Starring Emma Thompson, Stanley Tucci, Fionn Whitehead; Ben Chaplin; directed by Richard Eyre; 105 minutes; R for a sexual reference.

Emma Thompson plays God with convincing aplomb in “The Children Act,” an adaptation of Ian McEwan’s 2014 novel that fairly bursts with ideas about right and wrong, head and heart, and sense and sensibility.

As London judge Fiona Maye, Thompson is all crisp rationality and swift, no-nonsense alacrity. As the movie opens, she’s delivering a Solomonic verdict regarding a pair of conjoined twins, reminding the parents and their lawyers that “this court is a court of law, not of morals.”

That fact will come into even more high-stakes play when Fiona hears a case regarding a 17-year-old leukemia patient whose Jehovah’s Witness parents are refusing a blood transfusion that he probably will die without. When Fiona goes to visit the boy, she discovers a bright, seductively romantic adolescent who instantly bonds with her over literature, music and intellectual curiosity.

The legal parameters of Fiona’s decision are relatively straightforward; the title of “The Children Act” refers to the court’s responsibility to consider the welfare of minors its paramount concern. But Fiona’s decision forms only one strand of the story’s motives and misgivings.

For one thing, latent tensions in her marriage to Jack (Stanley Tucci), a classics professor, are surfacing in unexpected ways, offering another lens on Fiona’s avoidance of emotionalism, which has been one of her professional strengths. Once she delivers her verdict, its implications — the responsibility and expectations it confers — make themselves felt in increasingly unsettling encounters.

Accompanied by Bach piano pieces, “The Children Act” is a pleasure to watch and listen to, primarily as a showcase for Thompson’s gifts as an actress. True to McEwan’s literary vision, “The Children Act” allows all of its characters their dignity and good faith.