In 1968, an 11-year-old girl named Mary Bell strangles two toddlers in Newcastle's west end, telling one boy his throat hurt and she would make it better as her grip tightened. The film lays out the case that shook a city unprepared to see children killing children, tracing Bell's childhood in a slum where kids ran unsupervised and crime was routine. It examines her mother's abuse and a father described as a genetic wild-card, along with speculation that repeated drug overdoses in infancy may have caused physical damage. Psychologists and commentators walk through how a child denied loving attachment can build a substitute bond out of aggression instead, responding to others the way she herself was treated. The film treats this as a case study rather than a simple monster story, weighing genetics, neurology, and environment against each other while noting that most abused children do not become killers. Newcastle's response afterward, closer attention to children showing disturbing warning signs, closes out the account.