"Mother Nature is a serial killer." So claims a smart young doctor early on in Marc Forster’s new zombie-apocalypse blockbuster World War Z. The world’s in the grip of a terrifying pandemic, zombies are rampaging across every city, and we’re following that disaster-movie staple, the brilliant scientist charged with finding zombie zero and saving the human race. He’s trying to explain his theory of basic virology to the United Nations soldiers escorting him, but of course at the same time he’s putting forward the philosophy, or what passes for it, underlying the film as a whole. It’s an unscientific metaphor, ascribing narrative structure to natural phenomena - something like Tennyson’s "Nature, red in tooth and claw" for the Hannibal age, perhaps, but without his religious context to give it meaning.

And unfortunately, this unsatisfactory idea is about the only one the film holds onto from start to finish. Its opening sequences neatly illustrate the problem. There’s a montage of newsreaders and politicians talking about environmental disasters, and footage of armies of ants and flocks of birds. The first scene, a happy family breakfast, makes mention of the protagonist’s mysterious past working in "dangerous places", and it feels like the film is setting itself up to address some serious political issues. How does a pandemic affect the poorest areas of the world? Has humanity brought devastation upon itself by destroying the environment? Of course it’s a good thing that World War Z doesn’t go down the dull, po-faced Contagion route, beating you over the head with these issues; the problem is that it brings them up, then glides straight past them.

Sadly, how this came about is entirely understandable. Firstly, this is an adaptation of a rather complex novel. Secondly, it’s a zombie apocalypse film, and there have been a lot of those already. Thirdly, the production seems to have suffered a number of problems, and the screenplay was reportedly rewritten over and over again as writers (J Michael Straczynski, Matthew Michael Carnahan, Damon Lindelof, Drew Goddard) joined and left the project and substantial reshoots had to be done. World War Z is under pressure from several directions at once.

The source novel, Max Brooks’ 2006 World War Z: An Oral History Of The Zombie War, is a challenging prospect for adaptation. It’s a collection of personal accounts as told to ‘Brooks’ in the role of a United Nations agent. Characters report on disparate aspects - political, environmental, religious - of a decade-long war between humans and zombies. Brooks’ novel is part of a sequence (Paramount is set to release an adaptation of the first book, 2003’s The Zombie Survival Guide, next year, and a third is in the works) which is set in a highly detailed and exhaustively researched world, covering a multitude of characters and complex situations.

No film could aim to equal that kind of scope, of course, and the worst mistake would be to try. World War Z doesn’t - in fact, it only covers the very beginning of the war. What it does do, though, is lurch from sequence to sequence with only the presence of Brad Pitt as Gerry Lane, the analogue of Brooks’ UN agent to link them together. This makes sense in the context of the book’s structure, but the film makes no attempt to present itself as a collection of linked stories. Instead, it tries to create a coherent narrative out of quite disparate elements, and transforms the Lane character into an action hero at the centre of the story, rather than an observer and curator. It feels like three or four movies spliced uneasily together.

On top of the fact of adaptation comes the weight of genre expectations. World War Z has to come to terms with them, and does so through its zombies themselves. It’s firmly in the same family as 28 Days Later and the Walking Dead franchise; it’s modern. These aren’t Romero’s shuffling flesh-eaters. Modern zombies snap, snarl and rush. Infection is instant. They’re milk-eyed, unnaturally fast, creepily twisted. They’re drawn to sounds. They only settle down to shambling around, banging their heads against the nearest wall, when there’s no one left to bite. Otherwise they’re at the mercy of a raging megalomaniac of a virus that won’t stop until it’s taken over the planet.They’re utterly serious; there’s no traces of post-Shaun of the Dead or Zombieland humour here. And that’s fine. It’s effective. The most exciting scenes in World War Z feel like sequences from the video game Left 4 Dead, with Brad Pitt as Survivor Bill, carrying an axe, stalking zombies through claustrophobic corridors and stairways. The zombies aren’t the problem.

It’s interesting to see how the producers have papered over the cracks from all the reshooting and rewriting. For the most part, they show up as silly things, continuity issues and problems of the ‘but how did he get there?’ and ‘they would never do that’ variety. The trouble is that the deeper flaws are built into the structure of the film itself, its disjointed narrative and curious detachment from what should be its major concerns, given the choices of settings (Jerusalem, North Korea) and references (Lane is meant to have worked in Liberia and Bosnia). Worst of all though is the terrible, trite sentimentality of Lane’s family’s story. His wife (Mireille Enos) and daughters (Abigail Hargrove, Sterling Jerins) are portrayed as encumbrances, literal wastes of space and his devotion to them is cloyingly Hollywood. The film does at least seem to realise this isn’t good enough, and duly serves up a couple of strong female characters later on. Sadly, there’s no romantic tension of any kind, though; not that romance is obligatory, but its absence shows up the fact that there are no complex relationships at all in this narrative, and that’s why it’s ultimately always going to fall flat.

The film is, of course, beautiful in the way Hollywood does in its sleep. Golden skies, sparkling seas, spectacular crowd scenes. (Although it has to be said, very fast action sequences are still hard to follow in 3D, especially when they happen in the dark.) Sometimes, it takes on a haunting, dreamlike quality, helped along by Marco Beltrami’s lovely soundtrack, and there’s a glimpse of another, leaner, better, stranger film. Brad Pitt turns in a typically assured performance, dealing with the material with the straight face of years of experience. If you’re a fan, then his presence holds the film together. He’s eminently watchable. If you’re not, World War Z is just a forgettable, confused, potboiler of a thriller.