The film opens with a shot that shouldn’t be so radical, but is: Sam (Firth) and Tusker (Tucci) are asleep, and naked, spooning amid tangled bedsheets, and the way Sam hangs clings to his partner telegraphs a gulf ahead. It’s so tender a moment you almost don’t want the camera to flinch. Sam, we learn, is a decorated composer, and Tusker a celebrated novelist, and to mark 20 years of being together, they’re embarking on a road trip across England to a destination that will prove devastating. As intellectual equals, their snarky banter is a pleasure to watch, and the real-life friendship between Tucci and Firth is evident as they swing from bickering to more dour conversation. “Why didn’t you let me pack for you?” Sam says while they’re on the road, fiddling with Tusker’s glasses as he tries to follow an atlas. “Do you hear that? It’s the sound of me ignoring you.” Tusker says. Their back-and-forth has the lived-in quality that suggests a lifetime of reading each other’s minds.

Sam and Tusker are on the way to the North England lake country on a sort of goodbye tour to see their family. After a pit stop along the way, a panicked Sam loses sight of Tusker, who he finds lost and frittering down the road. Tusker is in the grips of early-onset dementia, but director/writer Macqueen doesn’t make a show of it. A moment back in the RV when Tusker can’t seem to remember how to put on a sweater, and which arm goes in what sleeve, is telling enough. In eloquently written two-handed scenes, they argue about how Tusker wants to take control of his deteriorating state before it gets worse — and in a movie like this, you can imagine what the execution of that decision might resemble from lightyears away. Otherwise Macqueen deftly avoids cliche, imbuing the film (and its title) with cosmic symbolism as Tusker shares his appreciation for astronomy as they travel. A supernova is an old star that burns bright, only to explode with a fantastic bang, and then it’s gone. That’s how Tusker would like to go. Not, as Tusker says, “as a passenger.” Tucci is masterfully understated, turning in a modulated performance that ranks with his work in “Big Night” and “Margin Call.” While Tucci is often relegated to the sassy sidekick, here he takes the lead as a man who is acutely aware of his mind’s erosion. As the desperate lover hanging on, Firth is calm and measured that only occasionally totters toward emotional outburst. “You just sit there doing nothing, propping up the entire world,” Tusker tells Sam, gradually inching his lover toward a life ahead without him. A letter written by Tusker to his family, which Sam reads on his behalf, guttingly charts their life together in ways no flashback or montage ever could achieve.

Sam and Tusker’s devotion to each other is deeply moving, and a rare interplay to see onscreen between two men, which makes the inevitable all the more agonizing. When Sam finds effects indicating Tusker’s heartbreaking plans for the very near future, “Supernova” takes on an almost suspenseful quality as the movie counts down to its elegiac final moments, even as cinematographer Dick Pope keeps the proceedings at a cool temperature. You wouldn’t be wrong for thinking Firth and Tucci rehearsed all their lives for these roles, because Sam and Tusker’s relationship has the textures of a real one, and with that comes real pain.

Grade: B+

A Bleecker Street release, “Supernova” opens in theaters January 29, and on digital February 16. Sign Up: Stay on top of the latest breaking film and TV news! Sign up for our Email Newsletters here.