Affinity is a captivating tale set in the grimmest jail of Victorian London, Millbank prison. The story follows Margaret Prior, an upper-class woman recovering from a suicide attempt. As part of her rehabilitative charity work, she begins visiting the women’s ward of Millbank.

Among the murderers and thieves, Margaret encounters an enigmatic spiritualist, Selina Dawes, who was imprisoned after a séance went horribly awry, leaving an elderly matron dead and a young woman deeply disturbed. Initially skeptical of Selina's gifts, Margaret is soon drawn into a twilight world of ghosts and shadows, unruly spirits and unseemly passions.

The narrative is a spellbinding ghost story, a complex historical mystery, and a poignant romance with unexpected twists. Sarah Waters brilliantly evokes the sights and smells of a moody and beguiling nineteenth-century London, crafting a tale that explores themes of confinement, betrayal, and the supernatural. Margaret's fascination with Selina leads her to concoct a desperate plot to secure both Selina's freedom and her own.