Helena is not happy living her parents' dream. She wants to run away from the circus and join real life, and after a screaming match with her mother, has a nasty wake-up call when her mother goes to the hospital with (apparently) a brain tumor. Helena's misery takes her over, and she finds herself displaced into a dark, grimy, dangerous and uncertain world.
Like in Labyrinth, Helena's adventure can be explained away as a dream, all its elements derived from the contents of her room and her life. Again like Labyrinth, there's more to the dream than just a dream. UNlike Labyrinth, Helena's dream isn't filled with whimsical, amusing muppets. It looks more like the dark end of the surrealism branch of the local grunge-art museum. The sense of not-rightness never lets up...one has to completely disengage ones mind from expecting things to make sense. Even the friendly creatures have a bit of 'ick' about them, and the unfriendly ones are chilling. (Those intense, feral, cat-beasts with human faces were scary,)
As a film, though, I don't think Mirrormask succeeded. Story...yeah, there was one. It was actually clever and intriguing. But my mind was too busy watching to see what appeared next, and then coping with it, to get too involved with the plot or characters. The whole otherworld sequence has an emotionless feel to it, and the emotional portion of the film occurrs all at the beginning. Given more time and a better budget, I'm sure they could have hammered out a film that had better timing and character development.
But even so, it's probably the weirdest thing you can see without taking anything illegal, and it's certainly worth seeing.