What the Dickens? It's a mystery-within-a-musical The Mystery of Edwin Drood Musical by Rupert Holmes, suggested by the unfinished novel by Charles Dickens. Directed by Wilford Leach.

Having most recently furnished the substance for a dramatic marathon, ``The Life and Adventures of Nicholas Nickleby,'' Charles Dickens now supplies the text for a mystery-within-a-musical. Dickens never finished ``The Mystery of Edwin Drood.'' Others have attempted completion, with varying results. The New York Shakespeare Festival puts the solution of the mystery to audience vote in its second and final production of the 1985 season at the Delacorte Theater in Central Park. Composer-librettist Rupert Holmes wisely avoids calling his free-wheeling extravaganza an adaptation. Instead, the Delacorte program merely claims that this particular treatment was ``suggested'' by the Dickens mystery. What the Dickens material has suggested to Mr. Holmes is a comic-opera version in a Victorian music-hall setting. The incomparable George Rose, as actor-manager and master of ceremonies, launches the entertainment by welcoming the audience to the Music Hall Royale at Greater Dorping-on-S ea, whose company of artistes is about to perform ``a musicale with dramatic interludes'' inspired by Mr. Dickens's unfinished novel.

Playing the spectator's guide and friend, Mr. Rose is comically broad, confidential, gossipy, and even conspiratorial. As general interlocutor, he delights in his little jokes, chides the occasional late-comer, and makes indiscreet remarks about fellow thespians. All of this cozy chatter sets the stage for what seems closer at times to a travesty of Dickens than a tribute to the eminent Victorian.

Mr. Holmes has retained the most necessary plot essentials of the mystery: the quickly and mutually dissolved relationship between Edwin (Betty Buckley) and Rosa Bud (Patti Cohenour); the sinister designs of Edwin's Uncle John Jasper (Howard McGillin); and the murky doings of Princess Puffer (Cleo Laine), to whose London opium den the addicted Jasper periodically repairs.

The comings and goings against Bob Shaw's changing pictorial settings are interrupted and augmented by a generous succession of song-and-dance interludes. Although Mr. Holmes has had an impressive career in the popular music field, ``The Mystery of Edwin Drood'' is his first work for the musical theater. His appealing score ranges from the pastiche of ``An English Music Hall,'' to the tender lyricism of ballads such as ``Moonfall'' and a scattering of comic songs that include the patter-y ``Both Sides of the Coin.''

An excellent cast of singing actors responds superbly to the lyricism of the score. As the male impersonator of the Dorping troupe, Miss Buckley makes a dashing Drood, with vocal delivery to match. Miss Cohenour's Rosa is a charmer in every respect. As Puffer, the den mother of the opium den, the magnificent Cleo Laine can savor the sardonic immoralities of ``The Wages of Sin'' or make a small morality tale out of ``The Garden Path to Hell.'' Puffer, incidentally, is one character on whose past Mr. Holm es attempts to speculate. The speculation pays off in audience appeal.

The Delacorte cast includes Larry Shue as the Reverend Mr. Crisparkle, John Herrera and Jana Schneider as Neville and Helena Landless, Jerome Dempsey as the bibulous Durdles, Don Kehr as his impudent Deputy, and Joe Grifasi as Bazzard, the unproduced playwright. Graciela Daniele's generally high-spirited choreography touches on darker matters in ``Jasper's Vision,'' a nightmare ballet in which Stephen Glavin's Statue comes violently to life. Lindsay W. Davis's costumes extend from Victorian high style t o lower depths tatters. At a crucial point, Paul Gallo's lighting and Otts Munderloh's sound create a storm effect that has a Delacorte spectator glancing warily skyward.

The musical direction of Michael Starobin and Edward Strauss (with Mr. Starobin conducting) is consistently lively and well tempered. It corresponds to the theatrical energy and pace of Wilford Leach's staging -- apparently inspired by the rousing ``Don't Quit While You're Ahead.'' The m'elange of pseudo-Dickens and British music hall left one spectator wondering whether Mr. Leach might not better have served the script with a less camp approach. But there was no doubt about the audience's enthusiastic response to this alfresco frolic as played and sung under the open skies of Central Park.

``The Mystery of Edwin Drood'' runs through Sept. 1 at the Delacorte.