Set in Victorian London, Miss Margaret Prior, an upper-class woman, becomes a "Lady Visitor" at Millbank Prison as a diversion from suicidal thoughts. She meets Selina Dawes, a spiritualist-medium who has been sentenced to five years imprisonment for fraud and assault. Miss Prior becomes intrigued by Selina because she is young and lovely and because things occur that cannot be explained - flowers mysteriously appear in her bedroom as well as a cutting of Selina's hair. Miss Prior finds herself falling in love with her and Selina returns her affection. Selina tells her that her spirit friends will help her escape but Miss Prior wonders how that will happen. The author covers such social issues as spiritualism, the double standard for women, prison life and lesbianism. Affinity is a grim story but a highly-entertaining one.