There's a whole lot of heavy lifting going on in London theaters, and the actors are doing most of it. Cast members can be seen dragging grand pianos (in War and Peace), hauling sofas and chairs (By Jeeves), rushing to cover the stage with a huge drop cloth while shouting as if they were devoutly hooliganish football fans (Stanley). And when they're not moving things, they are trying to keep from being clobbered by the fast-moving, computerized sets. At the National Theatre's crystalline revival of A Little Night Music, you don't have to hum the scenery; the scenery does its own humming, as the mechanized set whirrs and burrows its way below eye level. While the stagehands are playing a lazy game of poker, the poor actors must be fretting about hernias and collision insurance.

The real heavy lifter in this summer's London theater is Cameron Mackintosh, producer of the $5.7 million musical Martin Guerre, which opened last week. Mackintosh has to carry the show through the funk of its negative reviews. It's a nice challenge for the most successful producer in theater history: three of his shows (Les Miserables, The Phantom of the Opera, Miss Saigon) have together sold more than 100 million tickets.

The true story of Martin Guerre, a 16th century Frenchman who deserts his wife, stays away for years, then returns--or is it really he?--has inspired an opera (William Bergsma's The Wife of Martin Guerre, 1956), two films (the French The Return of Martin Guerre in 1982 and Hollywood's Sommersby, 1993), and no fewer than three musicals. Mackintosh's version is the work of the Les Miz/ Saigon team, composer Claude-Michel Schonberg and librettist Alain Boublil.

Stripping the legend of its suspense, they quickly establish that the new, improved Martin is a fraud--and a godsend to the virgin wife Bertrande. That's fine: we want to know why she connived in the deception. But the authors hoke up the plot with political pieties about heroic Protestants and a psychotic villain desperate to take the place of Martin, any Martin, in Bertrande's bed. The result is a Romeo and Juliet without the poetry, or an Oklahoma! that ain't O.K.

And why do serious musicals have to be so darn brown? As in Les Miz, earth tones predominate here--except in the peasants' garb, nicely creased and whiter than white. One thing about these villagers: they've all seen Riverdance; Bob Avian's choreography has the heavy-footed agility of that hit Irish show. The choral harmonies do edge toward magnificence as 30-some voices pump out Schonberg's anthems, many of which echo better tunes in Les Miz. But director Declan Donnellan (who staged, for his own Cheek by Jowl troupe, a superb As You Like It) can't make the drama sing. Martin Guerre is big; it should have been grand.

Say this for the Mackintosh team: they are ambitious. Too many other "new" London musicals fail on the small side. They are greatest-hits hagio-biographies of dead American pop stars: Elvis, Buddy (Holly) and, for Pete's sake, (Al) Jolson! Then there are the evergreen musicals that have haunted London and Broadway for years--in the case of Cats, 15 years.

Ah, Cats. Oh, Andrew Lloyd Webber! With five of his shows running in the West End, he's the main reason London has become a musical mausoleum. But in his latest, he almost offers atonement for the serioso musical style he helped create. Surprise, surprise. By Jeeves, a radical overhaul of his only flop (in 1975), is a delight in miniature. It recognizes that the source of the humor in P.G. Wodehouse's famous stories is a nonchalance that extends from the characters of Bertie Wooster (supreme upper-class twit) and his man Jeeves (Zen master of irony) to the very act of comic creativity. Never try too hard to be funny, old pip; people will just notice you're sweating.

almost arrogantly modest, by jeeves has a swank blitheness that makes the audience feel it's on a holiday from the new musical's huffery and puffery. alan ayckbourn's book builds, with labyrinthine comic logic, from a tangle of mistaken identities to the climactic remark, "there's a cat-burgling pig in my bedroom!"

Lloyd Webber's tunes--evoking both the Vivian Ellis musicals of the '30s and soft-hearted fluff of '50s pop charts--are inventive and sweetly chipper. But he allows Ayckbourn, who also directed, to undercut the loveliest of them (the duet Half a Moment) with some expert kidding. The overall effect is of two precocious lads putting on a public-school fete. If By Jeeves isn't lighter than air, it is surely lighter than Guerre.

In London, as on Broadway, it's musicals that stoke the box office. For nourishing fare, go to the National Theatre. Given the West End's conservatism and the virtual eclipse of the Royal Shakespeare Company, the NT is now the one reliable home for serious theater, handsomely mounted with incandescent stars.

Example: John Gabriel Borkman, the Ibsen revival with Paul Scofield as a man ruined in business and Eileen Atkins and Vanessa Redgrave as the two women whose love digs into his heart like talons. Last week's opening night had a few gaffes: actors blew lines; the audience giggled at the wild intensity of the emotions; and at one point--oops!--it began to snow in the living-room set. But then, by force of will, Scofield took over, wrapped the crowd in his majestic technique and brought to life the character, the play and the sacred vocation of acting.

Revivals are fine. But didn't Britain once have some decent dramatists--Shakespeare, Shaw, that crowd? Where has the English playwright gone? To the NT. Two new works, Helen Edmundson's version of War and Peace and Pam Gems' Stanley, show how epic themes can be addressed on an intimate scale.

Someone once walked past a restaurant table where Steve Martin was sitting and heard the comedian brag to his friends, "I've read War and Peace." This production, directed by Nancy Meckler and Polly Teale of the Shared Experience theater troupe, is so vital and engaging that you needn't congratulate yourself for sitting through it. Tolstoy's idealists and cynics are as compelling as if they were in the most intelligent and tumultuous of TV miniseries. The stage is bare, except for that all-purpose grand piano and 15 actors in twice as many roles. All is clear, brisk, deeply felt, beautifully conveyed.

But the real heartbreaker of the NT season is Gems' play, directed by John Caird, about the English painter Stanley Spencer (Anthony Sher) and his wife Hilda (Deborah Findlay). Writers often fail when trying to dramatize the artistic impulse or the agony of devotion; Gems gets both stunningly right.

Spencer has a knowing appetite for the world of the senses. Stanley lived in a perpetual state of aesthetic erection; he was alert to the beauty in a blade of grass, a woman's breast, the nearness of God. And though he lived among the 1920s bohemian set, he hated a term like avant-garde. "That's just another way to frighten people off their own ecstasy." He had a saint's star quality, which made living with him irresistible and impossible for a mere mortal. The heroic Hilda puts it to Stanley with a simplicity that brings tears: "You can exist without me. I cannot exist without you."

At its best--far from the bloat of a Martin Guerre or the pop nostalgia of a Jolson--theater is an exorcism you can't exist without. It locks you in a room with primal beliefs and griefs and makes them beautiful. The actor, like Stanley Spencer, becomes a priest of emotions. Moving all the scenery is a small conjuring trick; the big one is moving the audience. You can find that magic, in repertory, on the South Bank of London.