A retiree with a complicated past finds her relationship with her disapproving daughter and darling grandson derailed by an accident involving poison mushrooms.

A retired woman of a certain age (Hélène Vincent), who wants nothing more than to look after her perfect poppet of a grandson (Garlan Erlos), is devastated when her daughter (Ludivine Sagnier) removes access to him, all because of a silly mycological mistake in the delicious, sinister and deadly funny When Fall Is Coming. True to protean form, writer-director François Ozon (Swimming Pool, 8 Women) offers with this blackly comic thriller a tonal swerve into naturalism and away from the screwball energy of his last feature, the period-set courtroom caper The Crime Is Mine.

In fact, just about the only thing you could hold against this is the clunky English translation of the original French title, Quand vient l’automne. Surely When Fall Comes or, even better, When Autumn Comes, sounds so much better, crisper and more evocative, no?

Ozon’s script drops subtle hints in the dialogue about Michelle and Marie-Claude’s murky past, which dates back to a time when they both lived in Paris. Michelle had an apartment in the capital, but has given it to her grown daughter Valerie (Ludivine Sagnier, reunited with Ozon for the first time again since Swimming Pool) to live in with her angelic eight- or nine-year-old son Lucas (Erlos) during a contentious divorce.

In the tightly mapped first act, we see Michelle going off with Marie-Claude to pick mushrooms in the woods nearby as she prepares for a late-summer visit from Valerie and Lucas for a couple of weeks before the latter goes back to school. But when the two finally arrive, it’s clear that mother and daughter have a rocky relationship. Most of the hostility emanates from Valerie, the sort of person who is always steaming like a kettle with ill temper. She finds fault with everything Michelle does, and even mocks her for thinking Lucas would try the mushrooms she’s so lovingly prepared.

In the end, only Valerie eats the boletes, and when Michelle and Lucas return from a happy stroll together, they find an ambulance has come to take Valerie to the hospital. A poisonous mushroom had gotten into the dish, and although Valerie survives, she is incandescent with rage. She accuses her mother of trying to kill her, packs up Lucas and takes him back to Paris.

Let’s just note that when Vincent is finally released from prison and comes to work for Michelle as a gardener, the two of them form a strong bond. By and by, we learn that Michelle and Marie-Claude were once sex workers and that while Vincent’s instinct is to protect his mother and her best friend, Valerie has never accepted it and feels only shame and disgust at her mother’s earlier life. It’s in the gulf between Vincent and Valerie’s attitudes that Ozon (working once again with regular collaborator Philippe Piazzo on the script) finds the friction to light the film’s dramatic fire — one sparked by crime or a simple accident, though we never find out which for sure.

Perfectly calibrated to inspire post-screening debates over whether character X or Y is guilty, When Fall Is Coming dispenses clues and red herrings masterfully but always holds just a little something back. The cast does an immense amount of work with the smallest of expressions, like little pouts of disgruntlement or disgust, or an eyebrow raised just enough to convey unspoken understanding. But while Ozon and the cast draw a diaphanous veil over exactly what’s going on plot-wise, the emotional shifts and struggles are clearly visible and carry the movie forward to a powerful, well-crafted climax.

Everything is just so, from the rich autumnal palette in the production and costume design to the typically sparse but evocative score from the brothers Evgueni and Sacha Galperine (who seem to be everywhere from here to Emmanuelle to Baby Reindeer at the moment).