Olivia Colman and Jessie Buckleys dark comedy Wicked Little Letters is based on an unbelievable true story that played out (almost) how it plays out in the film. The movie opens in 1920 as devoted Christian and proud spinster Edith Swan, played by Colman, receives the latest in a long line of profane hate mail in the post. Upon receiving the 19th such letter, Edith finally reports it to the police, who immediately suspect her foul-mouthed neighbor Rose Gooding, played by Buckley, who vehemently denies being the one who sent the letters.

https://screenrant.com/wicked-little-letters-true-story-explained/