Some time in the near future, Earth faces an existential threat from the Buggers, an alien race that has already killed millions of humans. In the previous Bugger invasion (70 years before Ender’s Game takes place), the Buggers destroyed large chunks of China—they were only stopped from destroying the planet altogether because of Mazer Rackham, the legendary pilot who led a fleet of spaceships to destroy the Bugger fleet. In order to ensure that humanity wins the next encounter with the Buggers, the International Fleet (IF)—the most powerful military group on Earth—monitors young children around the world, hoping to find signs of military genius. The brightest children are recruited for Battle School, where they’re trained in combat.
At the beginning of the story, Ender Wiggin is a 6-year-old child. He is the third child of his parents—his older brother is Peter Wiggin and his older sister is Valentine Wiggin. One day at school, Ender is bullied by a boy named Stilson. Instead of accepting Stilson’s insults, Ender savagely kicks him again and again. Ender then goes home, where Peter bullies him. Peter resents Ender, while his sister, Valentine, loves him.
The day after Ender’s fight with Stilson, the Wiggins are visited by Colonel Hyrum Graff, who informs Ender’s parents that Ender shows signs of great military potential. Graff pressures the Wiggins into letting Ender go to Battle School, and tells Ender that he’ll be trained to protect the planet from Bugger aggression. Ender agrees to fight in Battle School, even though he’ll miss Valentine, whom he loves more than anyone.
Over his first months in Battle School, Ender builds up a friendship with Shen and Alai, two new students. Ender takes classes and plays many war games, including a computer simulation called the Giant game. Ender is soon transferred to fight in an army—at Battle School, older students are sent to different armies and made to fight one another in the battleroom, a zero-gravity chamber in which they can use laser guns to “freeze” their opponents. Ender is sent to the Salamander Army, where his commander, the arrogant Bonzo Madrid, immediately dislikes him. Bonzo orders Ender never to fire his gun during a battle, and Ender reluctantly agrees. Over time, however, he becomes frustrated with Bonzo’s orders, and in a battle, he draws his gun and succeeds in shooting many opponents. Bonzo is furious, and slaps Ender in the face. Soon afterwards, he trades Ender to Rat Army. In the Rat Army, Ender rises to the top of the battleroom “standings” and gains a reputation as a brilliant soldier.
Back on Earth, almost two years have passed since Ender left. One day, Peter, now an early teenager, tells Valentine (10 years old) that he wants her help to change the world. He explains that the Bugger Wars are coming to an end—when this happens, there will undoubtedly be a huge war on Earth (where the U.S. and Russia are still competing superpowers). Peter wants to use writing to control politics leading up to this war. He thinks that with Valentine’s help, he can rise to lead the world. In the coming months, Peter and Valentine use computers to write a series of popular articles. Peter, going by the name “Locke,” argues for peace and stability, while Valentine, writing under the name “Demosthenes,” promotes conflict and paranoia.
Graff plans to give Ender a series of increasingly severe challenges designed to make him the best commander possible. Ender is assigned command of an army consisting of young, mediocre students. He is tough on his soldiers, and picks on one of them, Bean. Nevertheless, Ender leads his troops to victory in battle after battle. Graff gives Ender more battles to fight. As time goes on, Ender feels himself becoming hated: his old friends barely talk to him, and when he easily defeats Bonzo, he learns that Bonzo wants to kill him.
One day, Ender is taking a shower when he turns to discover Bonzo, joined by a gang. Ender goads Bonzo into fighting him one-on-one, and he kicks Bonzo savagely in the groin. We learn that Graff was responsible for manipulating Bonzo against Ender, and will continue to give Ender tougher and tougher challenges. On the same day that Ender hurts Bonzo, Graff gives him a battle to fight. Although Ender is able to win the battle, he has a breakdown, and screams, “The game is over!” Afterwards, he is transferred to Command School. Before his transfer, Ender is sent to Earth with Graff.
On Earth, Valentine and Peter have used their writing to rise to prominence in politics. Peter plans to use Valentine’s influence ensure himself a strong coalition of supporters. Meanwhile, Graff convinces Valentine to talk to Ender one more time. Valentine visits Ender in Florida, and discovers that the International Fleet has turned Ender into a dangerous killer. Nevertheless, she tells Ender, honestly, that she’s not afraid of him. Ender is inspired to fight: seeing Earth and Valentine again has reminded him that he must beat the Buggers.
Ender and Graff travel to Command School, on the distant planet Eros. There, Ender meets his new teacher, the legendary Mazer Rackham. Mazer tells Ender what he knows about the Buggers: they’re capable of communicating with each other by ESP, and are controlled by a queen—Mazer defeated the Buggers by killing their queen. Mazer gives Ender a series of computer-simulated battles to fight. Ender is also reunited with his old friends, including Alai, Shen, and Bean. Ender proceeds with fighting against the computer simulations.
After months of difficult battles, Ender is told that he has only one battle left to fight. In it, he’ll face a large enemy force that’s protecting a simulation of the Bugger home planet, where the queen lives. Mazer stresses that Ender will have to make the right choice about what to do. Ender proceeds with his game. He decides to end the battle as quickly as he can by “bending” the rules. He sends a deadly missile, the “Dr. Device,” toward the Bugger planet, destroying the queen and thus wiping out all other Bugger forces. As he completes his game, Ender learns the truth: the game was always real. Ever since arriving at Command School, Ender was commanding real troops and fighting real Buggers. By making the decision to destroy the Bugger planet, Ender has won the war. Ender is horrified when he learns that he’s wiped out an entire species. He realizes that he is the killer he always feared he was—a monster.
Ender is then prevented from returning to Earth, to avoid being manipulated by other countries. One day Ender finds Valentine waiting for him in his room. Valentine tells Ender that Peter, writing as Locke, is quickly rising to power. Peter had plotted to use his family connection to Ender to gain more power—Valentine, however, blackmailed Peter into arranging for Ender to become the governor of a new colony far from Earth. Ender agrees to travel to the new colony (a former Bugger planet) with Valentine.
Ender becomes a popular governor there. One day, he goes exploring on his new planet, and is amazed to find a field containing recreated details from his childhood, as well as elements from the Giant game at Battle School. There Ender also finds a large insect pupa—the last surviving Bugger queen. Ender, communicating with the queen telepathically, realizes that the Buggers had no intention of attacking humans again after their first invasion. Ender also realizes that the Bugger queen has been watching him for years, and prepared this place so that Ender could help repopulate the Bugger species and atone for his crimes. Ender agrees to do this, and he and Valentine go off in search of a new world where the Buggers can live in peace.