Humanity is at war with a distance race of aliens (called “Buggers”) and for some reason, the adults are unable to discover the best way to fight this implacable ant-like enemy. (Apparently, Boric Acid doesn’t work.)
What it really requires is the sense of wonder and innocence that only a child can have, and so, the people in charge of Earth’s governments start a breeding program to turn out kids designed to be excellent space warriors.
The children are all tagged so the authorities can monitor all their thoughts and movements, while they are being evaluated for service in the International Fleet. (A device that is similar to an iPhone, but a little smaller, and you can’t download your own apps to it.) After he is un-tagged, a young Ender Wiggin is attacked by bullies, and he kills one of them, so that the bullies will no longer bother him. The IF realizes it has made a horrible mistake. This is just the kind of ruthless logic they need in their war.
The rest of the novel follows Ender’s rise through the ranks at Battle School and Command School, a marginally creepy shower scene, and eventually, the set up for an excellent sequel, The Speaker for the Dead.
It is worth noting that an anagram of Ender Wiggin is “Ending Grew I.”