When I was a kid, I used to watch Perry Mason every day after school. I was drawn to the show's black-and-white clarity. Perry always found out who was lying, who was telling the truth, who was guilty — and why.

As I grew older, I naturally discovered that things are grayer and more elusive in real world courtrooms. It's not simply that you can't always be sure who's telling the truth, but that sometimes nobody knows the truth well enough to tell it.

This ambiguity takes mesmerizing form in Saint Omer, the strikingly confident feature debut of Alice Diop, a 43-year-old French filmmaker born of Senegalese immigrants. Based on an actual criminal case in France in which a Senegalese woman killed her baby daughter, Diop's fictionalized version is at once rigorous, powerful and crackling with ideas about isolation, colonialism, the tricky bonds between mothers and daughters, and the equally tricky human habit of identifying with other people for reasons we may not grasp.

Saint Omer begins with Diop's surrogate, Rama (Kayije Kagame), a successful intellectual writer who has a white musician boyfriend and a Senegalese mother she can't quite stand. She heads off from Paris to the town of Saint Omer to watch the trial of Laurence Coly (Guslagie Malanda).

Laurence is a Senegalese woman who once dreamed of being a genius philosopher — she casually namechecks Descartes — but now confesses to causing the death of her baby daughter Elise. Rama plans to write a book about her titled, Medea Shipwrecked.

Like Rama, we get sucked into a trial that unfolds in the French manner, meaning that the judge — empathetically played by Valérie Dréville — questions most of the witnesses in a probing, expansive way reminiscent of a Ph.D. oral exam.

We see the self-serving slipperiness of Laurence's partner, a bearded white man 30-odd years her senior who wouldn't let her meet his family or friends. We hear the righteous words of the mother she always felt distant from, and we cringe at the testimony of her one-time professor who, with exquisite cultural condescension, wonders why Laurence had wanted to study Wittgenstein rather than a thinker befitting her African roots.

Of course, the trial's star attraction is Laurence who, in Malanda's rivetingly charismatic performance, is at once controlled and unreadable: She makes us feel that there's a whole universe in Laurence's head that we can never reach. Although her testimony is delivered matter-of-factly — even when she blames the murder on sorcery — she often contradicts her earlier statements. Asked why she left Elise to die on the beach, she replies that she doesn't know, adding, "I hope this trial will give me the answer."

If Laurence remains a mystery, even to herself, we gradually realize why Rama is so enthralled by her story. I won't tell you exactly why, but I will say that Saint Omer is as much about Rama as it is about Laurence. The film explores Rama's own cultural alienation, trouble with her mother, and intellectual analogies to Elise's murder that may or may not be accurate. She wonders if she may contain within herself the seeds of whatever has been motivating Laurence.

Now, it must be said that Rama's story is less emotionally compelling than the murder case, in part because Kagame, though haunted looking, is a less expressive actor than Malanda. That said, her story is important conceptually. Rama's identification with Laurence shows how the social and psychological issues raised in the trial go well beyond the courtroom. Saint Omer is about far more than just one murderous mother.

What makes the movie unforgettable are the scenes in the courtroom, every moment of them gripping. Diop started out making documentaries, and she looks at the trial with a born observer's unblinkingly rapt attention. Using superbly-acted long takes, she scrutinizes the characters for hints as to what made Laurence do it; she makes us feel the volcanic emotional pressure behind Laurence's largely unflappable demeanor; and she lets us see the complex, multi-layered network of social and psychological forces that led her to the beach. We keep asking ourselves whether Laurence is a criminal — or a victim. There are no easy answers, no simple explanation for her actions.

We're a long, long way from Perry Mason.