In 1960s Massachusetts, Eileen (Thomasin McKenzie) is a strange young woman – aloof, mousy and unfazed by the gloomy nature of her secretary job at the local youth prison. But something in her changes the day that the new counsellor, Dr. Rebecca St. John (Anne Hathaway) arrives. Instantly captivated by Rebecca’s glamorous, enigmatic presence, the two women grow closer, and Eileen is inspired to explore new facets of her own personality and desires. But her metamorphosis takes a twisted turn when Rebecca reveals a dark secret – throwing Eileen onto a much more sinister path.