First in a chilling two-part drama based on the extraordinary true story of Victorian poisoner Mary Ann Cotton. In Seaham, County Durham in 1857, 24-year-old Mary Ann returns home four years after eloping to Cornwall with Billy Mowbray, and Mary's mother Margaret and kindly stepfather George Stott welcome them back with open arms. It soon transpires that Mary Ann has lost four babies in as many years, prompting the young couple to come back home and make a fresh start with their one surviving baby girl, Margaret Jane. Determined to make Billy do right by his stepdaughter, George persuades him to take out life insurance, and Mary Ann soon reveals that she is pregnant again. The child is born and christened Isabella, but Margaret Jane soon meets the same fate as her elder siblings. Becoming increasingly suspicious of her own daughter, could Margaret and the rest of her family be in danger?