Veteran Seattle playwright John Longenbaugh has formulated a nifty conceit in "Sherlock Holmes and the Case of the Christmas Carol," now on stage at Taproot Theatre, and he runs with it.

Sherlock Holmes has never been a stranger to extreme mood swings. But in “Sherlock Holmes and the Case of the Christmas Carol,” the new John Longenbaugh play at Taproot Theatre, the great consulting sleuth is stuck in one of his darkest downers ever.

In the grand tradition of “Bah, humbug!” he makes it crystal-clear he’s in the Ebenezer Scrooge camp when it comes to Yuletide cheer.

And in the personage of Seattle actor Terry Edward Moore, Holmes expresses his utter disdain for all things Christmas (and for humanity in general) with an extensive vocabulary of scowls, sneers, upper-lip curls and eloquently grumpy invective.

Is there any redemption possible for this brilliant Victorian misanthrope?

Veteran Seattle playwright Longenbaugh has formulated a nifty conceit here, and he runs with it. At its best, “Sherlock Holmes and the Case of the Christmas Carol” stylishly conflates A. Conan Doyle’s iconic detective with the Charles Dickens chestnut.

The play’s premiere at Taproot was postponed a year, after an arson that damaged the theater’s Greenwood digs scotched its planned 2009 debut. Be that as it may, the script still is not yet all that it might be. But it’s smart, witty and fun, and has the makings of an annual event — the sort of family holiday diversion that many regional theaters, overdosed on seasons-past repeats of “Christmas Carol,” “Tuna Christmas,” et al., have on their wish lists.

This tale begins cleverly with a variant of the first line in the Dickens story. Instead of “Marley was dead,” we are told that Professor Moriarty, the evil mastermind who was Holmes’ longtime nemesis, is deceased.

It thereby follows that the ghost of Moriarty (Alex Robertson) materializes on Christmas Eve to haunt the severely bummed-out Holmes. And that the latter will next be visited by a spirit who whisks him back to his lonely, egghead youth, followed by visits with a couple of other illuminating spirits (a la Dickens).

Longenbaugh, a member of the local Sherlock Holmes society, The Sound of the Baskervilles, knows and relishes his Holmes lore well. And he neatly folds in references to the Conan Doyle oeuvre whenever possible.

Scott Nolte’s staging (on a too-cramped and overly dreary set designed by Mark Lund) benefits from the Shakespearean flair of Moore’s marvelously showy lead turn.

As Dr. Watson, devoted chronicler and amazingly patient assistant of Holmes, Stephen Grenley emits a robust gentlemanliness that suits the role well. But there’s room for more variety in his stock mannerisms.

Pam Nolte is very much in her element as Holmes’ long-suffering London landlady at 221B Baker Street, Mrs. Hudson. Nolte neatly and humorously balances the lady’s Victorian propriety with flashes of shrewdness, shock and nosiness, and she wears the heck out of one of Sarah Burch Gordon’s fine period costumes.

The play’s plot ingeniously foresees the horror of World War I, and a part Holmes might have played in it if not for his inevitable Scrooge-like transformation into a more charitable fellow.

But the “case” in the title is an awfully sketchy caper. And another subplot, or just a few additional colorful characters of substance, might give the story more heft and something echt-Holmesian it now lacks: suspense.

Holmes fans can cheer, however, at how nimbly Longenbaugh has captured the master detective’s uncanny powers of deduction. Here, as in the Conan Doyle classics, Holmes can size up your life and your character, after barely glancing down his nose at you.

Misha Berson: mberson@seattletimes.com