Elementary, My Dear Doyle : THE LIST OF 7, <i> By Mark Frost (William Morrow: $20; 320 pp.)</i>

A dull Christmastide turns cryptic as a letter slips under the door, imploring the doctor to attend a seance being staged the following evening. The letter is unsigned, but the subtext indicates a fair damsel is in distress. It’s more than enough bait to lure the bachelor surgeon, and fervent explorer of the occult, into a cunning--and lethal--trap. The year is 1884, the location is London, and the missive recipient is aspiring writer Arthur Conan Doyle.

In “The List of 7,” Mark Frost delivers a meld that’s part historical novel, part Sherlock Holmes mystery and part occult/horror tale. With cameo appearance by Queen Victoria, and pre-”Dracula” Bram Stoker, one of the treats awaiting readers of “The List Of 7” is the opportunity to discover, at the same time that Arthur Conan Doyle does, the heroic and villainous templates for all the key players in the Sherlock Holmes stories.

Unable to resist the invitation to an occult gathering, Dr. Doyle attends the Boxing Day seance. But when he unveils the medium’s offstage shenanigans, the event turns into a blood sport, and Doyle barely escapes with his life. Salvation comes with the timely intervention of a tall, rangy stranger, who appears from nowhere and foils the homicidal frauds.

As the villains scour the East End to finish the job, Doyle slips to sanctuary across Blighty in the stranger’s private coach. When Doyle solicits his savior’s identity, the enigmatic daredevil claims he’s just an antiquities professor, down from Cambridge. (Shades of Indiana Jones.)

The don knows much about Doyle, including the plot of an unpublished novel that Doyle has written. It has passed into evil hands, he reveals, but without surrendering further information, he dismisses Doyle outside his digs.

Doyle finds his flat burglarized and slimed (a la “Ghostbusters”). His elderly upstairs neighbor lies dead on the floor. Doyle contacts Scotland Yard in a panic, and a skeptical Inspector Leboux accompanies the good doctor to the scene of the seance. All traces of violence have been completely erased.

Only a few streets away, however, a prostitute has been murdered--gutted with the skill of a surgeon. Though he doesn’t know it, Doyle immediately becomes the prime suspect. Doyle hops a train to Cambridge to track down the professor. He fears he’s being followed. He is.

In his wanderings through the hallowed halls of the university, Doyle is stalked and attacked by transforming gargoyles. The public safety of a local inn provides little comfort; the cloaked villains from the London seance arrive at midnight. Once again, Doyle is rescued by the timely appearance of the tall professor, and led across the rooftops of Cambridge to safety.

The one truth that Doyle has learned in the medieval college town is that his benefactor isn’t the antiquities gownsman, as he claimed. During their off-road trek across the North Counties, he learns his companion is one Jack Sparks, special agent to Queen Victoria. It’s an unlikely story, but Sparks does appear to have access to resources far beyond the norm. Doyle’s tame existence is being forever altered.

The story is paced like a Hollywood thriller, with good reason: First-time novelist Frost is quite a clever chap with plot twists; he’s the co-creator (with David Lynch) of “Twin Peaks.” His skillful set-up for “The List Of 7” has all the action of a James Bond pre-title sequence in Victorian dress; and once he establishes his main characters, the story takes off like a runaway brougham. Doyle is the target of a diabolical coven of Satanists: the Dark Brotherhood. His anti-occultist writings, rejected by every publisher in Britain, have inadvertently slipped into dastardly hands. He’s unwittingly exposed the Brotherhood’s sinister intentions. The game is afoot--with tracks leading everywhere.

The pair unearth one major clue: a list of seven names--the elders of the Brotherhood. Pursuing their antagonists across the highlands and lowlands of Victorian England, assailed by forces of darkness both human and supernatural, Conan Doyle and Sparks unravel a conspiracy that threatens the very fabric of the modern civilization.

There is an evil mastermind who anticipates their every move. Equally important, Scotland Yard is on their heels. As Doyle continues to put his life in his cocaine-injecting rescuer’s hands, the question persists: Whose side is Sparks on?

For armchair sleuths, “The List Of 7” is a compulsive puzzler with a payoff as startling and unexpected as a mugging in Kensington Gardens. Although Holmes aficionados and Anglophiles will find a few sins in Frost’s depictions of Victorian England, these are mostly of the venial sort. Indeed, if Sir Arthur were alive today, he would probably be quite delighted to have “The List of 7” in his writing credits.