It is the classic agony aunt question. Your friend’s husband is having a fling: should you tell her? No, says her upper-middle-class mother: men will be men. Yes, says her progressively minded sister: she must punish him and demand a divorce at once. But what if your friend already knows, and is dealing with it in her own, rather creative, way?

This is the subject of The Constant Wife, the Royal Shakespeare Company’s sparkling new summer comedy by Laura Wade, based on the original play of the same name by W. Somerset Maugham. It is a play that also reflects more broadly on what happens to a marriage when two parties, however companionable, fall out of love (rather pessimistically characterising five years as a good innings), and on what on earth a respectable wife of a certain class, whose child is in the care of nannies then boarding school, is to do with herself after that.

The setting is ostensibly the 1920s, in keeping with Maugham’s intentions, but are we really there? The art-deco set, all angular geometric shapes in salmon pink and soft aquamarine, and costumes (including bias-cut gowns and a tangerine trouser suit) are decidedly more 1930s, though since all are magnificent, few will quibble about such pedantry. More significantly, the narrative has been quite heavily restructured by Wade – making use of flashback rather than letting the action play out in sequence – and re-envisioned, with the lightest of touches, through 21st-century eyes.

On the face of it, the idea of creating a play out of a play seems an odd one. Why not just reprise the original? Theatre producer David Pugh felt that Maugham’s play merited reviving, but wanted a female writer to give it ‘a modern remix’. Wade’s version makes nips and tucks to add energy, cut more quickly to the chase, and iron out a few dated wrinkles. It retains many of the original’s wittiest lines (‘It must be very tiresome to have three meals a day with the same woman seven days a week’), but also adds new ones, such as when our protagonist Constance reprimands her husband and his lover on their failure to cover their tracks: ‘I’ve had a mind to…ask you to please pull your socks up and get better at it because if there’s one thing I can’t stand it’s sloppiness.’

Wade’s adaptation also has a more pronounced sense of farce than Maugham’s original and a certain ‘meta’ playfulness. Constance and her friend Bernard are repeatedly impeded from going to see a play called The Constant Wife, and there are knowing references to not needing to go to the theatre when one’s own life provides so much drama. Wade also provides several ‘set-piece’ speeches to appeal to a present-day audience. The first is a whistle-stop plot recapitulation at the beginning of Act Two, delivered with virtuosic pacing by Constance’s sister Martha. The second is Constance’s decidedly modern monologue about experiencing pain on her own terms, and refusing to resolve her quandary in the neat way others might demand. The new version undoubtedly delves deeper into the emotions behind the crisp aphorisms.

The Constant Wife’s big selling point is the casting of Rose Leslie, the television actress known to international audiences from Downton Abbey, Game of Thrones, and most recently the BBC drama Miss Austen. In fact, Leslie was indisposed for the matinée I saw, but nobody in the audience could surely have been disappointed, because her understudy Jess Nesling’s blend of gazelle-limbed elegance, sharp, effortless delivery, and insouciant manner was charming from the off. Sometimes, you feel, a star only becomes a star because of a chance lucky break. Nesling is an actor eminently deserving of one.

Elsewhere, Kate Burton and Amy Morgan spark off each other drolly as Constance’s mother and sister, characters who, as the script tells us, ‘come as a pair’ and act as the two contradictory voices in the protagonist’s ears. Luke Norris is the husband, John, a handsome Harley Street surgeon with rather too many appendices to extract at odd hours. He plays the role as more silly goose than out-and-out cad, and rightly so: we need, ultimately, to be able to forgive him. Then there is Emma McDonald as Marie-Louise – the French name no doubt chosen by Maugham for risqué piquancy – John’s love-interest and, to complicate matters, Constance’s best friend. You can’t help but like her, too, especially in McDonald’s mixture of coquettish sex appeal and winning, wide-eyed naivety. Wade mercifully avoids heavy-handed 21st-century moralising, which would have drained Maugham’s play of its levity. We would not, after all, expect the woman behind the recent Disney+ adaptation of Jilly Cooper’s Rivals to be too po-faced on the subject of an inconsequential dalliance.

Wade has subtly updated some of the characterisation in order to speak to modern sensibilities. The butler, ‘Bentley’, is very much in the background and in his place in Maugham’s original script, but Wade takes a more democratic approach by allowing intimacies to be shared across the class divide. Bentley (Mark Meadows) becomes another sort of ‘constant’ character: Constance unburdens secrets to him that she is unwilling to disclose either to her mother or to her sister, and he hints at a secret of his own that a servant would certainly not have shared with his employer in 1926.

As modernising ‘interventions’ go, Wade’s play is a highly successful one, lightly and sensitively refreshed, anachronism-free, and with clever and amusing twists in abundance. Jamie Cullum’s swing-influenced score, all whining saxophones, captures the period flavour of the piece, metamorphosing into something more abstract for the hypnotic, slow-motion scene changes between the present and events of a year earlier.

The intimacy of the Swan Theatre, with audience on three sides of the stage, seems fitting for this very domestic comedy of manners, but this is a play that could also surely transfer with great success to one of the smaller theatres in the West End, or to regional theatres. With its witty script and charismatic cast, it is a production that deserves greater exposure than this short Stratford run can offer.

The Constant Wife  will be peformed at the Swan Theatre until 2 August.