Based on a true scandal that stunned 1920s England, the story centres on neighbours Edith Swan and Rose Gooding in the seaside town of Littlehampton. One day, a series of obscene letters begin to target Edith and others, with suspicion falling upon Rose. As the outrageous letters continue to escalate, Rose risks losing both her freedom and custody of her daughter. Police Officer Gladys Moss is determined to find the real culprit, and along with a group of other women, seeks to solve this perplexing mystery.