Alexandre Dumas père was an improbably productive writer. Between his debut as a dramatist in the 1820s and his death in 1870, hundreds of plays, novels, histories, travel journals, even cookbooks, poured from Parisian publishers under his name. He worked with dozens of collaborators, creating a kind of Dumas fictional workshop in which new stories could be collectively constructed. Now a modern publisher is adding yet another entry to his vast bibliography, proclaiming The Last Cavalier as a “lost masterpiece”. Well, it's certainly been lost.

Following hints in a letter he stumbled across in an archive, the Dumas scholar and biographer Claude Schopp searched through microfilm copies of a Paris newspaper from the late 1860s. In them, he discovered 118 chapters of a serialised novel (sadly unfinished) by Dumas, the existence of which had been forgotten for 140 years. Edited by Schopp and supplemented with suggestions of how the work might have concluded, these chapters have become The Last Cavalier.

There's much to enjoy in the book, which is set in the Napoleonic era. At the heart of its rambling story is the young Comte de Sainte-Hermine who has inherited a commitment to the long-lost royalist cause for which his father and elder brothers have fought and died. The comte is the epitome of the beau sabreur. Women need only glimpse him to become prey to weak-kneed longing. One lady even dies for love of him, expiring in his arms as he explains why honour forbids him from returning her affections. Men, stunned witnesses of his prowess with sword, pistol, lance and any other weapon that comes to hand, fall over themselves to become his followers. Condemned for his participation in a conspiracy against Napoleon, Sainte-Hermine is forced to fly from the woman he loves on the eve of his wedding. He becomes a wanderer, roaming the world from Italy to the Indies in search of wrongs to right and villains, usually English, to kill. He even pitches up at the Battle of Trafalgar, where his sharpshooting skills prove to be the death of Nelson.

This is all good swashbuckling stuff - the kind of action-filled narrative at which Dumas always excelled - and there is more to admire in The Last Cavalier. Around the central character of Sainte-Hermine swirls a cast of thousands. Real historical figures such as Surcouf, the daredevil corsair, Joseph Fouché, the sinister minister of police, and Georges Cadoudal, the Breton commander of royalist guerrillas, make memorable appearances. Dumas's vivid portrait of Napoleon - mentioned only in passing in his other novels - captures at least some of the intense energy and limitless ambition that must have driven the man.

The Last Cavalier, although it was not reworked in book form at the time, was part of the long-term plans Dumas had for his fiction. Soon after he made his name as a novelist, he wrote that, “The rest of my life is made up of compartments filled in advance, with future work already sketched out. If God grants me another five years to live, I shall have exhausted French history from Saint Louis up to the present day.” It was a wildly ambitious statement of intent and even Dumas's prodigious ability to work around the clock was not enough to make it a reality.

The rediscovered novel was clearly meant to take its place in what Dumas described as his “immense framework”, but, enjoyable though it often is, it cannot be ranked with books such as The Three Musketeers and The Count of Monte Cristo. By the time he came to write The Last Cavalier, French history sometimes appears to have exhausted Dumas rather than the other way about. Digressions and diversions that in earlier novels seem the results of an unquenchable urge to give readers more stories, information and historical colour, feel like padding, thrust into the narrative by a writer desperate to meet deadlines and word counts. Anybody who loves Dumas's fiction won't regret reading 118 previously unknown chapters of it but, amid all the excitement of discovery, it's as well to remember which novels really were his masterpieces.

The Last Cavalier by Alexandre Dumas
Fourth Estate £20 pp754