"Pieces of a Woman" has what may be the most memorable movie scene of 2020, but a memorable scene alone does not a great movie make.

Coming near the beginning and lasting more than 20 minutes, it appears to be one long take. Martha (Vanessa Kirby) is giving birth at home, with the help of partner Sean (Shia LaBeouf) and a midwife (Molly Parker), when something goes wrong. All of it — contractions, stumbling into a bathtub, complications, mounting concern from all three people — is captured in a seemingly chaotic but expertly choreographed scene that creates the illusion we are watching Kirby actually give birth.

That may sound like a stunt, but it doesn't play like one. The long take works because it suits the observational rhythm of Kornél Mundruczó's movie, which contains many long takes, and because it lends verisimilitude to a film that wants us to believe we are seeing a relationship fall apart in front of our eyes.

It's that stuff that doesn't work so well. We've met them in mid-relationship, but we can't believe self-described "boorish" Sean and elegant Martha are a couple. Their bond is strained at the start of the movie and there's nary a flashback to help us figure out what drew them to each other, much less to have a child.

Contempt seems to be the main thing they have in common, as indicated by thwarted sex, in the months after the childbirth scene, that ends with Sean telling Martha she takes too long to get in the mood, so "I don't want you anymore." (LaBeouf has been canceled due to abuse allegations since the movie was filmed, but the good news for the movie is we're supposed to think Sean shares traits with the LaBeouf that accusers have described.)

Further stacking the deck is Martha's rich, control-freak mother, Elizabeth, played by the great Ellen Burstyn. Elizabeth is supposed to come off as terrible, but circumstances outside the movie's control will have audiences inclined to concur with her disdain of Sean.

Burstyn is so convincing as a confident matron who can't understand why everyone doesn't agree with her that her villainy feels sympathetic. Yes, Elizabeth ignores her daughter's feelings, but on the other hand, the daughter is roughly as warm as the one Burstyn famously mothered in "The Exorcist."

Kirby, who won the best actress prize at the 2020 Venice Film Festival for "Pieces," is impressive, especially as her zonked character wanders around, disconnected from the world that's falling apart around her. Rightly, Kirby doesn't care if Martha is sympathetic or even understandable. Instead, her instinct seems to be that Martha's precarious situation is so painful that it's best to honor its uncertainty.

As real as that is, the movie often is phony. Despite Burstyn's efforts to humanize Elizabeth, the character feels plucked out of a 1950s movie where beautifully dressed people go to pieces, particularly in a ridiculous conversation with Sean at the climax. There's also a court case that makes no sense. That's the problem with a movie that begins well and goes south: The bad part is what sticks with you.

Chris Hewitt • 612-673-4367

Pieces of a Woman

⋆⋆½ out of four stars
Rating: R for graphic nudity, strong language and violence.
Streaming: Netflix.