Swordplay, romantic intrigue, comic escapades and desperate undertakings — Alexandre Dumas’ 1844 novel, “The Three Musketeers,” long ago set the standard for swashbuckling adventure. Even now, the thrilling pledge of “All for One and One for All,” coupled with the image of four raised swords crossed in eternal friendship, instantly brings to mind the book’s youthful heroes, Athos, Porthos, Aramis and their protege D’Artagnan.

While “The Three Musketeers” regales the reader with humor as well as derring-do, it nonetheless closes on a somber note. The scheming and cruel femme fatale, Milady de Winter, is judged guilty of the most heinous murders by an ad hoc tribunal consisting of the musketeers and her late husband’s brother. Following her secret execution, the once inseparable friends part and go their separate ways.

“Twenty Years After” — newly translated by Lawrence Ellsworth — opens in 1648 with D’Artagnan now a 40-year-old lieutenant in the King’s Musketeers. His has been a solid military career, but with nothing in it to match the dashing exploits of his youth. He still mourns his beloved Constance — poisoned by Milady — and has never married. Like so many middle-aged men, he wonders more and more, “Is this all there is?”

In the old days, D’Artagnan worked to thwart Cardinal Richelieu, but now misses that consummate politician’ statesmanship, generosity and personal finesse. (See Dumas’ “The Red Sphinx” for a sympathetic portrait of Richelieu, tirelessly working on behalf of France.) Louis XIII has also died but his heir, Louis XIV, is still just a little boy, not yet the Sun King. Queen Anne — who once loved England’s Duke of Buckingham, assassinated at the instigation of the seductive Milady — has become the secret mistress, possibly even the wife, of Richelieu’s successor, the avaricious, Italian-born Cardinal Mazarin. France meanwhile is riven by unrest, and there is talk of civil war. The Duc de Beaufort, leader of the Frondeurs, as the forces opposing Mazarin call themselves, has been imprisoned for five years. He has nonetheless vowed to escape and revenge himself on the cardinal.

Given such a threat, Mazarin needs loyal retainers he can count on, men with strong sword arms and dauntless hearts. But, alas, where can such heroes be found? Once, according to some half-forgotten rumors, Queen Anne was saved from disgrace by four extraordinary young champions, but that was 20 years ago. The queen has never revealed their identities.

By trickery, Mazarin eventually learns that D’Artagnan had been one of the four. Who and where are the others? Athos, Porthos and Aramis were simply noms de guerre assumed when these younger sons of distinguished families entered the Musketeers. Still, might they be found and persuaded to pick up their swords again? Somewhat hesitantly — nobody except the queen really likes the cardinal — D’Artagnan agrees to search for and, if possible, reassemble the old gang. Together, he knows, they bring out the best in one another.

Aramis, it turns out, is now the Abbe d’Herblay, head of a Jesuit monastery. Has that bon vivant become austere and devout? Not at all. Aramis resides in gorgeous chambers decorated with the souvenirs of war, drinks the most expensive wine and probably does more than just flirt with that beautiful Frondeur, the Duchesse de Longueville. Even though the genial, bearlike Porthos has inherited enormous wealth, his vanity yearns to add Baron to the name Porthos du Vallon de Bracieux de Pierrefonds. As the Comte de la Fere, Athos shows himself to be utterly devoted to his mysterious ward Raoul. The revelation of that youth’s parentage — in a delightful conversation between Athos and the free-spirited Duchesse de Chevreuse — might have been written by Boccaccio.

To all appearances, D’Artagnan’s old comrades have settled into quiet, provincial lives. Or have they? At this point, the plot thickens, as Dumas begins to develop three interlaced story lines: the rescue of the Duc de Beaufort from his maximum -security cell; Raoul’s entry into court society and his first military adventures; and — in an ominous Gothic turn — the introduction of a pale and sinister Englishman named Mordaunt, who is obsessed with discovering the names of Milady’s executioners.

Dumas’ original text of “Twenty Years After” makes for an enormous volume, so Ellsworth decided to reserve its second half for later publication. Not that this matters much. Dumas never stints the action, witty dialogue and surprising plot developments in what we have. As gradually grows clear, his overriding theme is loyalty —to friends, family and party but, above all, to living by a principled code of honor in a debased and chaotic world.

Alexandre Dumas is one of the world’s most enjoyable writers, though his artistry doesn’t always get the respect its deserves. William Ernest Henley — author of the stirring poem “Invictus” (“I am the master of my fate,/ I am the captain of my soul”) — long ago recognized the French writer’s genius, rightly describing him as “a prodigy of force and industry, a miracle of cleverness and accomplishment and ease.” Because Dumas started out as a playwright, his books always move along briskly, while emphasizing dialogue and action. He remains — again quoting Henley — “an artist at once original and exemplary, with an incomparable instinct of selection, a constructive faculty not equaled among the men of [the 19th] century, an understanding of what is right and what is wrong in art and a master of his material.”

That “material” would eventually include several volumes, beginning with this one, about the Musketeers’ later adventures, which reach their culmination in “The Man in the Iron Mask.” If you only know “The Three Musketeers” you owe yourself the pleasure of spending some happy evenings with “Twenty Years After.” Athos, Porthos, Aramis and D’Artagnan may be older and their hair starting to gray, but they’ve lost none of their romance and grandeur.

Michael Dirda reviews books each Thursday in Style.

TWENTY YEARS AFTER

By Alexandre Dumas

Translated by Lawrence Ellsworth

Pegasus. 456 pp. $26.95