We think we invented adultery — that as liberated 21st-century types, we were the first to wrestle with sexual fidelity. But in 1948, passions seethed below the surface, as David Lean’s newly restored The Passionate Friends reminds us. It is a very adult love story: it even opens with a lingering shot of sheets being changed on a double bed.

Hardly surprising when the very married Lean fell in love with his leading lady, Ann Todd, during the making of the film (they later married) and can’t keep his camera off her face.

Mary (Todd), a fragile Hitchcock blonde, is torn between romance with Steven (a lovely, teasing Trevor Howard) and unfulfilled, platonic companionship with her rich banker husband (Claude Rains). Mary and Steven have history — and, on holiday in Switzerland, find themselves, quite innocently, in connecting hotel rooms.

But it’s not that simple. Mary has already rejected him twice. They dated in their twenties, then later after her marriage she nearly eloped with him. But something held her back. Fear of sexuality (she talks wildly of the horror of clawing and possessing)? Snobbishness? A need to maintain her female independence? Certainly her jealous husband doesn’t believe she can ever know fulfilment.

The film is shot in confusing flashback (echoing Mary’s complex emotional state). It’s easy to laugh at the upper-class pretensions, the excitement about real butter and good coffee (rationing is still in place after the war). But there are extraordinary subtexts. Was Mary, like Hitchcock’s Marnie, abused as a child? Is her husband really gay (that combed-back hair)? Or a closet Nazi?

And does Steven really deserve her? In a feminist reading, Mary has the right to autonomy. When Steven tells her brutally that if she never marries, her life “will be a failure”, you have to resist the temptation to punch the screen.

Adapted from an HG Wells novel, and regarded as a companion piece to Lean’s Brief Encounter, The Passionate Friends looks gorgeous (Todd is never in the same gown twice) and has a feverishly modern intensity. I saw it on the same day as Sex and the City, and guess what? Lean is so much more lucid — and more subversive — on relationships than Big and Carrie.

BFI Southbank; Croydon Clocktower; Greenwich Picturehouse; Phoenix Cinema.