-product of the mining industry, it settled into the fabric of everyday Victorian life. It also accounted for over a third of homicides involving poison. Quiet, ordinary, yet lethal – the perfect, innocuous murder weapon for a woman like Mary Ann Cotton.

ITV’s new two-part series Dark Angel stars Downton Abbey’s Joanne Froggatt (Anna Bates) as Mary Ann Cotton, who was hanged in 1873 after being found guilty of murdering her stepson with arsenic. The drama follows Mary Ann through the death of three of her four husbands, her lover, her friend, her mother, her three stepchildren, and 11 of her own 13 children. They all reportedly died of stomach fevers – a then-fatal illness that shared all its symptoms with arsenic poisoning.

Based on the 2013 book Mary Ann Cotton: Britain’s First Female Serial Killer by David Wilson, the drama works on the assumption of Cotton’s guilt – even if some historians remain sceptical about the story.

Here’s the story as Wilson presents it in his biography. Mary Ann Robson was born, irresistibly, on Halloween, in 1832, in Sunderland. Her family moved when she was eight years old so her father could work at Murton Colliery – he died in the mine before her tenth birthday, and her mother remarried a year later.

Historians have speculated that Mary Ann did not get on with her stepfather, and that her childhood was an unhappy one – though she herself would refer to them as “days of joy”. She married at 20 to coal miner William Mowbray. They moved a lot throughout their marriage so William could find work – sometimes he was away for months at a time on a streamer ship. They had nine children together – eight of whom died from gastric fever. William too died of “gastric fever” in 1865, and Mary Ann received £35 in life insurance (about £1,500 today). After his death, their last surviving daughter went to live with Mary Ann’s parents. She would live until she was nine years old – longer than any of Mary Ann’s other children, aside from the two who outlived her.

She remarried later that year – her second husband, who also had life insurance, died of gastric fever barely a year after their wedding – although it was rumoured that Mary Ann was having an affair with a man named Joseph Nattrass throughout this marriage.

After the death of her second husband, Mary Ann took on work as a housemaid for a man named James Robinson – his child in died in her care of (yes, you guessed it) gastric fever less than a month into her post. In 1867, she took leave to visit her sick mother, who also died – this time from hepatitis, which sees symptoms including fever, abdominal discomfort, vomiting and diarrhoea – nine days after her daughter arrived back home. Mary Ann’s nine-year-old daughter, now reunited with her mother, and two of Robinson’s other children died of the same condition in a two-week period.

Soon after, Mary Ann married her boss. That’s when things started to go wrong for her. She and Robinson had two children, a boy and girl. Their first, Margaret, born in 1868, died at just five months. Robinson’s financial affairs took a turn for the worse, and he noticed money missing from his bank account. His son William told his father that his stepmother regularly sent him to pawn their belongings. Robinson grew suspicious of Mary Ann’s repeated insistence that he take out life insurance on himself and his three children. Mary Ann moved out shortly afterwards.

She found a fourth “husband” in 1870 – the brother of her friend Margaret Cotton, a widower named Frederick. In March, Margaret died of a mysterious stomach ailment, and Frederick and Mary Ann were bigamously married in September – many historians believe that, at this time, Mary Ann reunited with Joseph Nattrass. Frederick, his life insured, died of gastric fever in December, leaving Mary Ann with his two children and pregnant with his third son. All three children would be dead by 1872.

Nattrass moved in with her as her lodger – he, too became ill with fever, and died – but not before he had adjusted his will to include Mary Ann. A young neighbour who was present at Nattrass’ deathbed said, “I saw him have fits, he was very twisted up and seemed in great agony.... He said, ‘It is no fever I have’.... I have seen [Mary Ann] several times give him a drink.” Around this time, Mary Ann found work as a nurse to a man named John Quick-Manning – who would father her last two children.

Mary Ann had mostly avoided suspicion by moving from town to town between her different marriages. But doubts about her character grew when she prophesised the death of Frederick Cotton’s son Charles, telling a parish official that Charles would soon be dead – like “all the rest of the Cotton family”. When he died days later, the official went to the village police with his suspicions.

After the Reinsch Test confirmed that Charles had died of arsenic poisoning, Mary Ann insisted on her innocence, writing in a letter:

I hope you Will not Juge me rong As i have been on the Awfill crime of murder of Charles Edward Cotton Whitch i am not guilty of it those to reade the evidence that come sin against me you may think i am but ifie must Tell you I am not guilty I have been miss Lead

Her sentence was suspended until the birth of her 13th child, Margaret Quick-Manning Cotton. Her defence claimed that arsenic in the dye of the green wallpaper of the Cotton home must have been to blame, but Mary Ann was found guilty. She was hanged in 1873.

Retrospectively, it’s hard to know how many deaths were murder, and how many misfortune. Was Mary Ann driven to distraction by grief and poverty? A feminist anti-heroine dispensing with the men who oppressed her? A social climber ready to find her feet on the next rung of the ladder at any cost? Or a cold, calculating child-killer? Dark Angel seems unable to decide – making half-hearted attempts to humanise her in its opening, but dispensing with such endeavours halfway through the first episode, instead opting for a hard-hearted villain.

It’s a sensational tale, and yet, one that didn’t make a splash at the time. Wilson writes, “Mary Ann Cotton was never ‘prime time’”, as she “rarely made it onto the front pages of Victorian newspapers”. But with a 9pm slot on ITV tonight, it seems that’s about to change.