The proprietor and his adopted daughter Isabelle get caught up in Hugo's story, and, as one might expect, they have a deep and hidden connection to Hugo. All is revealed eventually, and everything wraps up more neatly than I'd prefer, though it is appropriate for a book for younger readers like this. The story ends up being in large part an excuse for Selznick to get into the history of a person he greatly admires, and to go on about automatons, another subject he's very interested in.
Without giving away important plot points, there's not much more I can say about the story. It's nice, but slightly flattened, the way some stories for younger readers are. The art is more cinematic than graphic-novelish, with obvious tracking shots moving across several pages. I think those effects work decently here, but comics have built up a visual language of their own over the last century or so, which can achieve many of the same effects without taking twenty pages to do so. There's nothing wrong with the way Selznick does it -- and his art is very nice -- but there's something odd and old-fashioned about Hugo Cabret, as if it were a book from 1931 instead of just being about that time.