Imagine a world where the US won in Vietnam, Richard Nixon is about to start his sixth term as President and nuclear Armageddon could be just hours away.

That’s the setting for Watchmen, Zack Snyder’s ambitious yet flawed film adaptation of Alan Moore’s cult graphic novel.

The sprawling tale revolves around a diverse group of superheroes whose number include the giant blue godlike colossus Dr Manhattan (Billy Crudup), business magnate Ozymandias (Matthew Goode) and masked detective Rorschach (Jackie Earl Haley).

They're not straightforward good guys, a point best shown by the Comedian (Jeffrey Dean Morgan) - a murdering rapist who acts as a parody for the dark, violent times.

Disbanded when we first meet them, they are gradually pulled back together to combat a hidden force which threatens to destroy humanity itself.

The film has numerous highlights. The dark, stylish, dystopian setting is reminiscent of Blade Runner while Haley excels as the Philip Marlowe-esque Rorschach, every line spat out from behind a moving cloth mask which changes to reflect his ever-fluctuating emotions.

Action scenes are as brutal as they are exhilarating while the CGI effects are certainly among the best you will see.

Where the film falls down is between these peaks of excitement. Snyder has been faithful to Moore’s expansive tome and while this is credible, it means huge chunks of time pass by with very little happening.

Characters are drawn vividly, almost too vividly in some cases to the detriment of any kind of pace or plot.

Another problem is the character of Dr Manhattan, one of the most sanctimonious, self-obsessed, morose bores ever to appear on screen.

The Morrissey of superheroes, by the end of the film his constant, monotone whining begins to ruin every scene he appears in. It’s a relief when, halfway through the film, he heads off to Mars for some solitude and “soul-searching.”

Watchmen is a film of too many contrasts. At times brilliant, at others boring, it is ultimately overlong and too frustrating to be called a masterpiece.

Moore once described the book as “unfilmable” and while technically he has been proved wrong, the result leaves a rather disappointing taste.