From its dark opening scene, Zack Snyder’s “Watchmen” establishes a complex blend of murder mystery, social commentary, and unrepentant action flick.

This mood is immediately set as brawny Edward Blake (Jeffrey Dean Morgan) fails to fight off an unidentified attacker in his home. Viewers—assuming they have not read the graphic novel—can only speculate as to the assassin’s identity, but Blake knows, telling his killer that their world is a joke before he is hurled out a high-rise window.

Creating this world has required years of imagination. The greatest challenge facing the creation of “Watchmen” has always been the legacy of its inspiration, the classic 1986-7 graphic novel written by Alan Moore. Moore, who deliberately divorces himself from all film adaptations of his works—including “V for Vendetta” and “From Hell”—left a daunting task for eager filmmakers. Any worthy onscreen adaptation would need to capture the gritty depth and scope that have made the book so influential, while preserving the pace and stylistic flair of a thriller.

In conveying the book’s events and emotion, Snyder largely succeeds. The high revenue of his last directorial project, “300,” gained him leverage in resisting Warner Bros.’ efforts to shorten and update the film. With an R rating, Snyder was able to embrace the sex and violence of the novel and maintain its setting: a fictional 1985 in which a fifth-term Nixon celebrates American triumph in Vietnam. Our insight into this alternate reality comes from primary narrator and protagonist Rorschach (Jackie Earle Haley), whose deliciously gravel voice, hangdog look, and uncanny resemblance to his novel counterpart lend the movie its most gripping and startling moments.

Snyder’s cast performs smoothly and largely carries the tone of the story—unabashed, raw violence coupled with an underlying black humor. Close behind Haley for outstanding performance in the film is Morgan, who appears to us as his alter-ego The Comedian in a series of flashbacks. Several other cast members invoke their characters with the appropriate panache and emotional range, notably Billy Crudup as Dr. Manhattan and Patrick Wilson as Dan Dreiberg, the former Nite Owl II.

The casting of British actor Matthew Goode (“Brideshead Revisited”) as Adrian Veidt—the billionaire who profits off his former life as Ozymandias, the smartest, quickest man alive—unnecessarily worried the novel’s fanbase. The slightly effeminate Goode is not the obvious choice to play the square-jawed super-athlete of the novel, but he delivers a solid interpretation of the character. The cast’s real weak link is its passive female romantic lead, Malin Akerman. Though the Swede’s delivery is mostly flawless, she at times seems bored or perhaps confused with her role as Laurie Jupiter, also known as Silk Spectre II. In her scenes of budding romance with Wilson’s Nite Owl, Silk Spectre II would benefit from an actress with a more charismatic presence, giving Wilson more maneuverability. His character’s gradual growth—overcoming social awkwardness, self-doubt, and even erectile dysfunction—would be one of the movie’s gems if paired with a better target of affection.

The film remains an immersive experience. Its heavy, disheartening examination of society’s periodic wish for self-destruction builds through long, brooding scenes and stunningly detailed panoramas—from Blake’s lonely, rainy funeral to Veidt’s ancient Egyptian-style retreat in Antarctica. This attention to detail, however, inevitably makes the movie feel slow at times. The viewer must embrace it and soak in the film’s visual extravagance, or reject it and battle moments of boredom, which at 163 minutes the movie cannot avoid.

The film’s saving quality is that when it seems to be losing momentum, Snyder injects humor to restore the tempo. And “Watchmen” certainly laughs at itself at times, from its over-the-top sex scene to some comical music choices, such as “Ride of the Valkyries” as Dr. Manhattan wins in Vietnam. Inclusion of Jimi Hendrix’s rendition of “All Along the Watchtower” and the overused Simon and Garfunkel hit “The Sounds of Silence” feels heavy-handed and forced, but Bob Dylan’s “The Times They Are a-Changin’” brilliantly caps a montage explaining the alternate world in which the Watchmen reside. This is almost enough to forgive Leonard Cohen’s painful original performance of “Hallelujah,” which plays to laughably awful effect as the Nite Owl rediscovers his libido.

“Watchmen” has flaws, but Snyder has proven that translating it to the screen could be done, and done well. The old caution, however, that the book should be read first, applies here more than ever. To fall asleep during this film would be an unfortunate waste, and though he is not credited in the film, one should pay respect to the brilliance of Moore’s original before enjoying this film as a second course.

—Staff writer Alexander R. Konrad can be reached at akonrad@fas.harvard.edu.