Gameplay is 8/10
Plot is 2/10
Unless you're a diehard fan and have played all the games the plot is completely incoherent. I'd only played KH I and II. The series apparently diverged from there, for the worst IMHO.. no more crossovers with FF, no more epic orchestral music, OP mechanics with rides, 2 dimensional characters.
In KH I & II the plots of each world were inspired by the universe they were in. In KHIII it's almost verbatim each movie with random messages to try and tie it all together in the overworld.