Let me start by saying you've never seen a feature film like Mirrormask. The visual style mixes live action with computer-generated imagery and other magic by Henson Studios. Several days after seeing the film, I'm still not sure what to think about the plot (which was a bit simple) but the creatures and characters are still front and centre in my mind.
Helena is at the centre of the story. Her parents run a small circus (somewhere in tone between a traditional circus and Cirque du Soleil) but she wants to run away and "join real life." To escape, she draws, endlessly; her walls are covered in fantastic charcoal sketches. When her mother falls ill and her world is thrust into flux, she again retreats into her drawings. On the eve of her mother's operation, she falls asleep and the dream world she enters seems all-too familiar.
Neil Gaiman penned the script after he was approached by Lisa Henson to create a story similar in tone and scope to The Dark Crystal and Labyrinth. They then approached Dave McKean (who has collaborated with Gaiman on several children's books) to take the director's chair -- they offered him almost no money but complete creative control. The result is uneven (due to a plot that is too close for my liking to the aforementioned Labyrinth) but still worth seeing -- on the big screen if you get the chance.
Buy Mirrormask on DVD
Buy Mirrormask: The Illustrated Film Script.