“A grown-up and a child shouldn’t be friends,” a brilliant but bonkers adult tenderly explains to a 10-year-old in Arnaud Desplechin’s glorious, exhilarating sixth film. Voluble and cerebral but never less than heartbreaking, “Kings and Queen” takes as its subject familial and romantic ties sundered and sutured, ending on a perverse grace note about the extraordinary powers of love and memory.

This sublime tragicomedy follows the entwined lives of two ex-lovers, Nora (Emmanuelle Devos) and Ismael (Mathieu Amalric). Delicate, damaged Nora, a 35-year-old art gallery director in Paris engaged to a frosty bourgeois, travels to Grenoble, where her saturnine father, Louis (Maurice Garrel), has been tending to her precocious fatherless son, Elias (Valentin Lelong). Almost immediately after Nora’s arrival, Louis is diagnosed with terminal bowel cancer and given a week to live; the dutiful daughter must track down her vagrant sister.

Meanwhile, wild-eyed viola player Ismael, a noose swaying at the ready in his dark Paris flat, is carted off to a sanatorium at the request of his sister. “Yes, my soul aches, but you can’t help me. Women have no soul,” the mad musician declares to the cool, reserved hospital psychiatrist Dr. Vasset (Catherine Deneuve, in a stunning cameo).

Unmoved by Ismael’s retrograde disquisitions and fits of pique, the imperturbable shrink orders a 10-day stay for the violist, who’s allowed out only to continue his sessions with his superstar psychoanalyst, Dr. Devereux (Elsa Wolliaston), a corpulent, regal French-African woman. Desperate for help from his own equally pixilated attorney, Ismael struggles valiantly, if not maniacally, for his freedom.

What ties the narratives of Nora and Ismael together is Elias, who was always especially fond of the neurotic string player. Nora wants her ex, with whom she shared several memorably miserable years, to be her son’s adoptive father. Recriminations and reconciliations, rapprochements and reversals all mark Mr. Desplechin’s emotionally rich view of what it means to love, forgive, and forge ahead.

Mr. Desplechin (who also co-wrote) is an actors’ director, generously attuned to the greatness of his players. Much of the force of “Kings and Queen” emanates from the astonishing performances of Ms. Devos and Mr. Amalric, Desplechin regulars who also happen to be two of France’s finest thespians. Playing damaged goods, the actors never succumb to broadly caricaturing mental anguish but instead gracefully balance grit and despair.

Beautifully shot by Eric Gautier, the film attains its supreme achievement in sound – or, specifically, speech. Mr. Desplechin’s films always acknowledge the power of words. “My Sex Life … or How I Got Into an Argument,” a masterful ensemble piece from 1996, is a garrulous tour de force; in 2000’s “Esther Kahn,” the titular character chews glass so she won’t have to speak onstage.

French cinema is often parodied for its prolixity, but in “Kings and Queen” speeches are the dramatic high points. In one 10-minute scene, Ismael explains to Elias why he can’t adopt him. Essentially a monologue by Ismael, this mini-masterpiece shuttles rapidly from paternal kindness to almost chilly distance. “The past isn’t what’s vanished – it’s what belongs to us now,” the mentally rehabilitated musician explains to the slowly comprehending boy. I don’t think I’ve ever seen a more moving breakup scene than this one, made all the more poignant because this is an adult saying goodbye to a child he once loved.