At the time Supernova went into production one headline read: ‘What did we do to deserve a love story starring Colin Firth and Stanley Tucci?’ Something right, I suppose. This is an intense, intimate, spare film about love, grief and dementia, and the two leads, who play a gay couple, are superb. There is now the argument that gay roles should be played solely by gay actors but, my heavens, you can’t deny this pair look irresistibly adorable tucked up in bed together. Like Eric and Ernie, but with a sexier vibe.

Tucci and Firth play Tusker and Sam, a couple who have been together for 20 years and possess excellent knitwear. Tusker works in an Amazon warehouse while Sam is a plumber… only kidding. Tusker is a novelist while Sam is a concert pianist, naturally enough. (In cinema, as in literature, the feeling seems to be: the more deeply cultural, the more worthwhile the life, and the more tragic the loss of it.) But their careers are now on the back burner as Tusker has been diagnosed with early-onset dementia. His symptoms aren’t yet too apparent. Occasionally he may lose the word ‘triangle’, for instance, and will have to draw it in the air.

But both know what is coming and have opted to take a last trip in their RV to the Lake District accompanied by their dog (Ruby). Sam drives while Tusker kvetches fondly: ‘How about just exploring the outer regions of fifth gear?’ They visit the lake where they first declared their love for each other, and look at the stars through their telescope — the film’s title will make sense by the end — and stock up at Spar, because no British holiday would be complete without a stock-up at Spar. They also spend a few nights with Sam’s sister (Pippa Haywood) and her family. Here they share Sam’s childhood single bed, sleeping in each other’s arms. It’s irresistibly adorable even as they continue to kvetch fondly. Sam: ‘You’re taking up the f-ing middle of the bed!’ Tusker: ‘I’m not… I’m on the edge!’

There are funny moments, but underlying it all is a terrible sadness. The film is written and directed by Harry Macqueen (Hinterland), who witnessed early-onset dementia in someone close to him. Sam is losing Tusker while Tusker is losing not just Sam, but also himself. ‘There will come a time when I’ll forget who is doing the forgetting,’ he says. Sam’s sister tells him that he is still the man Sam fell in love with, but Tusker knows this is not true: ‘I just look like him.’ Tusker frets about Sam becoming his carer. ‘It’s not fair,’ he says. ‘This isn’t about fair,’ Sam counters, ‘it’s about love.’ This is not like The Father— are ‘dementia films’ a genre now? — as it’s linear and focuses on loss rather than terror and rage. On occasion it feels manipulative and overly sentimental — do we need quite so much mournful cello on the soundtrack? — but it is still devastating. There’s a twist in the third act that you won’t see coming, and Sam didn’t see coming, that will have you bawling your eyes out.

Firth and Tucci, who have long been friends, have genuine chemistry and are each individually dazzling. Firth has that repressed, careful Englishness but with a single look he can convey what his character is experiencing, which is its own kind of torture. Tucci’s Tusker, meanwhile, is all bravery and determined resolve and yet we know how frightened he is, and how frightening it must be, even when you can remember who is doing the forgetting. Also, Ruby is excellent, as is the knitwear. A love story with Tucci, Firth, a lovely dog and top-class jumpers? We must have done something stupendously right somewhere down the line.