Saving D-Day From a Secret Weapon : CODE NAME: GRAND GUIGNOL <i> by Ib Melchior (Dodd Mead: $17.95; 318 pp.) </i>

“What if?” The resonance of those two words, particularly as they apply to history, intrigue the imagination of novelists about as much as historians are horrified when those same novelists start to play with implications of the question. Just consider for a moment the implications of a couple of history’s “What ifs?”

What if Picket’s men had broken the Union Center at Gettysburg? Would our geographical confines today embrace two nations rather than one?

What if Lee Harvey Oswald’s second shot had missed? Would we have the consequences of the Vietnam War and the ‘60s upheaval conditioning our national existence today?

Few of modern history’s “What ifs” offer ground more fertile to the novelist’s pen than those which surrounded the critical D-day landings in June, 1944. What if those landings had failed? Would we have met the Red Army on the Rhine or the Channel coast instead of the Elbe? Would the atomic bombs that fell on Hiroshima and Nagasaki have landed on Frankfurt and Cologne instead?

It is that rich vein Ib Melchior has set out to mine in “Code Name: Grand Guignol.” His “what if?” supposes the Germans wanted to plant in the Pas de Calais a manifold chamber gun capable of launching a 65-pound high-explosive shell over a range of 165 kilometers every few minutes. Would such a gun have been capable of defeating the invasion by devastating the ports essential to staging and launching the invasion fleet? It’s an interesting proposition, and Melchior sets out convincing evidence garnered from his own experience in the army’s Counter Intelligence Corps in World War II and from the Wehrmacht’s archives in Freiburg, West Germany, that such a project did, indeed, exist.

The answer to the question, could the gun, had it existed, have defeated the invasion? is almost certainly “no.” As Melchior has described its emplacement, its enormous barrel could not have been swung through the 45-degree arc needed to use it on the invasion beaches, and its range was too short to allow its shells to reach the ports from which most of the invasion fleet sailed. It would, however, have wreaked havoc on the ports from which the fake invasion fleet of the Fortitude deception scheme were to have sailed.

As the vehicle for moving his plot forward, Melchior has chosen to imagine a Resistance network in that great old Parisian horror theater, the Grand Guignol. The institution produced a repertory of horror plays from 1898 to 1968, most of them endowed with a dose of carnage and a macabre imagination to make Stephen King’s goriest scenes look like sketches from a morality play written by a disciple of Mother Teresa.

As a young man working for a British stage company in Paris in the late ‘30s, Melchior knew the stage manager of the Grand Guignol. He ingeniously employs the story themes of some of the more graphic plays in the theater’s repertory throughout the book to prompt the derring do of his Resistance heroes. The device also serves as a wonderful excuse to take us on tours of the Paris sewers, into secret rooms accessed by the spikes of an Iron Maiden and, of course, backstage at what was once a very special French institution.

Unfortunately, all too often those excursions are made to the accompaniment of the dreadfully banal sort of dialogue used in serials like “Laverne and Shirley.” Example:

“The Big Cheese himself.” He stood up. “OK,” he said resolutely. “As you theater types say, ‘Let’s get the show on the road.’ ” Or:

“You’re the doctor,” Kevin nodded, “as you American types say!”

“So we sit pretty,” Ted acknowledged, “for two hours.”

“It’ll be a damn long two hours.”

Melchior launches his three resistants, married, natch, to an American OSS officer, on a chase across Occupied France to Berlin and on to the village of Misdroy near Stettin on the Baltic Sea. Their mission: Get that gun before it can disrupt the invasion by sabotaging its prototype in a high- security installation outside the village.

As Melchior’s plot weaves its way from peril to peril, it begins to read a little bit like “The Hardy Boys Visit Nazi Germany.” Indeed, some of the escapades to which Melchior submits his heroes strike this reviewer, who possesses a certain knowledge of wartime Resistance activities, as being about as plausible as the exploits of Fenton Hardy’s sons.

Still, it’s all good fun, and Melchior serves up his reader along the way a ration of gore, mayhem and bloodshed worthy of that storied theater from which he took the novel’s title.