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.