It’s September, and construction engineer Sean (Shia LaBeouf) and his partner Martha (Vanessa Kirby) are preparing for the birth of their first child. Banners at the baby shower proclaim “It’s a girl”, before the couple head to pick up a car bought for them by Martha’s domineering mother Elizabeth (Ellen Burstyn). There are signs of unease between the three: the thwack of Elizabeth’s cheque book immediately denting Sean’s masculinity, Martha’s shoulders perceptibly tightening and locking as her mother’s silent disapproval sours the air between them.

These issues don’t pale into insignificance with what comes next. They’re simply the kindling for the slow-burning fire that follows and looks to swallow them all whole.

Martha’s labour is set to be a routine home birth. But when it starts, the midwife can’t come – she’s in the middle of a heavy labour with another woman – and a replacement arrives (Molly Parker). Martha howls, bucks, growls and swears in a 24-minute childbirth one-shot that comes close to body horror. Shot with a gimbal, rather than a handheld, it doesn’t have the frenetic, jumpy movement of a horror film, but it does have the intensity and immersive feel of one, and that’s not all it shares with the genre.

On the other side of this scene, in its wake, the film changes gear, tone and intent. Tragedy has struck, and if that half an hour was all about action, this is all about inaction; the paralysis in the wintery depths of grief. A grief that hasn’t just unmoored Martha from Sean, but from her mother too, who seems only concerned with seeing the midwife prosecuted.

While Sean, a remarkable LaBeouf, wears his agony on the outside – returning to drugs, cigarettes and alcohol to escape – Martha turns entirely in on herself. She’s numb and stripped raw, every nerve end alive with pain. A characterisation built from such seeming contradictions is entirely deliberate in performance, writing and direction. It’s the reality of a pain, a situation, so gigantic, illogical and impossible that you break completely just attempting to withstand it. And Kirby here is nothing short of breathtaking.