Meet Helena, our heroine. She crams the sickly King of France with Renaissance antibiotic, and then presents him with the bill for his cure, which is the hand of a nobleman she has no reason to suppose likes her any better than his horse. When he leaves the wedding reception in dismay, she follows him disguised as a nun, and tricks her way into his bed.

There she spends one of those curious Shakespearean nights in which the man has licit sex with his wife in the belief that he is enjoying an illicit fling with a mistress. Flaunting his ring, pregnant with his child, she then reclaims him in what everybody sees as a nice, romantic ending.

Meet Bertram, her husband and our hero. Dr Johnson, aghast at his lies and evasions, summed him up well: "a man noble without generosity and young without truth; who marries Helen as a coward and leaves her as a profligate; when she seems 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."

No wonder All's Well is numbered among the "problem plays". The truth probably is that Shakespeare altered Boccaccio's original story either too much or too little. He seems to expect our sympathy for people who might pass moral muster in a fairy-tale, but look pretty shoddy when they are as realistically treated as here. And that creates difficulties for the play's directors, the latest of whom is Peter Hall, returning after a 20-year absence to the Royal Shakespeare Company, the ensemble he founded.

His solution is to perform it all briskly, fluently, coolly and without obvious prejudice. If we regard Helena and Bertram as heroes, that is fine. If we don't, that is fine too if we conclude that, as one lordling says, people are "of a mingled yarn, good and bad together", that is best of all Shakespeare probably wrote All's Well after Hamlet and Troilus and Cressida, not a time when he dealt in blacks, whites, likes, hates, or anything easy.

There is too much of The Decameron left in All's Well for this argument fully to wash; but it helps to turn the play's internal strains into interesting ambiguities. No matter that Sophie Thompson is not the passionate, handsome Helena of tradition, but an intense girl who sometimes seems awkward to the point of being gawky, and ingenuous to the brink of gormlessness. That is presumably how she is meant to be. It is the same with Toby Stephens's Bertram not the usual emotional firebrand, but a supercilious, pouting cub, as lacking in charm as maturity.

Indeed, Hall invites a parallel between him and his army chum the cad and liar Parolles. Michael Siberry's braggart warrior lacks coherence—shouldn't he suggest an embryonic bitterness inside the fantasist if he is to evolve from Don Quixote into a raging Thersites? But his humiliation is decidedly uncomic, and seems meant to prefigure Bertram's own unmasking. Stephens may not be wearing rags weirdly plastered with straw, like Siberry, but it is an abject shattered husband Helena takes home.

That is not a happy ending, but it is striking—as is the whole production. Barbara Jefford, Bertram's mother, exudes gimlet-eyed aggression rather than the usual serenity. Anthony O'Donnell, her clown, is a daunting blend of Mr Punch and flaking skinhead. Richard Johnson's King—arm in gold splint, voice as thick and fuzzy as beard, but authority undimmed—is more conventionally excellent. Oh, and the period is Louis XIV or thereabouts, which explains the phalanx of white and maroon, lace and feathers. Altogether, a play to see.