The letters trickled in at first, and then they flooded the community. Nearly every resident of Littlehampton, a small coastal town in England, received an offensive note in 1920. Most of the libelous messages haven’t survived, but the ones that did display a dexterity with foul-mouthed language. They are a window into a different time — a rare look, in the words of historian Christopher Hilliard, at the “tangles of devotion and resentment, desire and manipulation” concealed under a working-class neighborhood’s genteel demeanor.

Mystery, tension, gossip and humor defined the Littlehampton scandal, but you wouldn’t be able to tell from Thea Sharrock’s odd screen translation. Wicked Little Letters, which premiered at the Toronto Film Festival, takes a one-note farcical approach to this slice of history, a peculiar move that deserts its formidable cast and squanders its thrilling premise.

Two years after The Lost Daughter, Olivia Colman and Jessie Buckley...