Rain in the rafters, song in their hearts Singin' in the Rain Musical comedy based on the MGM film. Screenplay and adaptation by Betty Comden and Adolph Green. Songs mostly by Nacio Herb Brown and Arthur Freed. Original choreography by Gene Kelly and Stanley Donen. Directed and choreographed by Twyla Tharp.

``Singin' in the Rain'' joins the assortment of revivals and recyclings that have brightened the Broadway scene in recent times. The renaissance has included such popular fare as ``No, No, Nanette,'' ``Irene,'' ``On Your Toes,'' ``My One and Only,'' and, of course, ``42nd Street,'' with its hundred dancing feet. So why not ``Singin' in the Rain,'' the 1952 MGM classic? Why not, indeed? The multimillion-dollar stage version at the Gershwin Theatre honors its source with a book adapted by Betty Comden and Adolph Green from their own playful screenplay, plus songs by Nacio Herb Brown, Arthur Freed, and others (some of which were not in the movie). The production has been staged and choreographed by Twyla Tharp. Making her Broadway debut in this dual capacity, Miss Tharp has achieved what is in many respects a homage to the film's original creators. Unfortunately, the resulting entertainment has too many rough edges.

After its fashion, ``Singin' in the Rain'' remains faithful to its inspiration. Comden and Green blend a lighthearted Hollywood romance with a broad spoof of the havoc caused when sound struck the movie industry in the late 1920s. If it matters to anyone, the plot tells what happens when Monumental Pictures decides to avert disaster by converting its silent ``Dueling Cavalier'' into an all-talking-singing ``Dancing Cavalier.''

At the insistence of screen idol Don Lockwood (Don Correia), the studio hires Kathy Selden (Mary D'Arcy), a silken-voiced unknown, to dub the voice of tempestuous leading lady Lina Lamont (Faye Grant), whose raucous tones are anything but pear-shaped. Needless to say, Don is smitten with Kathy. Ditto Lina with Don. In the end, it's no contest.

Except for an occasional love song, Miss Tharp and company seize on their borrowed material for an untidy but good-natured extravaganza that burlesques the silents, the early talkies, Hollywood tycoonery, and attendant lunacies. The dances are always energetic and frolicsome, particularly in Miss Tharp's evocation of a Warner Bros. musical (replete with rag dolls, a hoofing horse, etc.) and the all-out production number for ``The Dancing Cavalier,'' in itself a wild mini-musical.

It is the borrowed tap routines, however, that enable ``Singin' in the Rain'' to achieve the finesse quite frequently lacking elsewhere. Mr. Correia is a terrific dancer, who matches nimbleness with the charm of a genuine musical-comedy leading man. In the title number that climaxes Act I, he does a flashy, splashy, Kelly impersonation while buckets of rain fall from the flies.

Miss D'Arcy is a winsome, sweet-voiced Kathy, and Miss Grant's scheming Lina makes mischief with relish and mustard. For singing, dancing, and assorted tomfoolery, Peter Slutsker's Cosmo Brown proves an eminently valuable man to have around as the hero's partner from vaudeville days.

In a flashback to that bygone phase of their careers, Don and Cosmo get things moving with ``Fit as a Fiddle,'' a piece of old-timey razzmatazz. ``Make 'Em Laugh,'' ``Moses Supposes,'' and ``Good Mornin' '' also help enliven the evening. ``You Are My Lucky Star'' and ``Would You?'' strike the nostalgic chord of romance. And there is always ``Singin' in the Rain,'' with its delicately tantalizing obbligato, reprised at the end by the entire company decked out in yellow slickers and hats.

The producers have spent their money prodigally. ``Singin' in the Rain'' has what must be tons of scenic spectacle by Santo Loquasto (lighted by Jennifer Tipton), and racks and racks of costumes by Ann Roth. What the show lacks in Broadway slickness it at least attempts to make up for in extravagance, bustle, and goodwill. Credit for the production's musical authenticity is shared by arranger Stanley Lebowsky, music director Robert Billig, and orchestrator Larry Wilcox. The parodies of black-and-white film periodically projected on a lowered screen are by Gordon Willis.