A YOUNG couple, dressed in their wedding day best, walk hand in hand along a shore. It is England, 1962. Not too hot, not too cold. There is chatter, laughter, camaraderie. Hardly the stuff of watch through your fingers cinema, you might think.

But in this tale, adapted from the novel by Ian McEwan, it is not just the sea that has hidden depths. As we see later, there are some treacherous stretches for filmmakers besides.

Florence and Edward (Saoirse Ronan and Billy Howle) have come to the seaside fresh from the ceremony. Their next task is to have dinner a deux in their hotel room, served by a pair of waiters who hang around to provide silver service. It is horribly awkward, but it is the done thing apparently, and the two youngsters are very keen on the done thing, even if it is farcical.

“It was beautiful,” says Edward, recalling the wedding day. “Nothing went wrong,” agrees his new wife. The solemn ceremonies over, the sniggering waiters gone, the couple can now relax. Even though this is the Macmillan era, and sex has not yet been invented, we assume they will muddle through.

This is the point from which the tale begins proper. As the evening unfolds, director Dominic Cooke, working from a screenplay by McEwan, goes back and forth from past to present, filling in some blanks and leaving others to the audience’s imagination.

This first half of the story is delightfully done, with Cooke and his cast recreating the looks, mores and class tensions of the time. Edward is the son of a country headmaster and an ex-curator mother (Anne-Marie Duff). He has no money but he does now have a first-class degree in history. Florence has a first, too, in music. She lives in a very smart house in London with a family who eat mangetout and an academic mother (Emily Watson) who has telephone conversations with Iris Murdoch. The music by Dan Jones is excellent, moving the tale seamlessly back and forth across the years.

Different lives, different times, but as we have seen in flashback, young love has conquered all and taken the couple to the altar. Where they go from there forms the second, less successful part of the film.

The problem lies not in the telling of the wedding night story itself, or the lead-up to it, but in the epilogue. What works on the page does not necessarily work in cinema, with some things better told than shown, particularly if they involve your young actors “ageing up” in make-up.

Cooke also shows himself rather too fond of the flashback, so that the drama starts to drag just when it should be hurtling towards a resolution. A few scenes apart (the watch through your fingers ones in particular), there is a dullness hanging over the film. Nor does he make the most of his supporting cast, with Watson hardly featuring and Duff just slightly more. This is a tale of two families as much as it is a young couple, and he could have spent more time picking apart their backgrounds, Florence’s especially.

Ronan and Howle prove perfect casting as Edward and Florence. He plays the former like a character from Lawrence, while she, being the very model of middle-class poise, sophistication and, above all, restraint, could have sprung from the pages of an early Muriel Spark.

Howle, last seen playing a petty officer in Dunkirk, has a fraction of the experience of Ronan, but he does not let that hold him back.

After Atonement, Brooklyn and Lady Bird, Ronan is more used to featuring in Oscar nominee lists than small budget British dramas, but she is right at home here as the very English miss who knows what she wants, and does not want. She commands the screen but is never overbearing; she is chaste but never dull; open about her feelings yet fearfully buttoned up in other ways. Just the sort of enigma wrapped in a darling little jacket that the tale needs to hold the viewer’s interest when the going gets a touch too slow.