A funny thing happened to the French director Bruno Dumont a few years back: He discovered whimsy, or at least something like it. Mr. Dumont made his feature debut in 1997, with the striking “The Life of Jesus,” which built on the unflinching confrontation of emotional extremes pioneered by the filmmaker Maurice Pialat. His subsequent works — including the harrowing “Twentynine Palms,” the enigmatic war film “Flanders” and the gloss on Bressonian themes “Hadewijch” — are serious, searing pictures, replete with content and imagery that even the most adventurous viewer could find hard to stomach.

But his 2014 television mini-series, “Li’l Quinquin,” about an impish kid caught up in a grisly murder mystery, added an antic playfulness to the Dumont vocabulary. And his new feature, “Slack Bay,” widens that scope.

“Slack Bay” is set in a tiny seaside community in northern France, sometime in the early 1900s. Here live two families. The “haves” are the wealthy Van Peteghems, whose patriarch, André (Fabrice Luchini), a simpering hunchback, presides (after a fashion) over an ominous stone manse on a hill overlooking the bay. The “have-nots” are the Bruforts, whose own patriarch is nicknamed the Eternal. They run a ferry service in which the Eternal and his oldest son, called Ma Loute (also the film’s French title), often hand-carry passengers over a marsh. Sometimes they use a boat. And on these trips they sometimes bludgeon their passengers to death, take the bodies to their humble home and eat them.