Following the death of her father, Margaret Prior has decided to pursue some 'good work' with the lady criminals of one of London’s most notorious gaols. Surrounded by prisoners, murderers and common thieves, Margaret feels herself drawn to one of the prison's more unlikely inmates – the imprisoned spiritualist Selina Dawes who weaves an enigmatic spell. Is she a fraud, or a prodigy? Sympathetic to the plight of the innocent-seeming Selina, Margaret sees herself dispensing guidance and perhaps friendship on her visits, little expecting to find herself dabbling in a twilight world of séances, shadows, unruly spirits and unseemly passions.