Humanity is under threat from an alien species and they have started to recruit the best children from a very young age to be the future commanders of the human space fleet. Children are arranged into armies in which they battle in a gravity-free room to teach them proper three-dimensional thinking.

The book is partly a description of the trials an exceptional child has, and describes very well the kind of child who will not dumb himself down to conform. The main point, though, is showing how new situations call for unconventional thinking and how important leadership is to a military. But above all, it's an excellent tale in the tradition of science fiction that makes you think.