The film's setting is magnificently beautiful, there are a lot of stunning ideas and images and the main story is good enough to make the two-planet world look real. The one and the only one flaw of the film is that it pretends to be a sci-fi film rather than a fairy-tale in an unusual setting. If you judge it as a sci-fi you will find a lot of "how could it be" things that tear the imaginary world apart. The actors are great and it is fun to see Spall as a protagonist friend rather than foe. Both Sturges and Dunst are wonderful. So if you want to watch a sci-fi film you better avoid "Upside Down". This is a love story set in the fantasy world. No more, but no less.
Review of Upside Down
