You can call Xavier Dolan’s “Mommy” a domestic drama, but that would hardly suggest the ferocity of this film about a high-energy but lower-class Montreal woman, her turbulent teen son, and the mousy neighbor who befriends the pair. It’s a small-scaled soap opera on steroids, a portrait of familial crisis so pointed and wrenching that though nearly two-and-a-half hours long, it rarely fails to engross—and to an extent—exhaust.

The scale is emphasized by Dolan’s decision to frame the film in 1:1 format, which takes the form of a box in which the sides of the usual rectangular image are cut off—the very opposite of letterboxing—and creates a sense of visual confinement (or, if you prefer, intense focus) that broadens into full screen in only a few instances to register a brief, and frankly unreal, change of attitude. The technique is extremely calculated, of course, but it has the intended effect.

As to narrative, the film is at once simple but emotionally complex. Diana “Die” Despres (Anne Dorval) is a widow with a problem—her 15-year old son Steve (Antoine Olivier Pilon), whose ADHD makes him a very loose cannon. Summoned to remove the boy from a juvenile facility where he set the cafeteria afire and injured another resident, she has a car accident from which she emerges bleeding and belligerent, barely stopping to wipe away the blood before moving on to take Steve to their new digs. These opening sequences swiftly paint a picture of a troubled, volatile yet obviously codependent pair trying to get by and get along somehow.

Neighbors soon enter the picture. One is a lawyer down the block who spots Die and tries to strike up a relationship, without much success at first. The other is Kyla (Suzanne Clement), the woman who lives across the street. She’s a teacher on leave, dissociated from her own husband and daughter and suffering from a pronounced stutter. When Die tries—with a monumental lack of success—to home-school Steve, Kyla offers to tutor him, and though his volcanic shifts of temper are a challenge, he seems to make some progress. Kyla’s relationship with the pair appears to help her with her demons, too, and the three become close, even though one can sense they’re always on the edge.

It’s the complicated connections among the trio that dominate the film, which certainly has narrative elements—a major plot point involves Die’s calculated romancing of the lawyer in order to secure his help in defending Steve against a lawsuit, an arrangement that the boy will of course torpedo—but for the most part Dolan’s structure is deliberately ramshackle, mirroring the wild, unpredictable characters themselves. There are long scenes of Steve simply busting out—riding his skateboard, throwing around carts in a supermarket parking lot—and others, like a remarkable one with Kyla, in which his dangerous side suddenly bursts forth, only to be immediately followed by a downward spiral that evinces his vulnerability. Die is given the same sort of treatment, with scenes that show her exuberantly dancing with Kyla and Steve one moment, only to be arguing with her surly boss the next, or confronting Steve about his behavior. The emotions of the film change on a dime, just as those of mother and son do—all to the accompaniment of an eclectic assortment of pop songs, supposedly representing a mix that Steve’s dead father made for him and his mother before his death.

And hanging over the story like a sword is a news announcement that serves as a prologue, informing us that the government has just passed a law allowing parents with problem kids to place them in a state institution without having to go through the court system. The thought thus pervades all that follows: will the time come when Die will finally give up trying to control Steve’s darker impulses and simply turn him over to the state?

“Mommy” is, as the foregoing might suggest, a messy film, but though that might frustrate and annoy some viewers, others will find the very lack of structure a cinematic reflection of the characters’ turmoil. And certainly it showcases astonishingly forceful performances from the three leads. Dorval and Clement take pride of place, operating at two emotional extremes, the one highly extroverted and the other recessive beyond endurance. But it’s Pilon who’s the glue that holds everything together—and whose simmering unpredictability constantly threatens to blow everything apart. His cherubic features can instantly turn into a menacing glare, and yet one still senses the boyish fear beneath the bravado. The quicksilver shifts might remind you a bit of Edward Norton’s chameleon turn in “Primal Fear.”

The crew—particularly cinematographer Andre Turpin—are obviously in complete synch with Dolan’s vision, and though one might complain that at well over two hours the picture is self-indulgent (Dolan did the editing himself), even at that length it rarely feels padded. Bracing and intense even at its most rambling, “Mommy” is a cinematic scream of the primal pain and love shared by parent and child, and it won’t leave you unmoved. In fact, you probably won’t be able to shake it off even if you want to.