BURBANK, Calif.

WHEN Zack Snyder became the director of the film adaptation of “Watchmen,” the graphic novel about troubled superheroes in a declining age, he knew that he was taking on not only a seminal piece of popular culture but more than 20 years of unfulfilled expectations and competing agendas.

From his encounters with the original comics, written by Alan Moore and illustrated by Dave Gibbons, he was well versed in the creators’ weighty, grown-up ideas about the futility of heroism and knew that they had no enthusiasm for seeing “Watchmen” turned into a movie. He was also aware that many directors before him had been unsuccessful at the same endeavor, and he expected that he would have to fight his studio to make the film he wanted. (He did not anticipate, however, that one year before its release, a rival studio would sue to prevent his movie from reaching theaters.)

But Mr. Snyder said he believed that his greatest challenge would be satisfying the desires of the book’s devoted fans, who, like him, regard it as an exemplary work of postmodern storytelling and who would eviscerate him if he strayed too far from the original comics. And he believed that the only path to satisfying these viewers began by breaking from the source material.

“Watchmen,” which opens on March 6, begins with a scene depicted only in fragments in the comics: a lengthy fight between an unknown assailant and an over-the-hill avenger called the Comedian. This is followed by an unhurried opening credit sequence, largely of Mr. Snyder’s invention, that juxtaposes Bob Dylan’s “The Times They Are A-Changin’ ” with a montage of masked do-gooders with names like Dollar Bill and Hooded Justice as they participate in key moments of atomic-age history, like V-J Day and the assassination of John F. Kennedy. The scenes that follow will be familiar to readers with a panel-by-panel familiarity with the comic: the surreal dream of a costumed vigilante who is plagued by sexual shortcomings and fears of nuclear war; a man-god created in a scientific accident, strolling the red sands of Mars; the city of New York partly annihilated by a villain’s master plan  all connected by a story about heroes who are corrupted by the darkness they cannot expunge from the world.