It's 1857 and 24-year-old Mary Ann Cotton has come home to Seaham, County Durham after four years away, having eloped to Cornwall with Billy Mowbray. Her mother, Margaret, and kindly step father George Stott, welcome them with open arms.

On returning to Sunderland, Mary Ann discovers James has had a change of heart about their relationship and says he cannot marry again so soon after his first wife died, as it will unsettle his children. Furious, Mary Ann sets out to get rid of the apparent impediments.