FOG, DENSE AND LUSH, curling its way through London alleys and along seedy docksides, in and out of dingy shops and smudged and broken windows, obscuring the face of Big Ben himself and wrapping around the skirts of chilled and hapless prostitutes...

A sinister black coach drawn by sinister black horses in the sinister black night, the fog muffling the beat of hooves on cobblestones, and inside, one of Britain's most famous Victorians slowly savors the edge of a jeweled dagger, and waits...

Delicious. The perfect scene, the perfect villian, the perfect setting for some first-rate Sherlock Holmes sleuthing.

Unfortunately, in Murder By Decree Sherlock Holmes never shows up--Christopher Plummer does. Though dressed in all the right clothes and given the best Dr. Watson ever, Plummer never stops being Plummer, not for a minute. Who ever heard of a sexy, sauve, passionate Sherlock Holmes, a Holmes with blow-dried hair and visible muscles, anyway? Would the real Sherlock Holmes burst into tears at the sight of a beautiful, helpless woman unjustly committed to an insane asylum? Would the real Holmes leap at the throat of an official in an attempt to kill him? Would he sweat in front of the Prime Minister? Now really, would he?

This is a newer, more modern Holmes, a man not afraid to show his emotions, a man with nerves and real red blood, just like the rest of us. The idea is not entirely a bad one. Who among us has never wished the sleuth would experience just a little uncertainty, display a tiny bit of insecurity, or even just once show a little warmth toward dear, chubby old Watson? It's the execution here that is overdrawn. This Holmes at one point looks at the unsuspecting Watson with a gaze so rich in emotion and so reminiscent of Captain Von Trapp that you can almost here the Alps singing in the background. This Holmes really lusts for blood when he grasps that poor unfortunate official's throat. And this Holmes really means to save mankind, all mankind, with his impassioned plea to the powers that be at the end of the movie.

You really can't blame Christopher Plummer for this modified Holmes. Playing himself makes a great deal of sense. He knows he is handsome and talented. He knows he has that charming scar on his lower lip, that little half-smile, that direct and demanding gaze. It's easy for him to be Christopher Plummer, and besides, he gets paid for it.

You can, however, blame directer Bob Clark and screenwriter John Hopkins. The script is entirely their creation, and has about as much in common with Arthur Conan Dyle's stories as Plummer has in common with Basil Rathbone. Both Rathbone and Plummer wear deerstalker hats and speak with upperclass vowels, and both Doyle's work and this script rely heavily on fog for dramatic effect. And that is where the resemblance ends.

The film is so heavy-handed that you can almost see Clark's cinematic brain at work. First you take Jack the Ripper, a colorful killer with his gory methods, and popular with the masses as well. Jack does his work at the right place and in the right time, too, Victorian England--perfect. When Holmes meets Jack the Ripper, the temptation to indulge in gruesome special effects overwhelms the directors. No matter that the story line, a mish-mash of all the Jack the Ripper identity theories, is so complex and capricious as to make Conan Doyle's brand of deliberate and subtle terror impossible: Clark simply adds a sountrack of weird music and loud irregular heartbeats and heavy breathing and counts on these noises to create an atmosphere of horror an anticipation.

Now, to make sure that everyone knows that this is, after all, a Sherlock Holmes movie, Clark crams a lot of that vintage Holmesiana so dear to the hearts of Conan Doyle fans and provides the audience with radical plots, mysterious cults, rot in high places, missing persons, plus, of course, fog, cobblestones and Big Ben.

Then, as a final insurance against any misunderstanding in this film, Clark brings in some Famous People in Bit Parts to match his stereotyped characters with stereotyped actors. Donald Sutherland plays the weird guy; Genevieve Bujold, the beautiful child-woman-victim; David Hemmings, the man who is not quite what he seems; and the Prime Minister, is of course, John Gielgud.

James Mason as Dr. Watson is Clark's one brilliantly original bit of casting. Never has there been a Watson more long-suffering and tolerant of his friends' foibles, yet Mason projects a self-confidence and determination unknown to Nigel Bruce. He also provides the only comic relief in this overly melodramatic film.

CLARK'S B-MOVIE ORIGINS are obvious in this film. His psycho-killer Black Christmas clearly inspired the lurid murder scenes in Murder By Decree. And while one expects a little blood and weird goings-on in Sherlock Holmes' pursuit of criminals, such attention to violent detail is unnecessary.

The film isn't bad for an ordinary murder mystery, but don't go expecting to see Sherlock Holmes there. As the master sleuth himself would have said, "It's elementary my dear Watson. What we have here is an impostor. Would the real Sherlock Holmes ever stoop to such depths of passion? Never sir!"