In 1870s Victorian England, a woman of Margaret Prior's (Madeley) upper social class and age should be married. Instead, she lives with her mother and is suffering a deep depression after the recent death of her father and the marriage of her best friend (and secret lover) Helen (Young) to her brother. Encouraged to do good works, Margaret mentors the female inmates of Milbank prison and becomes curious about Selina Dawes (Tapper), a so-called spirit medium incarcerated for murder after a seance gone wrong. At first skeptical of Selina's alleged gifts, Margaret becomes fascinated by the allure of the unknown. Based on the novel by Sarah Waters.