Colin Firth and Stanley Tucci light up the end-of-life drama in ‘Supernova’

The premise of “Supernova” couldn’t be simpler, as a longtime couple grapples with the dire challenge of early-onset dementia. Having that duo played by Colin Firth and Stanley Tucci, happily, elevates the material, in a small but poignant story about love, loss and letting go.

Firth’s Sam and Tucci’s Tusker have been together for decades, and they’re introduced on a cross-country trek in a beat-up old camper. It’s what amounts to a last hurrah, with Tusker having pushed his partner to perform a piano recital, stopping to see family along the way.

Both are keenly aware that the hourglass is running out on the life they’ve known. Tusker’s condition is gradually worsening, with occasional moments where he wanders off or struggles to articulate thoughts. He’s mostly fine now, but his inevitable deterioration — and the unwelcome prospect of “becoming a passenger” in his own body, as he says — looms like a shadow over them.

As for Sam, the trip is dogged by the fact that he’ll soon be a full-time caretaker, a role to which he has committed himself that nevertheless scares him. “You’re not supposed to mourn someone while they’re still here,” Tusker observes, summing up Sam’s uncomfortable plight.

“Supernova” isn’t a great title for a movie like this — it’s a crafty play off the pair’s interest in stargazing — although it’s oddly appropriate, since the two stars keep things watchable even when there’s nothing much happening, which is most of the time. In that regard, the film joins a long roster of end-of-life romances, in this case unfolding in what feels like slow motion.

Marking the second writing-directing effort from actor Harry Macqueen, this British production doesn’t bother with flashbacks or much reminiscing about the couple’s relationship. All that history comes in the form of casual exchanges and small gestures that reflect a lifetime together, as touchingly conveyed by Firth and Tucci, whose real-life friendship surely contributes to that shorthand. (The latter will be featured in a CNN food and travel show premiering in February.)

As understated as the movie is, the emotion of the situation comes through loud and clear. While the pacing might have benefited from a few more detours or details, the audience has a pretty good understanding of where this road began and where it leads.

“Supernova” is by any measure a modest production, but it accomplishes what it sets out to do: Creating a touching, low-key showcase for its stars, one that allows them to cast a bright light.

“Supernova” premieres January 29 in select theaters and February 16 on demand. It’s rated R.