The heroine of All's Well crams the ailing King of France with late-medieval antibiotic, then sends him the bill for his cure, which turns out to be her marriage to a young nobleman she's no reason to suppose is more enamoured of herself than of his horse; and the hero, Bertram, seems even less admirable. As Dr Johnson bluntly put it, he's

a man noble without generosity, and young without truth; who marries Helen as a coward, and leaves her as a profligate; when she [seems to be] dead by his unkindness, sneaks home to a second marriage, is accused by a woman whom he has wronged, defends himself by falsehood, and is dismissed to happiness.

And it isn't just a question of botched and bungled sympathies. Several times one feels the plot is making grossly unreasonable demands on characters too strongly individualised plausibly to submit to them; and nowhere more so than at the end. Helena inveigles a ring and a pregnancy from Bertram by slyly exploiting his promiscuity, and the 'young cub' is promptly transformed into the once and future Darby to her Joan, passionately promising to love her 'ever, ever dearly'. How's a director who refuses to see the play as a symbolic study of providential reconciliation and regeneration (the sort of interpretation that always works better on the page than the stage) to prevent us rejecting its people as inconsistent, obnoxious, and worse?

Well, Trevor Nunn's first expedient is to make it clear that his Helena is actuated, not by any social ambition, but by a love that engulfs convention, tact, common sense, everything; and in this he's hugely helped by Harriet Walter, half-fighting the husky longings that insist on competing with the grief she wants to feel for her recently-dead father, yet increasingly managing to convince herself that the quality and quantity of the passion she has to offer Bertram make marriage with him feasible. Why not? Besides, isn't there a suggestion, the sort wishful thinking can easily magnify, that the might just care for her? We understand, assent, as Nunn also persuades us to do when he comes to Mike Gwilym's Bertram, whose excuses include the instinctive arrogance of his class ('a poor physician's daughter my wife!!!!'), his caste's callow preoccupation with military glory, and, of course, the perversity of the way the king pays his debts. Why should a spirited young blood submit to having his name substituted for the pounds, shillings and pence on the cheque an old man sends his doctor?

Here, Nunn gets what help he can from Lindy Hemming's Edwardian-Ruritanian costumes and John Gunter's splendidly adaptable set, a great Victorian conservatory capable of becoming a ballroom, a gymnasium, a cafè, Florence railway station and (it seems) the field of the battle that occurs on or around Platform Two. This was a period when the officer code would permit or even encourage Bertram to behave as he does; and, thanks to Mr Nunn's unerring feel for detail rather subtler than the planes, trains and cars we hear buzzing or puffing offstage, it also turns out to be a period so rich in atmosphere, so evocative and captivating, that it seems almost ungrateful to point to the one place where the production does not work.

The problem, God knows, isn't Bertram's mum, who is Peggy Ashcroft serenely exuding a melancholy benignity. It isn't Bertram's caddish fellow-officer Parolles, played by Stephen Moore first with a swaggering growl, later with a rueful, self-recognising shrug, both keeping him safely this side of caricature. Nor is it the unusual affection Gwilym is allowed to show Moore, and, hence, his greater-than-average disillusionment after his chum's exposure as a cheat, drone and coward. Nor is it even the suggestion that Bertram, having spotted the dross behind Parolles's gold, becomes maturer, more clear-sighted, better able to recognise the gold behind Helena's apparent dross. All this is intellectually admirable, and more; but it still fails to solve the eternal problem of the ending, at which our hero needs, not just to see the heroine's value, but actually to love her. Indeed, the production is so admirable that it tends to prove the ending unworkable, the fault Shakespeare's.

The production's excellence leaves me little space either for Natasha Morgan's sensitive and visually arresting tone-poem about mothers and motherhood, or David Rudkin's more coarsely comic handling of some of the same ambiguities and contradictions. Having recently read a Siberian version of the same myth, in which the Hansel-surrogate gobbles the dying witch's streaming fat, only to be transformed into a kid and eaten by the witch's daughter, I expected something robust; and, on the whole, I got it. But Rudkin isn't content to suggest that Brenda Bruce, playing the demon-mother, represents dark and baleful forces which our 'rational' society ignores at its peril. She also seems meant to be the unacceptable face of feminism, the arrogant British establishment, Lady Bountiful, and much else. I was held, entertained, intrigued; but not quite enough to convince me that the confusions I felt were worth disentangling.