“Watchmen” was a 12-issue comic book published in 1986 and 1987 and subsequently compiled into a single graphic novel that has since become a widely respected and rabidly scrutinized work of fiction. Time magazine called it one of the 100 best English-language novels of the century. Its influence can be seen in a variety of works, including Pixar’s “The Incredibles” and TV’s “Lost.”

I provide this background for the benefit of readers who have never heard of “Watchmen” and who, after seeing the new film version, might wonder what the big deal is, and why it’s sparked such heavy anticipation within the nerd community. The movie, directed by Zack Snyder (“300”), is mostly faithful to the characters and story of Alan Moore and Dave Gibbons’ book, but the original is so complex and multi-layered that perhaps no adaptation — even a very, very long one — could do it justice.

It’s set in an alternate version of 1985, one where “costumed heroes” (i.e., Batman-style vigilantes) once protected America’s cities before being outlawed some years earlier. Now the retired crusaders, most of them having kept their secret identities secret, live out their lives quietly, but they are reunited when one of their own — a brash mercenary named Eddie Blake, aka the Comedian (Jeffrey Dean Morgan) — is murdered. Rorschach (Jackie Earle Haley), a masked, paranoid misanthrope whose face even his fellow crime-fighters have never seen, is convinced there is a conspiracy to kill all of them, and that the Comedian was only the first.

So Rorschach makes the rounds, warning everyone. There’s Dan (Patrick Wilson), formerly known as Nite Owl, a nerdy schlub who desperately misses the action and still has his Owlmobile in working order down in his Owlcave. There’s Laurie (Malin Akerman), aka Silk Spectre, who took her persona from the original Silk Spectre, her mother (Carla Gugino), who got too old to fight a couple decades ago. Laurie is the girlfriend of Dr. Manhattan (Billy Crudup), a physicist who suffered a lab accident and came out of it with immense control over time and space — the only one in the group, it would seem, with actual superpowers.

Finally, there’s Adrian Veidt (Matthew Goode), whose costumed hero was named Ozymandias, and who is frequently referred to as “the smartest man in the world” as if this was some kind of formal title bestowed upon him, though little is done to demonstrate its validity. (Unless “smartest” means “thinnest.”) I don’t know what his crime-fighting gimmick was, but he has a thing for Egypt. He’s one of the few superheroes to reveal his true identity, and he’s made a fortune capitalizing on it. This also makes him a prime target for assassination, though.

All of this is set against a backdrop of imminent nuclear war between the United States and the Soviet Union, theoretically raising the stakes so that it’s not just the Superfriends we’re concerned about, but all of humankind. Dr. Manhattan helped America win the Vietnam war in this alternate reality, creating a surge in popularity for Richard Nixon that has kept him in the White House for another three terms, but the Cold War continues. Dr. Manhattan sees possible nuclear destruction in the near future, and Rorschach wonders if this might be someone’s motive for killing superheroes, to keep them from preventing it.

The screenplay, by David Hayter (“X-Men,” “The Scorpion King”) and Alex Tse (his first film credit), doesn’t deviate significantly from the graphic novel until the end, when there are some major changes that, frankly, I like better. For the most part, the dialogue and story remain intact. The problem is that the book is packed with backstory: We learn the history not just of the most recent batch of superheroes, but of their predecessors, the original Watchmen, called the Minutemen. Supplementary material at the end of each chapter fleshes out their motives and personalities. Most of that must be eliminated from a movie version unless you want it to run seven hours — and even then, some things are simply better expressed in words and still images than in cinema.

I suspect that if I hadn’t read the book beforehand, I’d have been baffled by a lot of the movie. The process by which “costumed heroes” became outlawed is barely referred to, and the scheme behind the Comedian’s murder, when finally explained, is absurdly elaborate and improbable. And what’s that crazy thing Dr. Manhattan is building on Mars? Why does Rorschach occasionally speak in Tarzan English, as if dictating a telegram? Does anyone other than Dr. Manhattan have super powers? (The answer would seem to be no, except that there’s one guy who moves awfully fast and has incredible strength.) Why does everyone jump to the conclusion that the Comedian’s death means someone is targeting all former superheroes?

More important than these questions is this one: How can I have spent 2 1/2 hours with these characters without ever caring about anyone other than Rorschach? (He alone, played by the terrific Jackie Earle Haley with a seething ferocity, elicits sympathy and compassion — which is a little unsettling, considering he’s a psychopath.) For all the enormity and magnitude of what happens in these people’s lives, and for as much ground is covered in the plot, there’s almost no emotional engagement. Laurie has drama involving her parents, and it feels like it was tacked on at the last minute. The psychology of most of the characters — what sort of person puts on a costume and fights crime? — is given scant attention. Everyone talks and talks (and talks and TALKS), but to what end?

Indeed, the film is often ponderous and self-serious, almost tipping over into silliness. Snyder’s camera moves slowly and deliberately, so that even the action scenes — infrequent to begin with — feel weighed-down, the brutal violence coming off as sickening rather than exciting. The frenetic, rapid-cut style used in many other films would have been tiresome, too, but surely there’s a happy medium.

Speaking of medium, that’s about how I feel about the film as a whole. It’s too weird to ever be boring, and Snyder’s knack for stylish visuals has been amply demonstrated. (Even people who hated “300” have to admit it looked pretty cool.) The plot is compelling enough, the acting serviceable (and in certain cases excellent), the special effects suitably astonishing — it just doesn’t add up to much, that’s all. It’s a movie to see once, more or less enjoy, and then shrug off.

B- (2 hrs., 43 min.; R, some harsh profanity, a lot of graphic violence, a fair amount of nudity and some strong sexual situations.)