Two's not enough for a trend, but I think it's terribly fascinating that 2015's two best cinematic love stories were both directed by gay men, are both not about gay men, but are both nonetheless depictions of modes of sexual and romantic desire that tend to be marginalised in mainstream society. Todd Haynes graced us already with his stirring lesbian love story Carol; and now comes 45 Years, a story of lust and jealousy and companionship between two straight sexagenarians, from Andrew Haigh, director of the extraordinary and unfairly overlooked Weekend. That was a great film, one of the key romantic dramas of the current decade; that 45 Years manages to surpass it pretty effortlessly is enough to confirm Haigh as one of the most exciting filmmakers in British cinema right now.

The ingredients sound unbelievably contrived and helplessly trite, so bear with me: we meet Kate Mercer (Charlotte Rampling) and her husband Geoff (Tom Courtenay) about a week before their 45th wedding anniversary (like Weekend, an ominously slow-moving ticking clock is the driving engine of the whole movie - will the marriage implode or heal before they parade it in front of all their friends?). Geoff receives a letter, from Switzerland, and it contains a quietly shattering piece of news: the body of his lover, who fell into a crevasse while they were in the Alps vacationing 50 years prior, has just thawed out and been discovered. Geoff, who has obviously started to slip away with age, doesn't seem to react very much to this at first, but over the course of the next handful of days, he lets out more and more fragments of half-century-old memories to make it clear that the woman - Katya, whose symbolic parallel to "Kate" is thankfully left un-stressed by Haigh's script, adapted from a David Constantine short story - was not merely some girlfriend; she was maybe the love of his life.

That's really all you need to know, and it's easy to feel a little hackles-raised about this: it's very Literate, this scenario, and if you suspect you can already trace out how the film proceeds to ask and then leave pointedly half-answered questions about the way that the past can sneak along and sucker-punch us, how we never know the people we think we love, how galling it is to realise that things that seem very important to you weren't at all important for your lover to share, guess what? You absolutely can trace out how this happens. And that's maybe the other parallel to Carol: they both compensate for their distinct lack of ingenuity by being fucking great in the execution. Though I fear that 45 Years is perhaps doomed to be overlooked in that regard, given how whisper-quiet Haigh's preferred aesthetic is.

This is an extraordinarily restrained film, mapping the glacier-slow verbal dance of its two utterly British leads: scenes play out in marathon-length unadorned long takes, letting Rampling and Courtenay crawl their way through a script where he never volunteers anything and she is petrified to ask. A film that is concerned with nothing else whatsoever than the space between two humans and the long-dead girl that fills up that space does well to take place primarily in two-shots. Nor are its ruthlessly unadorned interiors (Sarah Finlay was the production designer) any accident: if the film ever raises its voice enough to harangue us on any point, it would be the way that the Mercers have a solitary, unsentimental kind of life: no kids, few photographs, nothing but memories that Kate is amused to repeat with friends while Geoff somewhat laboriously reconstructs them. Fervid style is far down on the film's list of priorities, though Haigh and cinematographer Lol Crawley fire off some stunning landscape shots to go along with the dingy interiors.

The film takes the fullest possible advantage of Rampling and Courtenay's combined decades of screen experience and old age, resulting in what is simply one of the best depictions of old age that I have ever seen in cinema, all the better because the film is surprisingly devoid of any musings on mortality. With the showier arc, the audience identification role, and the greater accumulated goodwill from the world's cinephiles, it's little surprise that Rampling dominates the film and scores all of its finest moments - I think the scene that I knew I was watching a clear-cut masterpiece comes when Kate is looking at Katya in some old film slides up in the attic, where she heard Geoff snooping about. The contrast between Rampling's aged face and Katya's eternal youth, preserved by celluloid and death, placed next to each other in the frame, is perfectly stunning, and then Rampling adopts an utterly gutted look that just does it - it's the greatest shot of a human face in 2015, and maybe the best single piece of screen acting, too.

Still, it wouldn't do to let praise for Rampling swallow up Courtenay, who's really very nearly as good (Oscar season sidebar: in the sterile desert that Best Actor has turned out to be this year, it's criminal that Courtenay hasn't been able to get even a scrap of awards attention). And that's with a more slippery role: the film's perspectives oblige Geoff to be actively and specifically unknowable, and yet Courtenay is still able to make him feel complete and living and driven by his own demons, not just the needs of the story. Some of the strongest moments in the film are those where Geoff and Kate intersect, and we get to experience the heady rush of realising how little overlap there is between how they're perceiving the events of the week. This is particularly clear in the extraordinary last scene, where one of the two plainly feels that they've weathered this crisis and can move on with the rest of life, and one of them plainly hasn't decided yet.

It's soul-stirring and acute in its diagnosis of how a well-established couple negotiates its own continued survival, in tender scenes (the two awkwardly and with shy enthusiasm have sex), in strained and clipped scenes of non-communication, and in tense, even terrifying ones - for this is, in essence, a ghost story, right down to the mysterious business up in that attic. Perhaps calling it a great love story is a stretch, given how much ambivalence underpins everything that happens in the movie; but the movies haven't shown us a couple as thick with lived-in detail as the Mercers in a long while, nor can I immediately recall the last time a movie depicted the passion, comfort, irritation, and pettiness of a long-term romantic partnership with such rich complexity. Aeshetically pared down to the bone, choking on soft lighting and mist, and yet wholly warm and affectionate throughout, 45 Years is an outrageously good human story in every way.