Okay, this trailer has a very annoying song sung by a 5 year old girl, plus some morning news crew talking over it. You may want to turn the sound down.
The story, "Ponyo on the Cliff," from what I gather, is a fish (goldfish, I am told, but he doesn't look like one so much as a sea cucumber with a boy's head) named Ponyo who desperately wants to be a human. In pursuing her goal, she befriends a five-year-old human boy, Sosuke. It's loosely based on "The Little Mermaid" as far as the theme goes, and the boy is loosely based on Miyazaki's son.