A hundred years ago, before email and social media found ways to slap us in the face with unsolicited obscenity on a daily basis, the quiet English town of Littlehampton was scandalized by an outburst of poison pen letters — a nasty case of epistolary terrorism that today might be lumped under the heading of “trolling.” Someone with lovely penmanship and a very salty vocabulary dashed off dozens (if not hundreds) of blisteringly offensive notes to members of the seaside community, igniting a police investigation and a series of trials breathlessly covered by the local press, then largely forgotten for almost a century.

A bawdy black comedy that isn’t nearly as “outrageous” as it would have you believe, “Wicked Little Letters” offers a tongue-in-cheek retelling of those events for the Merchant Ivory set. Titillating profanity aside, it’s a relatively tame critique of 1920s gender dynamics, focusing on the two women at the center of the affair — a sour-puss spinster named Edith Swan, who received the bulk of the harassment, and her disruptive Irish neighbor, Rose Gooding, whom she accused of sending the raunchy missives — as well as the female detective responsible for untangling the mystery.

It doesn’t take much of a detective to realize that adds up to something fairly rare: a period film with three substantial leading roles for women, set (in the words of the local priest) at “a time when morality is threatened and women everywhere are losing their decorum.” Small wonder, then, that director Thea Sharrock attracted such a strong cast.

Edith is played by Olivia Colman with an exaggerated piousness that tips toward cartoonish, while the part of force-of-nature Rose proves perfectly suited to “Wild Rose” star Jessie Buckley. As a single mom with a Black boyfriend (Malachi Kirby) who drinks and swears and makes love at all hours, Rose challenges the puritanical patriarchy to which her neighbors kowtow (in one scene, her “furious jumping” nearly dislodges the crucifix hanging from long-suffering Edith’s wall). The two characters could hardly be more different, and yet we’re told they were once best friends.

Edith lives at home with an insufferably strict father (Timothy Spall), who spouts off about women’s suffrage and other perceived threats to his authority, while Rose doesn’t hesitate to tell people what she thinks of them. For a time, Edith found a kind of vicarious satisfaction in Rose’s liberated attitude. But now that Edith imagines herself on the receiving end of Rose’s insults, she can abide it no longer. “She’s heinous,” Edith complains a bit too enthusiastically to the police, “and she’s what we feared would come after the war.” For their part, the authorities show an alarming lack of curiosity when presented with what seems like an open-and-shut case.

Only Gladys Moss (“We Are Lady Parts” vet Anjana Vasan) suspects otherwise, representing the weaker leg of the central trio. As Sussex’s first “woman police officer,” she’s confronted by sexism and racism every day on the job: Her male colleagues use the word “woman” the way they might “canine,” for example — as though astonished the opposite sex can be of any help in a professional setting — interrupting their locker-room banter to put Gladys in her place whenever possible. It’s an insufferable work dynamic, which Sharrock and screenwriter Jonny Sweet are none too subtle about calling out.

The movie feels very of-the-moment (almost frustratingly so) in its critique of religious hypocrisy and backward gender dynamics, and yet, one longs for a little more nuance in the clownish way these bigots and blowhards are depicted. In truth, the so-called “Littlehampton libels” built to a twist, which a decent contingent of the audience will surely see coming. The English courts of that time might not have taken handwriting analysis seriously, but the evidence is clear as day to our eyes. Plus, the culprit is hiding in plain sight.

Meanwhile, Edith seems to relish all the attention the indignity brings as the case drags on, collecting the newspaper articles written about the shame she’s suffered. (As her mother, Gemma Jones scolds Edith, lest she grow too prideful.) Who knew that enduring such abuse could turn this dowdy old maid into an unlikely local celebrity? Contained in the conflict between these two women is a deeper commentary about the media and how the public relishes a good scandal, rushing to judge with only a fraction of the facts. In Sharrock’s hands, “Wicked Little Letters” is an entertaining account of what feels like a primitive form of today’s online flame wars, where people take sides as commenters openly disparage one another.

Ironically, however hurtful Edith and others found these personal attacks to be, they merely compounded the humiliation by going public with what had been written about them. Good that they did — for our sake, at least — as it’s a hoot to hear Armando Iannucci-caliber insults being lobbed in this conservative 1920s milieu. Amid all that bullying, it is free-spirited Rose who shows what dignity looks like, rising above the slander.