The story, which has been adapted for a BBC drama, has a big impact and will be riddled with killer clues – but will you spot them?

This year’s big Agatha Christie adaptation, The Pale Horse, is one of her less well-known murder mysteries – but in some ways it has had a much greater impact than the others. The novel is credited with saving two lives and has also been cited in a murder trial.

The novel centres around a series of apparently unrelated deaths that have all been attributed to a wide variety of natural causes. Mark Easterbrook becomes suspicious when a young woman dies just weeks after he saw her in a cafe, apparently in the prime of life, and he decides to investigate. Christie’s tale of contract killers, witches and black magic is a fantastic example of her brilliant plotting and superb ability to keep you guessing whodunnit right until the very end.

The novel, as would be expected of Christie, is packed with poisonous information and killer clues. It is sufficiently detailed that on two occasions readers recognised symptoms in people who were being poisoned and were able to intervene. It seems reading Christie could save your life. But the same novel has also been accused of inspiring a murderer.

Christie often used real life cases of murder as sources for her plots but on this occasion it was allegedly the other way round. There are certainly some uncomfortable parallels between The Pale Horse and the crimes committed by Graham Young. The novel was published in 1961, the year before Young began his poisoning activities, but at his trial in 1971 he denied having read it. It is doubtful that Christie, scientifically accurate though she was, could have taught him anything anyway, as Young had made his own extensive and detailed studies of all things toxic.

I don’t want to give away too much about The Pale Horse and spoil your enjoyment of the drama but I do want to discuss just how brilliantly Christie used her scientific knowledge to construct the plot. So to avoid giving away whodunnit but to offer you lots of clues, this article is all about Young, one of Britain’s most prolific poisoners, and some of the uncomfortably similar circumstances of his real-life murderous career and Christie’s fiction.

Young’s fascination with the macabre and toxicology started when he was a schoolboy. He excelled at chemistry but nothing else, despite being a bright student. His extensive knowledge convinced a local pharmacist Young was much older than he appeared and enabled the schoolboy to get hold of some unusual and dangerous compounds. The pharmacist thought he was being encouraging and supportive, but Young was using the pharmacist for his own devious ends: poisoning his family.

Young began adding different compounds to different foods in the family home. He stirred atropine into his sister’s tea and added antimony to the jars of jam in the cupboards and watched the results.

The atropine experiment was not a success from Young’s point of view. It made the tea taste bitter, so his sister only drank a mouthful before leaving for work. During her bus journey she started to feel dizzy. She got to her office but by then she was having trouble focusing her eyes. Her condition was serious enough that she was taken to hospital, where the doctors recognised the symptoms and were able to treat her successfully.

The antimony in the jam also had an effect on the family: lots of vomiting and painful cramps in the legs. Most of the poison would have been removed by the vomit but the small amount that remained slowly built up in the body with repeated doses. Disappointed with the slow progress the poison was making on his stepmother, Young added a large dose of thallium to her tea one day. He was found by the kitchen window watching her writhe in agony in the back garden. She died the same day but her death was attributed to natural causes.

Young continued to dose his father with antimony, often sneaked into his pint of beer when he wasn’t looking. Eventually his father was taken to hospital, where he recovered, but he remained suspicious of his son.

Encouraged by his initial successes Young also poisoned a school friend, Chris Williams. Williams’s deteriorating health sounded alarm bells at the school and a psychiatrist was brought in to question Young under the pretext of being a careers advisor. Young’s evident fascination with toxicology, and nothing but toxicology, was the key to discovering the truth of his poisoning habits and he was sent to Broadmoor psychiatric hospital.

More poisonings occurred during his time in Broadmoor. He added toilet cleaner to the nurses’ tea and sugar soap to the sugar bowl but fortunately no one touched either. Another patient killed himself using cyanide – a fast acting but horrible poison that can cause blinding headaches, disorientation and convulsions before collapse, coma and death in rapid succession. Young had talked about how to extract cyanide from laurel leaves, which grew abundantly in the grounds of Broadmoor. He was not accused of killing the patient but the laurel bushes were cut down nevertheless. After these early incidents Young turned himself into a model patient and after nine years was deemed safe to be allowed to return to society.

Shortly after his release he got a job in the storeroom of Hadland’s, a company that made photographic equipment. He volunteered to take on the tea round, a job no one else wanted. Everyone had their own mug and as Young pushed the tea trolley around the premises there was one point where he was completely unobserved. It was the perfect opportunity for him to selectively poison his co-workers. Some were given antimony, others thallium. Young varied the dose depending on whether someone had annoyed him that day and noted down his observations in a diary.

Young managed to poison eight people, two of whom died before he was caught. The eight people were examined by a total of 43 physicians during their illnesses and not one of them diagnosed poisoning of any kind. The seemingly random symptoms could not be attributed to a single cause. Graham’s victims suffered agonising pains, hallucinations, one went blind, others had their hair fall out. The illnesses were attributed to conditions as varied as broncho-pneumonia and a mystery virus.

The truth was only discovered because Young could not resist showing off his toxicology knowledge. At a meeting at the factory to discuss the “bug” that seemed to be afflicting so many of its employees, he spoke at length about heavy metal poisoning and their symptoms. A search of his room revealed an extensive collection of poisons, toxicology books and the diary where he had noted the symptoms of his co-workers. In the dock Young claimed the diary was notes he was preparing for a novel. No one was convinced, and he was sentenced to life for murder.

Young evaded suspicion for so long because he used poisons whose symptoms mimicked other illnesses and were so varied that no single cause was immediately obvious. It is the same problem that faces Easterbrook in The Pale Horse, and just as in the real-life case of Young, it is only a chance discovery and some bragging that reveals the truth. Will you spot the links between the real and fictional cases? And, armed in advance with so many clues, will you be able to work out whodunnit before Easterbrook?

The Pale Horse, written by Sarah Phelps, will air on BBC One in the coming months; a screening date has not yet been confirmed.