How do you save the human race from destroying itself? That's the big question posed by an unstintingly complex and smart Hollywood movie. Oh, such joy to be able to write and really mean those words. Watchmen is the rarest thing of all; a grown up movie for adults whose subject matter seems, on the surface, the stuff of childhood comic fantasies. Costumed super heroes, ordinary men and women with highly developed physiques and combat ability to be sure, standing for and fighting for justice. Some are dumb enough to get their capes caught in revolving doors and shot to death (see where Brad Bird and his Incredibles got at least some of their inspiration?) At the height of the cold war in a society that has outlawed masked vigilantes, the two immense superpowers (the US and the USSR) are on the brink of nuclear engagement. Itchy nuclear trigger fingers hover over 'the button' and it seems just a matter of a short amount of time before both paranoid leaders bring about total global annihilation.

It's 1985 and Richard ('Tricky Dicky') Nixon is still President because, with a little superhero help, America won the war in Vietnam and it's implied that Woodward and Bernstein, the Washington Post Watergate reporters, were 'dealt with' by a right wing thug whose death kick-starts the movie. He does the same service for the powers that be from a grassy knoll in Dallas. The recreation of Abraham Zapruder's home movie of Kennedy's assassination is staggering in its accuracy. Now we start straying into adult territory. Nixon may be, to the younger generation, that floating head in Futurama with a low, jowly voice but the prosthetic nose in Snyder's epic of a comic book movie is about half an inch too long for reality but just about perfect here. This is Watchmen World, an alternate timeline from the extraordinary mind of Alan Moore and, even more so than V For Vendetta, it does the graphic novel proud.

The set up opening title montage is just sublime; my favourite moment, a lesbian kiss captured in a delightfully familiar photographic context. In fact you could virtually age the sections of the audience by the ripples of knowing laughter that these images prompted. As Dylan belts out "The Times They Are A Changing", we get the Watchmen historical background illustrated in as playful and creative a fashion that I've seen in a supposedly 'mainstream' movie for years. Remember those character portraits in Koyaanisqatsi, where people just stood stock still in front of their workplace? This has the same stylistic bent. That particular movie's soundtrack (Track five, 'Prophecy' on the CD) is also raided by Snyder and (wash my mouth out), it snuggly fits better here than it did in its original context. That could be the enthusiasm talking. Well known cultural figures loiter in frame begging to be identified and pored over when the movie makes it to DVD, look-alikes as guides to a past time only to be fully appreciated by those who lived through that time. Did I mention it was adult entertainment? There's nudity, ultra-violence and (gasp), politics, metaphysical philosophy and just all round grown up fare (and children eaten by dogs). Uh, what's not to like?

Even given the limitations of its own form, this is marvelous film-making in the true sense of the word, not the meaning hijacked by the fashion industry to mean just about anything from 'acceptable' to 'good'. This is something to marvel at. So how does Zack (300) Snyder manage the regurgitated worms, the hundreds of densely packed pages of a graphic novel that made it in to the top one hundred on Time magazine's list of the best works of fiction in the 20th century? Really rather well is my answer. He had a near impossible task (duh) but like V For Vendetta, he's distilled the essence of the source and delivered two hours and forty minutes that race by, filled with the most wonderful imagery and ideas. If you get the chance, see it at an IMAX theatre, as it's a movie so full of detail that rewards the vigilant. The bigger the screen, the better. And please forgive a shameless plug for a great venue (IMAX at London's South Bank) and another great 'comic book' movie. The Dark Knight starts its last week in IMAX days after this review is posted. If you haven't, go on, go on, go on... You know how he got those scars? Go and find out. I have to add a small but necessary caveat. Watchmen was originated in 35mm and the IMAX presentation is extremely detailed and vivid but it's not IMAX per se unlike The Dark Knight which had many sequences shot in full IMAX format. You know the stuff. It makes your synapses scream with shock at the detail.

So how come the guy who wrote the damn comic book isn't even credited? "Alan Moore – a synonym for 'disgruntled from Northampton'". Given how Hollywood players have treated him over the years, it's profoundly understandable. Taking most of the movies produced from his work this disgruntlement is also depressingly explicable. Writer and leading doyen of the graphic novel, Moore has already damned both seen and unseen Hollywood interpretations of his mammoth narratives. He perseveres with the idea that graphic novels are essentially unfilmable as single movies (he convinced Terry Gilliam that Watchmen would only work as an epic TV series). He also makes the point that you can go at your own pace, revisit things you may have missed – stuff impossible to do when you're wrenched forward at breakneck speed in a cinema. You'd need way too much time to tell the story and it's not that the images are impossible to capture.

We know that's no longer true. Just take a look at Watchmen's trailer. If you've read the source material, these images make a Watchmen fan salivate. Unlike contemporary writer/artist Frank Miller who was conned into collaboration in the nicest way by Robert Rodriguez on the startling Sin City, Moore remains wedded to his craft. He even forgoes any profit he would normally earn, preferring his share to go to his collaborators. Miller, tasting movies, found them rewarding and went on to helm Spirit, a work I've not seen but doesn't seem to have made the same brash splash as his and Rodriguez's bloody and stylized extravaganza.

I cannot but admire Moore's skill, his moral integrity and intelligence. This is a man whose handshake seems as binding as a cement Hollywood contract. My introduction to Moore and the whole world of graphic novels was the astonishing Batman work, The Killing Joke. Anything with Moore's name on it and I'm there. In his place, I wouldn't have gone down so hard on V For Vendetta (to be fair, he's probably not seen it). If the rights of your work are up for sale and Hollywood is a buyer, you must accept that the wonderfully dense and labyrinthine 'graphic novel' will never translate without an eighteen hour running time. Moore is famous enough to maintain copyright on his work, no? Perhaps not. If it's up for sale then apart from distancing himself from the effort, there's not too much more he can do.

When is a Soprano or West Wing producer going to mine that rich vein? It's not as if you need a storyboard artist... Instead, these overtly political stories are whittled down and presented as abridged and therefore simplified versions (how could they not be?) As a punter, you either accept this and see how well the movie works in and of itself or you bemoan the necessity for one art form to shamefully leech off another. Sometimes an Alan Moore fan can be a fan of a movie built from the shards of the original work, a fan that can embrace both. If you're one of those, then you simply have to see Watchmen. If you know nothing of the source material, you should see it for one reason. It's a grown up movie with grown up ideas and boy, do they not flinch on the 'graphic'. But you should also have the experience of reading the original. You think this art form is for kids? That's a ludicrous and frankly puerile supposition. Am I appreciating Moore's work too much? I don't care. Like Joss Whedon, his is a world view that needs to be stamped into the largest walls like giant graffiti and taken in to one's soul like osmotic wisdom. So there.

Essentially, it's 1985 and a group of costumed heroes has been disbanded with one of their more twisted number, Rorschach, still roaming the streets righting wrongs in his eerie flannel mask infused with moving, symmetrical ink blots (hence the name). He's played in a mask most of the time, but when he is revealed, the actor's likeness to the comic's rendition is scary and also is the fact that this is little Moocher from Peter Yates wonderful Breaking Away, a film I am inordinately fond of. How Jackie Earle Haley transformed from the spotty, mop haired underdog into Rorschach, the violent and moral absolutist isn't as beyond me as I was going to write. It's called 'thirty years'... Jesus... The hero known as the Comedian (played gleefully and amorally by Grey's Anatomy's Jeffrey Dean Morgan) is brutally killed and Rorschach suspects someone is targeting super heroes. His suspicion is fuelled as another hero is attacked. Old romances are stoked up and each character's back-story is presented with real flair. With a lightness of touch, director Snyder goes back and forward in time like Marty McFly on steroids but you are never left reeling with incomprehension. Each flashback informs and enriches the movie's current events (this is footnote film-making at its finest and yes, I enjoyed the alliteration too). The scope of the imagery is vast doing justice to the source material if not in density of idea, then ambition of vision. As you would expect, the effects are seamless and to even mention them these days seems redundant. One effect needs mentioning because it was always going to be a contentious issue with studios and while we have reached about a far as we can go in the mainstream with bloodletting and sex, we still have problems with penises, up and down and from top to bottom so to speak.

Dr. Manhattan is perhaps the film's signature character not least because he's a real gifted superhero, blue and naked most of the way through the movie. After a few cuts which almost convinced me we were never going to get a full frontal (not that I was actively desiring this, you understand. I was keen to see Hollywood grow up about this last physical taboo). And then "Ta da!"! No fanfare. It's a penis. Live with it. As Dr. Manhattan is entirely computer generated, I read that actor Billy Crudup was quite satisfied with his resulting genitalia. I only mention this trivia for two reasons. It was always going to be a big deal and a deal breaker to show a naked man in a mainstream movie and the fact it was done the way it was confirms that Zack Snyder is a grown up. And more importantly, he treats his audience like grown ups. Iced water to a man in a desert...

Crudup plays physicist Dr. John Osterman who, by accident, gets trapped in an a test vault that separates objects from their intrinsic fields (no, me neither). He forgets his girlfriend's watch that he'd fixed, something significant as the word and image are central to the meanings and designs of both comic book and movie. After he is taken apart, molecule by molecule (it looks damn painful), he eventually comes back to Earth, naked and blue with the power over atoms themselves. With the development of this power, his concern for human beings diminishes and in the story, he represents the argument against the continuation of the inherently savage human race. He's there to witness this and actively help by being the American answer to the conflict in Vietnam. It seems there's not much to stop you if you can make atoms dance to your tune.

He alone has the power to stop Armageddon but is the model of indifference. The relevant line is on a lot of the advertising; "I believe that life is a highly overrated phenomenon." That's us told, then. Crudup, again, playing a rather cool and unsympathetic character, does a great job. I don't see him in a lot of movies but he always goes further than convincing and as a sometimes enormous, blue, naked man, that's giving him acres of well deserved credit. I'm assuming the Dr. Manhattan effect was done with motion capture (a la Polar Express) but my copy of Cinefex hasn't arrived yet.

One of the joys of Watchmen is without question its production design. Yes, Dave Gibbons' original artwork was there for all to see, copy and extrapolate from but let's give some credit. Making these designs in 3D utterly convincing, is an astounding feat. The success and reality of these creations should make all those who contributed to the movie feel proud. But there is one creation that didn't make it to the screen and in not making it, its exclusion alters the thrust of the ending that Moore came up with in the source material. The inclusion of the opening titles to the 60s TV show The Outer Limits was a deliberate nod to parallel creationism. Moore came up with an idea and then was informed that this idea was also put forward on the afore mentioned TV show. It's a damn good one but not one that Snyder felt belonged in his real if alternate Watchmen universe. It was another case of reflection of the original 'event' and an honest attempt to improve it. We've seen this before. It is possible to improve upon source material (Goldfinger being the best example) but I'm not sure Snyder improved upon the source but he made it more relevant to 2009. If this skirting around the squid is pissing you off then go and see the movie. You will not be disappointed unless in your mind Watchmen is spelled B-I-B-L-E.

Doesn't that stand for "Basic Information Before Leaving Earth"? Over to you, Dr. Manhattan.