You should go see Pan's Labyrinth. You absolutely should. But, if you're reading this blog because you picked up my book in the fantasy section and fantasy is your favourite genre, be warned: it's not a fantasy film, not really. Note, for instance, that the publicity has shown pretty much all the fantastical stuff: you'll see it in action in the film, but there will only be a couple of creatures that you didn't know about before you went in. And while the imaginative spectacle is just marvellous in all senses, most of the film is a grim and frightening story about a child stuck in the home of her new stepfather, a psychopathic Fascist captain at an outpost in Franco's Spain, savagely hunting down rebels. There are a lot more naturalistic than mystical scenes. It's not a monster movie, or at least, not non-human monsters.

But it's one of the most superbly structured films I've ever seen, and it's harrowing and angry and beautiful. Evil is a human thing here, and Vidal, the Fascist stepfather, is busily creating a hell on earth while Ofelia, our little heroine, is trying to step out of earth to fairyland. Both worlds are dark and questionable, and they weave in and out of one another in a symbolic dance that's flawlessly managed.

I have the suspicion it's going to be one of those films that a section of the audience gets passionate about, and the rest kind of shrug at, because it doesn't resonate on the same frequency that they do. For my money, the deciding factor is probably going to be this: what are your nightmares like? Vidal's treatment of prisoners, for instance, is horrifically brutal, in a way that I'd describe as nightmarish. But that's because it's how people in my nightmares behave. Other people's nightmares have a different quality. Talking the film over with people whose nightmares involve, say, being chased by something scary, I found that they were less convinced than I was. Vidal wasn't one of their demons. Given how dream-like the logic of the story is, it seems that some people will find that, intuitively, it makes perfect sense, while other people will like it less, because their dreams are different.

Some people, I have the impression from reading comments and reviews, feel let down because they thought it was going to be a special-effects fairytale, and found that the plot was driven by something else. Being the kind of writer I am, I'm automatically up in arms against anyone who says 'It was okay but it would have been better if we'd had more of the monsters because it was supposed to be a monster movie', so I'm on del Toro's side on principle. But I'm on his side anyway, because my goodness that was an amazing film.

I don't know del Toro, but the feeling the film gave me was that he didn't feel the need to turn it into more of a fantasy film or more of a realist film, because the two weren't incompatible. Each got as much screen time as was necessary to tell the story - but separating them out wouldn't work. Each was part of the other, part of the same sensibility, which didn't split the world along regular lines but along personal ones.

That takes imaginative freedom. It's a sad fact about human nature that the imagination is often conservative: many people will accept only so much, and then get frustrated. Fantastical art, which is supposedly free from the confines of reality, develops its own conventions quickly, and it takes a big effort to get away with breaking them. Conventions become entrenched. And one of them is a meta-convention: that if there are fantasy elements, it automatically makes the entire work officially Fantasy, entirely Fantasy, and not supposed to mix in other elements. I've seen that look myself that people get when you do something that's no odder than having, say, a talking dragon, but less well-established. 'I don't understand why you did this,' is the common refrain, and the look is nothing so much as accusing. This look doesn't just come from fantasy fans, it comes from people who don't follow fantasy especially but are equally convinced that fantasy is a jealous genre like champagne is a jealous wine, and shouldn't be mixed with other things.

You can get entangled in that, or you can stop worrying about it and pursue your own vision. Del Toro did the latter. And I'm so glad he did: the result stands entirely on its own terms, and is close to perfect. Visually, it's spectacular; emotionally, it's gruelling, and it just ripped my heart out. By the time the credits came round, I went from being frightened for the heroine to worrying about myself: I didn't want to go out into the street crying, and I wasn't sure I could stop. That hasn't happened to me in a long time. Forget about genre and see it.