Many of us are familiar with the pain of someone becoming unfamiliar even when they are right in front of us. It could be a relationship gone awry, a friendship past its expiry date.... Very often, and this is the most heart-wrenching, it’s what happens when you lose a parent even before you have actually lost them.

That’s what happens in The Father, a poignant portrait of a patriarch who feels his world falling apart around him. It’s just that Anthony, an octogenarian widower with a twinkle in his eye, and played remarkably by Anthony Hopkins, doesn’t quite remember his world anymore. For Anthony is in the throes of acute dementia. On most days, he can’t recall whether he’s had his dinner. More often, he doesn’t remember what his daughter looks like.

Ann (Olivia Colman) experiences her father fading away every day, but is largely stoic in the face of it. She breaks down occasionally, especially when he flies into a rage and then good-humoredly tells her that she’s the one being repetitive. The main cause for her exasperation, even as she helplessly watches her father turning into a stranger, stems from his refusal to allow a caregiver into his plush London flat.

Debutant director Florian Zeller, who adapts his own play of the same name from stage to screen, makes Anthony’s mind the canvas from which to tell this story. As a viewer, we are taken right into the deep recesses of Anthony’s mind, and his ever-shifting reality. And Anthony’s mind is, well and truly, a labyrinth, with his memories constantly slipping through his fingers. Is Ann married? Is she moving to Paris with a new boyfriend? What happened to his other daughter? Where is his watch? Anthony’s memories are misty, and the unreliable narrator — a favourite, and sometimes overused, cinematic trope of late — acquires a new dimension as the narrative of The Father flows along.

Apart from being a gut-wrenching depiction of what dementia does to once-sprightly minds, Zeller crafts a brilliant piece of cinema in which deep emotions meet a thriller tinged with a touch of horror. For Anthony’s mental maze is complemented by his home, which we see through his eyes. As a result, the furniture keeps shifting, the piano morphs into the bar, the paintings vanish from the walls.... Faces and places mix up and one doesn’t really know what’s true and what’s not. Zeller places the audience in a vulnerable, questioning position, through sound design, camera movement and perspective, making us empathise deeply with Anthony’s plight.

But even as Zeller and co-writer Christopher Hampton tug at our tear ducts with this moving narrative about what it means to grieve for someone even while they are alive, The Father is also a kind of gaslight thriller where the mind acts as both predator and prey. The horror is, well, not real here. Or is it? Anthony’s mind constantly keeps us guessing.

One would expect the mood in a film like this to be elegiac, but The Father is anything but. That is courtesy a staggering performance from Anthony Hopkins, who at 83, lithely potters around his apartment, his physical movements being a reflection of the constant ebb and flow of his mind and its memories. Hopkins, who won a Best Actor Oscar for The Father earlier this year, plunges headlong into the part, even while making it seem that he’s improvising as he goes along. It’s a part full of anger and helplessness, but also humour and a certain sprightliness. And Hopkins makes Anthony a man who brings on a surge of emotions, examining them all with magnificent equanimity.

Olivia Colman, one of the best we have in the business now, is equally good, but this is Hopkins’ film and she quietly and gracefully allows him to take centrestage.

For those who deal with dementia on a first-hand basis, The Father may be triggering. This is a film that is at once staggeringly effective and profoundly upsetting. But it is also cinema that demands to be watched. Make time — and heart — for it.

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The Father

Director: Florian Zeller

Cast: Anthony Hopkins, Olivia Colman, Mark Gatiss, Imogen Poots, Rufus Sewell,Olivia Williams

Running time: 92 minutes

The Father is now streaming on the Lionsgate Play app