I'm not going to tell you about the plot -- it is ridiculous. For me to even discuss it would take away whatever pleasures might be there. But if I were you, I would do what most audiences have done: not go.
I'm sure that Tales from Earthsea, taken from the science-fiction writing of Ursula LeGuin, will entertain undiscriminating young viewers and will probably thrill anime lovers. It left me bored and impatient.