This film is based on a true story about a British teenager who allegedly poisoned family, friends, and co-workers. Graham is highly intelligent, but completely amoral. He becomes interested in science, especially chemistry, and begins to read avidly. Something of a social misfit, he is fascinated by morbid subjects such as poisons and murder. His family environment is intolerable to him and, in particular, his stepmother torments him. He decides to poison those who annoy him, first with antimony and later with thallium. He smugly thinks himself cleverer than all those around him, but nevertheless he is caught and sentenced to 'rehabilitation' at a psychiatric institution. Once there, he undertakes to deceive the new eminent psychiatrist sent there to 'cure' him, thereby securing his release.

This is a dramatisation of the true story of Major Herbert Rowse Armstrong, a solicitor and magistrate's clerk who lived in the small Welsh town of Hay-on-Wye. In 1921 he was arrested and charged with poisoning his domineering wife, Catherine, and later attempting to poison a business rival, Oswald Martin, by administering arsenic to them. At his trial, Armstrong claimed that he had bought the arsenic simply to kill the dandelions on his lawn. However he was convicted of murder and executed in 1922.

A descent into Hell is triggered when "Ex-Lord" Donald Brocklebank finds that he must leave Longleigh House for London to find a way to pay for the medical treatments for his wife Nancy. Alone, his over-protected, delusional, adult son, James, fancies himself in charge of the manor house with his terminally ill mother, and barricades the two of them into the house for a series of ever more panicked home treatments, mistakenly protecting her from the arrival of Nurse Mary and any outside help.