The following version of the novel was used to create this study guide: Horowitz, Anthony. A Line to Kill. Harper, October 19, 2021. Kindle.

In the murder mystery A Line to Kill by Anthony Horowitz, when the sponsor of a literary festival is brutally killed during the event, Detective Inspector Daniel Hawthorne and his biographer, Horowitz, are luckily on hand to solve the crime. Horowitz both narrates and is the central character in this third novel in his Hawthorne and Horowitz Mystery series. Horowitz believed he would finally be able to impress Hawthorne with his life as a writer, but Hawthorne took center stage by solving the crime with only a few clues and a cast of cagey suspects.

Strange things have happened at the festival on the island of Alderney even before the murders occur. Horowitz saw Maïssa, one of the writers, having a tense argument with a man wearing a leather jacket. When Horowitz asked, both denied knowing the other. A five-pound note Horowitz left as a tip was stolen. Anne Cleary, another writer, reported a pen with sentimental value had been stolen from her. Also, during a session given by Elizabeth Lovell, who billed herself as a psychic, Elizabeth made a point to deliver a message to Anne from Anne's son, William, who had died by suicide. The mention of William and his death badly upset Anne.

On the morning following a party at the home of Charles and Helen le Mesurier, whose online gambling company Spin-the-Wheel was hosting the festival, Charles was found brutally murdered. A few hours after Charles’s body had been found, Helen went for a walk. She never returned. Her body was located in one of the many caves on the island. She had been hit multiple times in the head with a rock.

Those attending the festival, as well as the local police, believed the crime was solved when Derek Abbott, a man who had worked for Charles, seemed to admit to the murder. Abbott, who had just been fired by Charles, had been arrested and questioned by Hawthorne in the past for child pornography and having sex with minors. The police were only able to convict him of possession of pornography. While Abbott was in police custody, his left leg was permanently injured when he fell down a flight of stairs. He claimed Hawthorne pushed him.

After Hawthorne, Horowitz, and others questioned Abbott about Charles and Helen’s deaths, Abbott left what appeared to be a suicide note in his home. He jumped to his death from a cliff, making it appear that he had committed the murder.

Only Hawthorne was not convinced and later went to visit Anne. He discovered her son William had not died from a drug addiction. He had chosen to die by suicide because he had spent all of his savings gambling online. He had started gambling with Charles’s onsite gambling company. Anne, with the help of her daughter, Kathryn, killed Charles to get revenge for William’s death. They also killed Helen because she served as the model on the website, luring young men to gamble.

Even though Anne showed Horowitz and Hawthorne a letter stating that she was dying of heart disease and she claimed Kathryn had little to do with the murders, Hawthorne still insisted that they turn themselves in to the police.

Because of the circumstances, Horowitz wondered why Hawthorne insisted the women admit their crimes. Horowitz believed justice was served when Anne died before she could go to trial. Kathryn’s case was heard by a jury, but they sympathized with her and found her not guilty of any crime.