In the novel The Mystery of the Blue Train, Dr Arthur Harrison and his wife are Katherine Grey’s good friends in St. Mary Mead. Dr Harrison's youngest son is Johnnie.
Dr Harrison is described as being a "big, middle-aged doctor", and he has bushy eyebrows. When he visits Katherine, he bustles in with a lot of energy and breeziness.
Dr Harrison visits Katherine early, explaining that he did this in case she had heard from some of Mrs Jane Harfield's relations. When he reads Mrs Samuel Harfield's letter, he gives "snorts and grunts of violent disapproval", and dashes the letter down on the table.
Dr Harrison disagrees with Mrs Samuel Harfield's suggestion that Jane's mind had been failing, saying that her intellect had been as good as anyone else's. He tells Katherine not to let the letter worry her, and not to think it is her duty to give the money she had inherited to Mr and Mrs Samuel Harfield.
Dr Harrison expresses the opinion that the letter from Mrs Samuel Harfield is "odious", and Katherine says that under the circumstances it is quite natural. Dr Harrison says that he has "the gravest suspicions" of Katherine sometimes, because of the things that she finds perfectly natural.
Dr Harrison later tells his wife about the money Katherine has inherited from Jane Harfield. When Mrs Harrison points out that Katherine is extremely good-looking, Dr Harrison is "honestly surprised", although he had noticed that Katherine has "very nice eyes". He also asks Mrs Harrison to visit Katherine.