An upper-class woman recovering from a suicide attempt, Margaret Prior has begun visiting the women's ward of London's Millbank prison as part of her rehabilitative charity work. Margaret finds herself increasingly fascinated by an apparently innocent inmate, the enigmatic spiritualist Selina Dawes. Selina was imprisoned after a séance she was conducting went horribly awry, leaving an elderly matron dead and a young woman deeply disturbed. Although initially sceptical of Selina's gifts, Margaret is soon drawn into a twilight world of ghosts and shadows, unruly spirits and unseemly passions, until she is at last driven to concoct a desperate plot to secure Selina's freedom, and her own.