Ender Wiggin, the third in a family of child geniuses, is selected by international military forces to save the world from destruction. Before being chosen Ender wears a unique monitor that allows the heads of the military to see things as Ender does. Ender's brother Peter and his sister Valentine also wore this monitor, although neither was selected, nor did they have it for as long as Ender, and Peter will never forgive Ender for this. Peter hates Ender, and even when the monitor is taken out it does nothing to decrease Peter's anger. The same is true of Ender's schoolmates, and he is forced into brutally beating the leader of a gang of bullies in order to protect himself. Although Valentine tries to protect Ender from Peter, he is only saved from his brother when Colonel Graff of the International Fleet comes to take Ender away to Battle School. Ender leaves behind Valentine, who loves him, in order to help save the world from the buggers.
Battle School is located on a ship far from earth. On the flight there Ender demonstrates his ability to brilliantly visualize gravitational effects, and Graff begins to isolate him from his fellow classmates. Ender lives with the new recruits, called Launchies. He makes a few friends among the recruits and ends his isolation, although his brilliance will never cease causing resentment. Inside the battleroom, Ender figures out how to maneuver in null gravity, along with another recruit named Alai. Ender and Alai become friends, and this helps Ender fit in with the rest of the group. Ender manages to get farther in one of the computer games, called the mind game, than anyone ever had before him, and although he does not know, the military commanders take notice.
Ender is abruptly promoted to Salamander Army. There Ender is befriended by Petra Arkanian, the only girl in the army. The army commander, Bonzo Madrid, does not like Ender and does not want him around. Ender practices with Petra and begins to teach his launch group what he knows. Ender helps the army by disobeying Bonzo, who hates Ender, but fortunately Ender is traded to Rat Army. Ender is put in Dink Meeker's platoon. Dink is an excellent toon leader, and Ender does well in battles. He continues teaching his Launchies, even though this causes resentment among other soldiers. Ender gets into a fight and he hurts four older boys who attack him. It is clear to Ender that the teachers are leaving him to fend for himself.
Meanwhile, Peter has convinced Valentine that it is time for them to assert themselves. Peter has unlimited ambition, and he is intelligent enough to try to manipulate world events. Valentine agrees to work with him because she wants their impact to be a positive one. Disguised on the information networks they soon begin to exert an influence. By this time Ender is nine years old and a platoon leader in Phoenix Army under Petra's command. Everyone respects his brilliance and his practice...