As courtroom drama, this is such a departure we don’t even hang around for the verdict, although the facts are not in dispute. In the dock is a young French woman of Senegalese origin, Laurence Coly, who is as baffled as the prosecutors about why she killed her child. She tells the court she hopes this trial might give her the answer.

Saint Omer is based on a true case of infanticide in France in 2016, when a woman left her baby to die and blamed it on witchcraft, with the argument here running on similar lines. Coly, played with stoic grace by Guslagie Malanda, is on trial for abandoning her daughter on the beach in the path of the incoming tide. She is seen by witnesses and on CCTV, yet she pleads not guilty, citing sorcery as a cause.

What follows is the eerie unravelling of an intelligent young woman from a former French colony who attends university and is pressured to rise above her immigrant status. Watching proceedings each day is another young woman, Rama (Kayije Kagame), who is also Black but has found her place in France and lectures her students about the adored French novelist and essayist Marguerite Duras.

Rama attends the trial hoping to write a book about Medea, the granddaughter of the sun god in Greek mythology who murders her children for revenge. Instead, she is felled by Laurence’s story, which has echoes of her own; she is also pregnant.

Laurence describes events without emotion: the moon lit the way as she lay the baby on the beach where she thought the waves would swallow her up. Instead, the tiny corpse was found by a fisherman who thought she was a seal.

As others give testimony, we learn Laurence had dropped out of university, became homeless and moved in with a much older man who was incapable of love. For months she hid in his apartment, never going out or being acknowledged by him in public. After leaving her baby on the beach, she went home and slept like a log.

The film, France’s entry in last year’s Academy Awards, is quintessentially French in its angsty exploration of reasons for madness. Racism, exclusion and elitism conspire against Laurence, who the French press single out for her educated, formal French, and who is questioned in court about her thesis on the Austrian philosopher Wittgenstein. She is asked: Why not choose someone close to your own culture?

The staging and casting are impeccable, down to Laurence’s cheap wig that hides her African hair compared with Rama’s confident, free-ranging plaits. The judge, bewildered by Laurence’s ambiguous replies, is a study of compassion and patience in the service of justice.

The director is Alice Diop, a French Senegalese woman who studied at the Sorbonne and whose films focus on the margins of society. Diop followed the real trial that inspired the film, and could see the complexities of the case that were largely overlooked. Here, she takes what was unspoken or ignored and puts it on the stand.

Saint Omer is in cinemas now.