This movie is inspired by the book "enders game." I'm not going to go into too much detail because I don't want to spoil the movie or book but to generalize it. Ender is a boy that has been chosen to lead a fleet against the alien race they call the buggers. The book and movie explains what he had to go through in order to claim this position. I recommend to see the movie first before reading the book because movies are mostly not as good as the books. Take a look at this trailer for a further understanding of the story.