Fiery Rose Gooding (Jessie Buckley) has little in common with the pious Edith Swan (Olivia Colman) aside from them being neighbours in the seaside town of Littlehampton in the 1920’s. But one day, Edith starts receiving scandalous letters written with foul language as eccentric as it is obscene, and she immediately accuses Rose of being the poison pen writer behind them. As Rose stands to lose her freedom and the custody of her daughter, and outrageous letters continue to blight the town, police officer Gladys Moss (Anjana Vasan) begins to suspect something is amiss. Together with a group of resourceful women, Gladys endeavours to solve the mystery, liberate Rose, and catch the real culprit.