THE TWENTIES musical can be a demanding genre. It usually requires the audience to accept a ridiculous plot, a troupe of two-dimensional characters, and a rapid succession of bad jokes. But it's worth making allowances for all these things. Not only do you get an occasional gem of a character and a few priceless one-liners, but thrown in with the prices of admission are the things that make the twenties musical one of the crown jewels of the American theater--songs and dances that transcend the plot breathe life into the characters, and float the audience into a dream world where anything is believable if you want if to be.

Oh, Kay' which was first produced in 1926, is not perhaps the Hope Diamond of the twenties musical--not exactly, the specimen one would choose if one were putting together a museum exhibit on the subject. But it does have its transcendent moments, and the time in between is at last, steadily enjoyable. The Loeb's production has been mounted faithfully in the true twenties style with a vaguely art, does set lavish costumes and lost of energetic dance numbers. Director Josh Rubins has carried this historical faithfulness over in the acting style, too: the broad, farcical characterizations second forced at, first, but once you've gotten used to the production-after the first scene or so--slapstick and mugging seem like the only techniques that would make the play work.

The plot is one of those casually outrageous frameworks a pith which all the essential trimmings can be hung--lots of love interest, sophisticated hero and heroine, comic minor roles with plenty of room for hamming it up. Our hero the dashing and debonair if lightly befuddled Jimmy Winter (John Witham), returns to his palatial Southampton estate with his new bride, an insufferably prim young woman named Constance (Innes-Fergus McDade). Unbeknownst to him, however, his mansion has been appropriated by a gang of enterprising bootleggers who have managed to charm their way into the good graces of "The Girls," a sort of Platonic harem that takes care of Jimmy's house while he's away and takes care of Jimmy while he's there. In a little under 24 hours, Jimmy discovers he's committed bigamy, obtains two annulments, and takes yet a third wife--the Lady Kay (Shella McCarthy), sister of the Duke of Durham (Dan Strickler), a down-and-out nobleman who has been forced by estate and inheritance taxes to dabble in a bit of booze running.

JOHN WITHAM and Shells McCarthy, as the romantic leads, work well together. William often sounds more like an announcer than an actor, but every once in a while he exhibits a flash of comic timing. ("Is my face dirty, dear," Constance asks him prissily as she gets ready for her wedding night, "or is it just my imagination?" "Your face is clean," he says with a smirk, "but I don't know about your imagination.") McCarthy, as the arch and witty but secretly vulnerable Kay--a role that was written for Gertrude Lawrence--has a some what more difficult job; by the end of the play, all the male characters are pledging her their undying devotion, she plays the part with the languid voice of Joan Greenwood and the elegant mannerisms of Maggie Smith, but it's only during her solos--particularly the hauntingly beautiful "Someone To Watch Over Mc"--that her character develops the necessary magnetic appeal. Both Witham and McCarthy have excellent voices, and their duets are some of the high points of the show.

But the real scene-stealers are the supporting characters. Dan Strickland as the Duke is a walking cartoon of the stereotypical stiff-upper-lip Englishman (there a even a number called "Stiff Upper Lip"): he slinks around the stage in an unhealthy slouch, his face frozen in a mournful sneer. Another cartoon character with a face to match is Jansen, a Revenue Officers (Timothy Wallace), who rushes in and out pursuing those clever bootleggers, the scowl across his bulldog J. Edgar Hoover jowls growing deeper each time he's outwitted.

Michael Ricardo, who plays one of the Duke's two partners in crime and doubles as choreographer, gracefully sings and dances his way through some dazzling production numbers. Ricardo has an elastic body and a fertile choreographic imagination--he's even managed to design a dance for seven people linked together in handcuffs. Two of the best numbers in the show are the dance routines that Ricardo does with the Girls--they're and empty-headed trio that does nothing but run around and giggle and snuggle up to Jimmy, but they make a great chorus line.

But the most successful characterization in this production relies neither on fancy footwork nor on a funny face. James E. Maxwell turns in a masterfully understated performance as Snorty McGee, a third bootlegger who masquerades as a butler and surveys the frantic goings-on with a mildly amused detachment, politely refusing to adopt anything resembling a servile manner. Calmly observing a violent quarrel between his newly wedded "master" and "mistress," he helpfully supplies the words to finish off the sentences they are sputtering and in the tense full that follows the outburst, says with infuriating matter of factness, "Well, I suppose you're waiting to hear what I think of all this."

THE SCENE in which luncheon is served by Snorty and Kay, now disguised as a maid, is the funniest in the whole play. Soup is inevitably spilled, plates dropped, and strange crashes are heard from the direction of the kitchen, "Oh, that must be the salmon." McGee explains to the guests. "The cat's had it on the floor three times already." Snorty's calm, almost scholarly manner makes a nice counterpoint to all the chaotic running around, and Maxwell's subtle performance is a welcome break from the usual mugging.

But mugging has its place in this show, where most of the acting (and most everything else) is subordinated to the singing and dancing. And that's as it should be. This is the kind of show you walk out of furtively trying to execute a two-step and humming under your breath, "Do, do, do what you've done, done, done before, baby...Do, do, do what I do, do, do adore baby." You may get a few stares, but anyone who has ever fallen under the spell of a twenties musical will understand where your head is.