Looking at the London Film Festival schedule without knowing much about the films there, a sensible person would assume that Supernova, of all the programme, would be in for a good shout at awards glory. Starring Colin Firth and Stanley Tucci as a couple facing the impending terror of early-onset dementia, it’s every bit as tender, heartfelt and devastating as you could imagine.

What helps Supernova tick so well is the electric performances from the two leads. Tucci is particularly astounding as Tusker, an American author whose dementia diagnosis sets him and long-term husband and musician Sam (Firth) on a nostalgic trip to visit old friends and family before Sam’s long-awaited piano recital. The chemistry between the two is so natural: both feel like old married lovers, with natural quarrels and brilliant back-and-forth dialogue, and while the issue of casting two straight men as gay partners is undeniably problematic, both pull off their roles with aplomb.

Yet under the clear affection between them is the simmering and impending burden of Tusker’s condition. Tucci conveys Tusker’s dementia so sensitively, with a great physical performance that charts his slow descent into less mobility. Seeing him unable to give a speech, or gradually lose his ability to write, is truly heart-wrenching, yet Tucci’s portrayal of Tusker as constantly and unwaveringly upbeat – with some darker secrets – is masterful. There’s no way we don’t hear his name tossed around for BAFTAs and Oscars in the months to come – it’s one of the best performances of the year.

This is all aided by a beautiful, subtle screenplay from director Harry Macqueen – in only his second directorial role – which so delicately handles incredibly painful conversations. The dialogue feels so natural, the back-and-forth so real, that it makes the more difficult moments even more harrowing. And without giving anything away at all (this is a film best experienced knowing as little as possible), when it takes turns for the bleak, Macqueen handles it so expertly, with subtlety and care. The second half is so exceptionally sad but incredibly heartfelt, and is sure to touch anyone who has lost a loved one.

Yet throughout, Supernova feels so homely. Maybe it’s the British setting – it’s odd seeing SPAR in an Oscar-worthy film – or the back-and-forth and dialogue that feels so intrinsically British, but it’s such an easy film to attach oneself to – which makes the tough moments even harder. Macqueen knows exactly how impactful his story is, and does everything to convey maximum emotion from his audience, from the stunning string-and-flute-induced soundtrack to the long takes that really allow the cast to sink their teeth into the characters and make them their own.

It’s a truly exceptional film, and one of the best of the festival so far. It’s so raw, so sad, and so totally and unwaveringly human. Expect total uproar if Stanley Tucci isn’t nominated for an Oscar – his performance is but one gem in one of the year’s most heartbreaking and unforgettable films.