The movie is fantastic.
Jared, Simon , and Mallory Grace (played by Freddie Highmore and Sarah Bolger, respectively) have just moved into the old mansion once owned by their great-aunt Lucinda Spiderwick. Her father, Arthur, had vanished one day, and it seems to have driven her mad. She ranted on that fairies had kidnapped him and was found with slices on her arms. Thinking her to be out of her mind and suicidal, she was, for lack of a better phrase, put away.
In the old house, strange things begin to happen. Objects being stolen, frightening noises, and messages written in dust show something is not right. Jared investigates and finds an old book of Arthur's. There is a note on the front telling him not to open it. But, being Jared, he starts to read.
His eyes are then opened to the fantastical world around him, a world of sprites, fairies, hobgoblins, brownies, and griffins. But there are also darker creatures, led by the ferocious ogre Mulgarath, who wants the book for himself. If he gets the book, it will mean death for all those nearby.
This movie has magic. The creatures of the books have been fully realized, and the plot moves as fast as ever. Go see it. You'll be glad you did.