The film “Supernova” is a small and superficially tidy thing, notwithstanding the astronomical implications of its title, which augurs the sudden explosion of a star or – more metaphorically – some brilliant light, often heralding its extinguishment. It seems, at first, an odd allusion for a road-trip story that takes place largely inside a boxy old RV trundling along the one-lane roads of rural England, with two middle-aged men at the helm.

But the novelist Tusker (Stanley Tucci) and his longtime partner Sam (Colin Firth), a musician on the way to give a piano recital for the first time in a while, are actually taking the long way toward a destination of a different sort. It’s one whose unexpected appearance on the horizon breaks the silence of the film – a silence filled with the familiar, almost cozy bickering of all loving couples – with the tiny cracking sounds of a breaking heart.

Tusker has early-onset dementia. It isn’t obvious, except when we see his forgetfulness: He wanders off from a roadside convenience store with the couple’s dog. Or when we get a glimpse of dysphasia, watching him struggle to find the word “triangle.” But otherwise Tusker seems OK, not just to us, but to Sam. “Can you tell that it’s gotten worse?” Tusker asks his partner, gently prodding him, as if to talk him down from his pedestal of denial. Tusker, still fit, still handsome, still funny, is not the person he used to be, he insists: “I just look like him.”

Mostly, what we see is the love between these two people, as they chat, fall asleep together, take a pit stop at the home of Sam’s sister (Pippa Haywood), where Tusker has organized a surprise party for Sam, and, later, overnight in a small, rented cottage. Their talk is about the past – and, for Sam at least, their future together.

It’s on this journey – which would be enough, just to spend time in the company of two such fine actors – that a revelation is made. When it comes is less important than how it comes, and what it precipitates. It brings about an emotional revolution and a resignation of sorts, a momentous transition signified not with the shouting and histrionics of melodrama, but the pangs of deep feeling that seems to throb, at times, with an ache too real to bear. Tucci and Firth have never been better than they are here, and they earn every superlative that has been laid on them in early reviews.

What transpires in “Supernova” doesn’t stay on the screen. Written and directed by the English actor and filmmaker Harry Macqueen, with an understatement more powerful than cheap sentimentality, it’s a love story that feels raw, especially if you have watched a loved one deteriorate.

“You break my heart,” Sam tells Tusker at the film’s climax. In a way, it’s the only time when “Supernova” is guilty of stating the obvious. But it’s also a moment in which those words need to be said, not because we don’t know what’s happening, but because they articulate – they echo – what the audience is already feeling.

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