The Children Act offers Emma Thompson her best onscreen role since Saving Mr. Banks — and also, not coincidentally, marks the first time since then that she’s been given a lead part worthy of her talents.

As high-ranking English judge Fiona Maye, calm and confident while making rulings for complicated cases, she commands each courtroom and domestic scene under the direction of Notes on a Scandal’s Richard Eyre, who brings a similarly layered and distinctly British emotional tone to these tightly scripted proceedings by Ian McEwan, adapting his own novel.

Strengthened in certain ways while weakened in others once her husband Jack (Stanley Tucci) decides he wants to have an affair, Fiona proves less composed when it comes to the matter of Adam Henry (Fionn Whitehead, Dunkirk), a Jehovah’s Witness on the cusp of legal adulthood who refuses a blood transfusion on religious principles.

Dealing with the consequences of her legally-binding decision, the Mayes and Henrys — as well as Fiona’s painfully loyal assistant Nigel (Jason Watkins, FX’s Taboo) — react in unexpected ways that elevate the relatively small story to something nearing minor epic status, at least for these characters.

Such ends are fairly typical of a McEwan yarn, and though it’s been a while since Thompson was last afforded the chance to electrify the screen in such a profound dramatic way, The Children Act is of a piece with her finest work, too.

Grade: B-plus. Rated R. Starts Sept. 28 at Grail Moviehouse

(Photo: A24)