Then, the night when her mother is being operated on, she falls into the world she has drawn (basically, McKean drawings stuck to the wall of her bedroom). Things are Not Right in this world, as The Shadows are encroaching on to the City of Light! It befalls to Helena to search for the Charm that Will Set Things Right, the fabled Mirrormask...
Sure, that's a description of the plot of the movie. But it doesn't do justice to the look and feel of it. The movie oozes CGI, and it recreates the unique visuals of McKean into a moving world. Let me stress this again: the visuals are amazing.
But that's pretty much the only amazing thing about the movie. The scenes don't serve to further the plot -- they serve to showcase a particular design. Sure, there are neat finds in there, but the deus ex machina of the Really Useful Book gets kind of old by the third time it is used to get Helena and Valentine out of a pinch.
If you are a fan of McKean and/or Gaiman, then this movie will delight you. If not, then it will leave you confused and indifferent.