Those two stark monosyllables in the title are a callback to Leigh’s debut from 1971, Bleak Moments: they lay down an uncompromising and yet also enigmatic challenge. This movie isn’t going to be an easy ride; of course not. But if it promises to reveal hard truths in their simplest and most irreducible sense, then what are these hard truths exactly?

Perhaps the hardest and most obvious truth is that the lead character is suffering from clinical depression and urgently needs to see a professional about it. But no one actually puts that hard truth to her, or maybe they did, long ago, and had it angrily thrown back in their face. And so the second hard truth is that no one can help someone who doesn’t want to be helped.

Hard Truths is a deeply sober, sombre, compassionate drama about a black British family, with flashes of fun and happiness that are emollient if not exactly redemptive. After his two large-scale historical dramas in 2014 and 2018, Mr Turner and Peterloo, this is a return to Leigh’s classic style, inhabiting a contemporary world shot by cinematographer Dick Pope in cold, clear London daylight, with sad family scenes and vignettes of disillusionment and quiet day-to-day courage divided by dreamy, melancholy woodwind melodies composed by Leigh’s longtime musical collaborator Gary Yershon.

Most importantly it reunites Leigh with that overwhelmingly powerful female lead, Marianne Jean-Baptiste, whose name was made by her electrifying performance in Leigh’s 1996 film Secrets and Lies, and might well get made all over again with her formidable appearance here, demonstrating the terrible connection between depression and anger. And yet her character is at the centre of what for me is this film’s most challenging aspect: the fact the film arguably withholds, or amputates, the inner core of her emotional truth, pitilessly denying the audience the catharsis, the extended cloudburst of revelation, that they have perhaps been longing for.

Jean-Baptiste plays Pansy, a woman in middle-age living in a spotless but featureless suburban house; very clearly, she is in the terrifying endgame of depression. She begins the drama by waking with a melodramatic gasp from another painful dream and carries on with her day by shouting – or rather speaking angrily and 15% too loudly – at everyone with whom she is annoyed; that is, everyone she sees. She berates her husband, her son, her doctor, her dentist, the hapless person on the till in a convenience store, the person trying to sell her a new sofa (she is angry about the condition of the one she has at the moment), and the guy in the car park asking her if she is about to vacate her spot so he can have it. She is angry, but also voluble and articulate about her grievances, real and imagined, making objection or interruption almost impossible, because it sounds at first as if she might have a point. (“Don’t patronise me! I’m not a child!”) She is frantic at the thought of insects and animals getting into the house and the sight of a fox in the garden brings her to hyperventilation; a scene which is funny in an unfunnily awful way.

Pansy’s nonstop rage has reduced her husband and son to a longterm stricken silence of their own. Curtley (David Webber) is a plumber whose taciturnity clearly speaks volumes about his own swallowed despair, and their son Moses (Tuwaine Barrett) is a morose guy in his early 20s who provides Pansy with a perennial rage-pretext by having no idea what to do with his life other than slope and mope about the house or go for low-morale walks on which he is bullied by local guys.

So why are they like this? Leigh brings us a little closer to the answer by showing us Pansy’s good-natured sister and her family, who are shown by Leigh to exist in a Jekyll-and-Hyde opposite state of happiness to Pansy’s misery, a happiness which is mysteriously part of Pansy’s ecosystem of wretchedness. Chantelle (a lovely performance from Michele Austin) is a salon hairdresser and single mum much beloved by her clients for her wisdom and gentleness, and who loves her two smart grownup daughters; Aleisha (Sophia Brown) is a trainee lawyer and Kayla (Ani Nelson) works for a skincare company presided over by a somewhat cantankerous chief executive, played by Samantha Spiro. Leigh shows both Kayla and Aliesha messing up in unserious ways at work, but also learning from their mistakes and basically impressing their employers with their attitude. Chantelle’s relationship with her daughters is filled with happiness and laughter. It couldn’t be more different from Pansy.

We get still closer and harder to the truth when Chantelle and Pansy go on a Mother’s Day visit to the grave of their mother, who appears to have got along a lot better with Chantelle, but who was perhaps looked after more in her latter years by Pansy, who bitterly remembers the trauma of finding her dead. Yet even this does not fully explain what is happening. The film’s menacing weather is set by Pansy, and after a while it almost begins to resemble some kind of deadpan psychological horror … and, yes, perhaps that is exactly what this is.

But where is it all leading? I was expecting a great climactic relief, like the one at the end of Leigh’s 2002 film All or Nothing, but perhaps he thinks that this is too easy, too contrived, too fictional. Pansy has a scene of hysterical laughter and tears at the thought of a certain bunch of flowers, which is also to be the focal point of some bitter anger from Curtley. But this is an extended moment of relief or release. It could also be that Leigh took an artistic decision to delete a scene of this sort that he had initially conceived or even fully filmed. It is certainly an arrestingly deliberate storytelling choice. Perhaps this is the hardest truth of all: that in our real world, in all its mess and pain and unresolved confusion, there is no final reckoning; just moments of happiness and truth that we have to hang on to. It’s an exceptional performance from Jean-Baptiste.

• Hard Truths screened at the Toronto film festival.