A slightly sinister masquerade; Anthony Shaffer's playful parody on whodunits; Whodunnit Comedy by Anthony Shaffer. Starring George Hearn, Hermione Baddeley, Barbara Baxley, Gordon Chater, Jerome Dempsey, Ronald Drake, John Glover, Fred Gwynne. Directed by Michael Kahn.

Like most other popular stage fare, murder mysteries and psychological thrillers have been in short supply so far this indifferent season. For the record, stage criminality has risen slightly of late.

Playgoing amateur detectives are being challenged once more by old hand Anthony Shaffer. Mr. Shaffer's previous record in the who-killed-cock-robin department has included ''Sleuth'' and ''Murderer.'' For the first-mentioned skulduggery, Mr. Shaffer received a resounding commendation from the critical bench and an Antoinette Perry (Tony) Award from a Broadway jury.

This time Mr. Shaffer has struck with ''Whodunnit.'' The ungrammatical title hints at the playful parody surrounding his outlandishly enigmatical plot. Or does the plot surround the parody? No matter. The object of the slightly sinister masquerade at the Biltmore Theater is to collect every possible cliche from murder mysteries in the Agatha Christie manner and then pull his own switcheroos.

For starters, the action is set in the library of Orcas Champflower Manor on an evening in the 1930s that happens to include a thunder storm. Besides walls of books, the decor includes the usual fireplace, French windows, bric-a-brac, nooks and crannies, and a case of seven lethal antique swords.

The caricatured characters are The Butler (Gordon Chatter), An Oily Levantine (George Hearn), A Respectable Family Lawyer (Jerome Dempsey), An Old Sea Dog (Ronald Drake), A Dotty Aristocrat (Barbara Baxley), A Sweet Young Thing (Lauren Thompson), A Black Sheep (John Glover), An Eccentric Archaeologist (Hermione Baddeley), An Unconventional Yard Detective (Fred Gwynne), and A Stolid Copper (Jeffrey Alan Chandler).

The object of the unmannerly exercise is blackmail. Each of the guests and their host harbor the kinds of guilty secrets that might motivate murder. Seasoned thriller buffs will be prepared to take it from there. Besides strewing the scene with cunningly hidden clues, Mr. Shaffer allows the disguised voice of the murderer to intrude occasionally with tantalizing hints delivered via the amplification system. The hints didn't help me at all.

Under Michael Kahn's direction, the enthusiastic cast at the Biltmore plays the burlesque with broad histrionic strokes. During the longish act one, the parody tends to run on like an extended review sketch well after Mr. Shaffer has made his point. But the author of this comic-cartoon crime tale sets matters right in Act II when he brings off ''the perfect double bluff'' and eventually permits Mr. Gwynne's splendidly bumbling sleuth to announce, ''I believe it's a trifle old hat, but I'm going to reconstruct the crime.''

The design credits belong to Andrew Jackness (setting), Patricia Zipprodt (costumes), Martin Aronstein (lighting), and Richard Fitzgerald (sound). Nor should one overlook Patrik D. Moreton, the hair and makeup man. Thanks to him . . . Well, enough of that.