I’d been briefed that Portrait of a Lady on Fire was good. That it was cerebral as well as sexy, beautifully shot and acted and brilliantly scripted. That it didn’t flinch to examine the subservience of women in 18th-century France – and, perhaps, rather more recently. But that any manifesto was only gently waggled.

I hadn’t expected it was quite so devastating. Céline Sciamma’s glorious romance is as much a Brief Encounter as it is a nouveau-Vertigo. It ends with a long shot of a woman deeply moved and publicly weeping; it might as well be a mirror back at the audience – albeit quite a flattering mirror, with dynamite hair and top corsets.

Héloïse (Adèle Haenel) and Marianne (Noémie Merlant) are our doomed couple: the first a pouting noblewoman angry at her mother hauling her from a convent to be married off to an Italian artiso; the second a portrait artist employed to paint her in secret.

She can’t – or, at least, she can’t very well. Partly it’s tension, partly it’s the artifice. The two women become close, Marianne comes clean about not just being a companion for cliffside totters. Héloïse is angry, and uncomplimentary about the finished picture; Marianne destroys it so she can have another try – and stay a little longer.

When Héloïse’s mother departs for a few days, the relationship intensifies, the women united by desire and an appreciation of sudden liberty, and by compassion, helping a young maid have an abortion. Often movie romances gain false agency by the knowledge they must be short-lived. You’re reminded so frequently these are moments the lovers need to cherish that their relationship’s power derives solely from its fleetingness.

Here, it’s different. The clock ticks, yet the pulse of their attraction has its own rhythm and resonance. These are two people who really could remain together forever. This is a real and impossible future.

Sciamma’s heroines are conscious of this horror and exist independently of it. But the way in which her film manages to etch into the memories of its central characters and its viewers those small, enormous keystones of the affair – this look, that smile, that page in the book, that music – is deeply moving. She takes what is familiar and makes it fresh as a paper cut.