In the novel The Mystery of the Blue Train, Mrs Polly Harrison is Katherine Grey’s good friend in St. Mary Mead. She is the wife of Dr Harrison. Her youngest son is called Johnnie.
At the beginning of the novel, Mrs Harrison goes to visit Katherine, intending to talk about how Katherine had inherited Mrs Harfield's money, and about her plans. However, she beings telling Katherine a long story about Johnnie instead.
When Katherine says that she does not have any plans yet, Mrs Harrison expresses horror at the idea of Katherine staying on in St. Mary Mead. Mrs Harrison is of the opinion that it must have been an awful life for Katherine, being cooped up there for so many years. When Katherine says that she has always had a lovely feeling of mental freedom, Mrs Harrison says that she cannot understand that.
Mrs Harrison is of the opinion that Katherine could look "possibly positively beautiful" if she tried, and that she just needs some good clothes.