It’s become old hat to refer to any Emma Thompson performance these days as a crowning achievement in her screen career.

But if Helen Mirren was born to play “The Queen,” she simply had to age into the part, the actress’s actress Thompson is so naturally imperious as an I-won’t-be-second-guessed children’s court judge “The Children Act” that you’d swear that’s what she’s been doing, between movie, these past few years.

As Mrs. Justice Fiona May she is a workaholic, taking her Solomonic duties and herself seriously. She’s deciding custody, weighing court efforts to retrieve children removed from Britain illegally and when we meet her, wrestling with a very public case about conjoined twin babies.

“The welfare of the child is paramount,” according to the opening lines of Britain’s “Children Act,” and it is her cover for a wide range of publicly controversial rulings — deciding that the parents don’t get to choose not to separate twins who won’t live long without such surgery. As thoughtful as “My lady’s” rulings are from the bench, “the logic of the lesser of two evils” and all that, when she gets home it’s “I’ve given instructions to slaughter a baby,” she tells her American husband, a college classics professor played by the great Stanley Tucci.

Jack, however, is feeling neglected. And when he announces “I think I want to have an affair,” Fiona — “Fee” — is aghast. She doesn’t have time for this. He has a paramour picked out, and that’s that. Her only dismissive riposte is “I can’t believe how cool we are.”

Weekend duty at the court gets her mind back on work, the sudden call by a hospital trying to save an under-age-of-consent leukemia patient. He’s a Jehovah’s Witness and he’s refusing a blood transfusion.

The judge and the court hear from a hematologist, irritated by the boy and his parents’ posistion, and from the kid’s father (Ben Chaplin, sensitive, working class and passioate) who explains that they all believe the soul is in the blood, and “transfusions pollute it…It belongs to God,” and therefor it is wrong to accept blood from another.

This is going to be a tricky one, something the distracted judge — she is also an enthusiastic piano accompanist at annual legal profession parties — will have to weight carefully. She’s always short with her all-serving clerk. Now she’s downright snippy.’

The court case is the most interest section of this Richard Eyre film, based on a script by Ian McEwan, who wrote the novel it’s based on. The arcanna of the British legal system — the robes, the wigs, the works — is showcased, as is the belief system of Jehovah’s witnesses.

A passionate lawyer for the parents cites “Common Law, privacy and precious dignity” as what’s being debated here. The lawyer for the hospital reminds one and all just how secular Britain can be. arguing that “a religious cult” has convinced the kid to become “a pointless martyr” based on their interpretation “of an Iron Age text (the Bible).”

It’s when the judge takes the unusual step of going to see the lad (Fionn Whitehead) that Eyre’s film — he did “Notes on a Scandal” — takes a turn for the loopy. Adam is charming, breathlessly arguing for his sanity, his personal sanctity and against her acting as “an interfering busybody.”

He reasons with her, explains the merits of belief, that deep-seeded sense of right and wrong his upbringing gives him. And will not let her leave without playing her something on his guitar.

They duet on a folk song setting of a Yeats poem. Seriously.

And after she high-handedly does what she always does, rules from on high and thus “saves” his life, Adam only gets clingier. She has closed one world off to him and he’s scrambling to replace it with another, built on her “wisdom.”

The moral quandary of the film is interesting, but Mrs. Justice Maye’s role in it is more so. As she struggles to treat her marriage’s problems with the same edict-like finality, she fends off Adam’s post-court pleas and moves on to the next pronouncement.

Thompson plays the judge with a brittle, icy edge, firm with Adam even as you can see his plight has reached her humanity. Her performance pulls it back from “loopy” and the movie passes muster on the strength of that.

The domestic melodrama has sparks thanks to the fortuitous pairing of the Oscar winning Thompson with the formidable Tucci. What could play as trite and trivial next to the Big Issues weighed in the other scenes has actual gravitas thanks to the performers.

Thompson also plays the piano and sings, here and there, showing this iron-willed judge’s vulnerability as she does.

Young Whitehead (“Dunkirk”) makes Adam almost laughably overbearing, in an overeager boy’s way. The writer McEwan gets across what the kid’s about in a scene or two, and gives us one or two more for good measure.

This certainly played differently in the UK than it will in the US, where children’s rights appear to have more latitude, even if they can seem even more at the mercy of the caprices of the judiciary.

But what translates on both sides of the Atlantic is the acting, especially Thompson, finally starting to get the roles the Great Mirren began to land at her age, crowning yet another film with the latest in a long line of subtle, layer performances.

MPAA Rating:R for a sexual reference

Cast: Emma Thompson, Fionn Whitehead, Stanley Tucci, Ben Chaplin

Credits:Directed by Richard Eyre, script by Ian McEwan. An A24 release.

Running time: 1:45