It was said of the 1987 superhero deconstruction graphic novel Watchmen that it was unfilmable. Originally released as a 12-issue comic book, its story was epic, marshalling science fiction, Egyptian and Greek mythology, politics, (reworked) history, mystery; its illustrations were packed with clues, symbols and games; its characters complex and nuanced. Written and illustrated by Brits Alan Moore and Dave Gibbons, respectively, Watchmen grabbed the steering wheel of the comic book world and yanked it in a whole new direction.

Talk of movie deals flitted about for 20 years, and dissolved over the daunting prospect of adapting the monumental work, which Time recently crowned one of the 100 best novels of the 20th century, alongside Animal Farm and Invisible Man. And fans are zealous and protective. So it’s to director Zack Snyder’s credit that he not only took a swing, but that he knocked it out the park. Watchmen kicks ass.

After fans breathe a sigh of relief that their beloved comic book was adapted with such fidelity, they will be hyperventilating over how good the movie is. And newbies to the whole shebang will be in for a smart, sprawling and shocking thrill.

The opening credits masterfully lay down a lot of back story in an economical and ingenious montage. Then the story kicks in. It’s 1985 in America – well, a kind of alternate America where JFK was assassinated, but Richard Nixon never resigned, instead going on to five terms in office and presiding over a fractious and violent country teetering on nuclear war with Russia. The biggest deterrent to nuclear annihilation is Dr. Manhattan (valiantly played by Billy Crudup), a former physicist who – Bruce Banner-like – was transformed into a godlike being by an atomic accident. Nearly omniscient, he’s blue, glowing and naked throughout nearly the entire movie – FYI.

Dr. Manhattan is the only “hero” with super-powers among a medley of his former teammates, who are all retired. But the murder of one triggers all into action again to confront a growing and ominous conspiracy. One of them is central character and narrator Rorschach, a grumbling, violent vigilante with a Travis Bickle-like repulsion of the city around him, played spot on by Jackie Earle Haley. Other cast include the lovely Malin Akerman as the Silk Spectre, Matthew Goode as the “smartest man in the world” Ozymandias (get it?), Patrick Wilson playing Nite Owl 2, the most relatable character, and Jeffrey Dean Morgan as the cynical Comedian.

Although the story is deeply about superheroes, it’s equally deeply human: The motley crew is wracked with anger, paranoia, loss of faith, narcissism and other pathologies. Silk Spectre gets angry at her mother for fondly recalling her past in crime-fighting. Her mother’s reply: “I’m 67 years old. Every day the future gets a little darker. But the past? Even the grimy parts get a little bit brighter.”

The soundtrack is faithful to the spirit of the comic – uh, graphic novel – and it rocks with energy from Nina’s “99 Luft Balloons,” Tears for Fear’s “Everybody Wants to Rule the World,” Hendrix, Dylan, Leonard Cohen, and the Smashing Pumpkins’ “The Beginning is the End is the Beginning.” It shoots rays of pop-culture light into the many shadows roaming the picture.

This movie is for adults. It is unflinchingly violent, which is likely to get less play than shots of a blue man doing full frontal. But in the hands of an anti-authoritarian writer like Moore, the violence is supposed to repulse, which it does. The heavy and sophisticated ideas at brew also recommend it to the grown-ups.

For a movie so big and imposing, it’s packed with finite, faithful details – it practically uses the panels of the graphic novel as a shooting script. Fans in the theater could recite lines, verbatim, at an early screening. Flashbacks and narration tell big chunks of the story. They’re done with intelligence – especially when Dr. Manhattan describes his past, which veers over biography, space-time continuum physics and philosophy. Snyder uses a lot of the same folks who worked on 300, so the gears behind the movie glide effortlessly on sumptuous slo-mo shots, clever editing, fight sequences and gorgeous cinematography.

Comparisons to The Dark Knight are almost obligatory: both were published by DC Comics; Watchmen director Snyder also directed Dark Knight writer Frank Miller’s 300; Warner Bros. released both films; and while Dark Knight swelled into a phenomenon of a blockbuster last year, Watchmen looks poised to do the same. But no matter how Watchmen performs at the box office, it can stand proudly next to the Caped Crusader in terms of its superhuman spectacle, substance and smarts.

Watchmen (3½) Directed by Zack Snyder. With Billy Crudup, Jackie Earle Haley, Patrick Wilson, Malin Akerman, Edward Blake. • R, 163 min. • At the Maya Cinemas, Century Cinemas Del Monte Center, Lighthouse Cinemas, Northridge Cinemas..