It's a film about Arthur the 10-year-old method of working to keep his granny's house from destruction. This young man tries to search for those mythical hidden treasures in the land of Minimoys, a small people perfectly compatible with that nature around them.
Luc Besson's half-baked live-action/animated fantasy looks like it was invented on the hoof: it's erratically plotted, poorly animated, overly derivative and too insufferably cute to interest anyone above undemanding toddler age.
Luc Besson has made a fair share of artfully bad movies. Arthur and the Invisibles -- half-live-action, half-CG kid's adventure -- is (by a hair) more bad-bad, like The Fifth Element, than good-bad, like The Big Blue.