Courtesy of TIFF

A typically marvelous performance from Emma Thompson elevates what might have otherwise played as melodrama in this sophisticated Ian McEwan adaptation.

In the arena of law, it’s commonly known — and widely derided — that one can unfairly “win” any debate by using the so-called “Helen Lovejoy defence,” named after the self-righteous wife of the town reverend in “The Simpsons,” whose fallback argument on any issue amounts to the inarguable emotional plea, “Won’t somebody please think of the children!?” As it happens, British law has someone tasked with doing exactly that, and she is the subject of Richard Eyre’s beautifully complex “The Children Act,” adapted by Ian McEwan from his 2014 novel of the same name, which is itself christened after a 1989 U.K. law known as the Children Act, dedicated to the welfare of minors.

From the moment she awakens till her head hits the pillow at night, family court judge Fiona Maye does little more than think of the children, ruling on whether to separate conjoined “Siamese” twins with one case (a tricky decision, as it means sentencing one to certain death so that the other might live) before turning around to evaluate whether to allow a 17-year-old Jehovah’s Witness to reject a life-saving blood transfusion that violates his religion.

Fiona is played by the great Emma Thompson in her best role since 2001’s “Wit” — and one that ought to remind her remind the casting gods that Thompson’s good for more than playing magical governesses (“Nanny McPhee”) and the women who write about them (P.L. Travers in “Saving Mr. Banks”). It’s not that Fiona doesn’t have other things on her mind, but her job is such that she takes each case rather personally, leaving her very little energy to tend to her personal life, including a marriage to her devoted, yet woefully neglected husband Jack (Stanley Tucci), who asks her after 11 months of bed-death whether he might have her permission embark on an extra-marital affair.

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Told with a depth of empathy so profound — and so British — that a rather sizable segment of the viewing public will either reject or ignore it outright, “The Children Act” is that rarest of things: an adult drama, written and interpreted with a sensitivity to mature human concerns — not just the quite personal complexities of maintaining a 30-year relationship with no children of their own, but the more broad-reaching tension between the law and firmly held religious belief. That’s something we’ve reliably come to expect from McEwan, whose novels include “Atonement” and the newly adapted “On Chesil Beach,” which have collectively yielded some of the richest female screen roles this century.

More restrained than director Eyre’s earlier work (“Iris,” “Notes on a Scandal”), yet driven by an energy for which he is directly responsible (as when the camera chases a pair of tardy reporters through courtroom corridors in order to arrive breathlessly just in time for a verdict), the wonderfully nuanced film concerns Fiona’s attempts to reconcile these two weighty challenges: There is the fate of the Jehovah’s Witness, Adam Henry (Fionn Whitehead), which rests in her hands, and there is the future of her marriage, which she has successfully shifted to the back burner for so long, but now hangs in the balance.

For Fiona, it’s clear: As someone who embodies the governing principle of the Children Act — which states that “children’s welfare should be the paramount concern of the courts” — it would be selfish of her to think of anything above the life of a child, even one she has never met. And yet Jack knows that her work will always take precedence, and that a disheartening pattern has taken hold at home, where their love holds strong, but he can scarcely pull her attention away from her court briefings for even a moment. Perhaps unwisely, he goes through with his plan to seek physical intimacy elsewhere, to which Fiona reacts harshly, but not unreasonably, by initiating divorce proceedings.

Through it all, Fiona’s attention remains as focused as the can possibly manage on the case at hand, Adam’s blood transfusion, calling an emergency session of court and making the highly unconventional decision to visit the young man in hospital. In testimony, Adam’s religious-minded parents (played with earnest conviction by Ben Chaplin and Eileen Walsh) describe their son as an exceptional person, and Fiona soon discovers for herself what they mean.

Adam is no robot, blindly reciting the beliefs of his parents and church. However medieval it may sound, he sincerely believes that a person’s God-given soul resides in his blood, and that to contaminate it with another’s is to reject the sacred gift of life — a naïveté that co-exists alongside a precocious interest in music and poetry. An easily avoidable tragedy waiting to happen, Adam sits in his hospital bed holding a guitar, and in the first of several unabashedly sentimental such scenes, he and Fiona sing “Down by the Salley Gardens” together, a folk song whose words were written by Yeats.

The encounter sparks something in Adam, who becomes obsessed with Fiona, making things awkward after she has moved on to other cases. Whitehead (who played the childlike English soldier who survives the opening scene of “Dunkirk”) is clearly quite the discovery, a young actor with desperate eyes and an early ability to access so many levels, and he manages to sell the risky, some-might-say overripe romantic fixation Adam develops for Fiona, who is the first adult other than his parents ever to have taken an interest in him.

Their song, “Down by the Salley Gardens,” and music by extension serves as a recurring motif here, as Fiona indulges but one extracurricular pastime: rehearsing piano with her friend Mark (Anthony Calf), a High Court barrister. That hobby sets up the film’s climactic emotional scene — one whose raw, wrenching power depends entirely on how successfully audiences consider every preceding element to have worked. Thompson interprets the moment beautifully, and indeed, the entire film hinges on how deeply felt her performance comes across. The actress is playing someone whose brain is constantly working, which she depicts as a kind of distraction: While her body is there in frame, her mind is often miles away, thinking of the children — those she’s saved, those she’s lost and those of her own which she and Jack will never have.