"In the moment when I truly understand my enemy, understand him well enough to defeat him, then in that very moment I also love him." - A.E. Wiggin
50 years ago, an alien race known as the Formics invaded Earth. A war ensued, and tens of millions were killed. A daring commander named Mazer Rackham flew his jet into the Formic queen ship, destroying it and bringing all the other Formics down. In the decades since, the International Fleet has recruited children to train them in case the Formics ever return.
We meet Andrew "Ender" Wiggin (Asa Butterfield) playing a game on a tablet with another boy, Stilson (Caleb Thaggard), at his training school. He is being observed by Colonel Hiram Graff (Harrison Ford) and Major Gwen Anderson (Viola Davis) through a monitor in the back of Ender's neck. Ender destroys Stilson's ship in the game, leading him to accuse Ender of cheating. He demands a rematch, but Ender leaves calmly and says perhaps tomorrow.
Ender is called to the infirmary. He worries that he's done something wrong, and the monitor is taken out of his neck soon after, which means he is being removed from the school. As he leaves the infirmary, Stilson and his friends corner him in the hallway. They grab Ender and push him into their classroom so they can have their rematch, one-on-one. Ender wonders aloud why Stilson's friends need to back him up if it's one-on-one, so Stilson orders his boys to step aside. He advances toward Ender, and he reacts by grabbing an item off the table and then hitting Stilson with it. Stilson falls to the ground and Ender continues kicking him furiously. Graff and Anderson watch all this unfold on screen.
At home, Ender laments getting his monitor removed to his sister Valentine (Abigail Breslin) and then tells her about the incident with Stilson. He had thought "What would Peter do?", as his brother Peter (Jimmy Pinchak) was also removed from the school. Ender thinks it has to do with him being a "third" (third-born child when there is only a two child policy on Earth). Peter enters the room and bugs Ender about what has happened. Valentine steps out of the room while Peter gives Ender a mask resembling a Formic to play a game they call "Buggers and Astronauts". Peter tackles Ender and has his hand around his throat, angry at Ender for keeping his monitor longer than he did by the same point.
During dinner, Graff and Anderson visit the Wiggin home to tell Ender's parents about the fight at school. Ender explains that he continued to kick Stilson because he thought it would prevent future conflicts. Because of this strategy, Graff believes Ender is suited for Battle School, where the best recruits will be trained to lead an army against the Formics. Ender is surprised since his monitor was taken away, but it turns out this was a test to see what kind of motivation Ender has in conflict. He and Graff speak in private, and Graff is able to appeal to Ender by telling him he can save mankind by joining Battle School.
Ender agrees to join Graff. He narrates that Valentine was not chosen because she was too compassionate, and Peter was too quickly prone to violence. Ender and Graff go to a rocket filled with other kids, or "Launchies", as Graff refers to them. Ender introduces himself to a kid named Bean (Aramis Knight), so named because he "grew up on the streets, not worth a bean." The rocket launches into space, putting everyone in zero gravity. One kid, Alai (Suraj Partha), becomes sick and pukes into a bag. Graff comes floating in horizontally, causing Ender to grin. Graff asks him if something is funny, and Ender replies that him floating is funny because in zero gravity, nobody is actually vertical or horizontal, so Graff could see himself as vertical and the kids as horizontal. Graff then asks the other kids if he thinks this is funny, to which they all reply, "Sir, no, sir!" Graff replies that they're wrong, because it IS funny, and then states that Ender is the only intelligent person on board. This instantly creates animosity among the kids toward Ender.
The recruits arrive at the Battle School where they watch videos of Mazer Rackham's heroic defeat of the Formics. The kids cheer while Ender seems to be quietly observing the video. The kids are brought to their quarters where one boy, Bernard (Conor Caroll) refers to Ender as the "smartass". Entering the quarters is the hard and abrasive Sergeant Dap (Nonso Anozie). He barks the rules to the kids and tells them to bring any questions to him.
Graff brings the kids to a zero gravity battle room to test their skills and strategy for taking down the enemy. Ender and Bean play with the guns they're given, which, when fired, freezes the nerves in the area the enemy's been hit and renders them motionless.
In the first three months at school, Ender tries to e-mail Valentine, though he realizes the e-mails are not going through. He also spends a lot of time isolated from the other kids, and the rest of his time training in combat and in the battle room with other teams. Graff and Dap enter the Launchies' quarters one evening, and Ender asks Graff about the e-mails. Graff explains that communication to Earth has been cut, which upsets Ender because he was told his privacy would be restored if he joined. When Graff leaves, Dap orders Ender to never speak unless spoken to, but Ender argues he had a legitimate question. Enraged, Dap tells Ender to drop and give him 20. The other kids watch silently as Ender never shows any emotional reaction. Dap tells Ender he will never be a commander and that he will never salute Ender.
During another class, the professor makes a note that Ender, Bean, and Alai were the only ones who passed a given test, which Bernard thinks was a cheat. The professor calls Ender to come up and explain how he did it, but Ender says Alai is better suited to explain. Bernard writes a message in the computer for the other students to read, calling Alai "captain of the vomit comet." Everybody laughs, while Ender retaliates by writing "Bernard is living proof that they still send chimps into space." This gets a bigger laugh from the kids, and the professor sees it. She replies that if he can't take it, don't dish it.
Bean, Alai, and other kids start to befriend Ender. They start sitting with him during meals while Bernard is the one left alone. One evening, Ender plays a mind game on a tablet in which he navigates as a mouse through an area, running before a giant (Gavin Hood) who tells the player that there are two cups before him, one of which contains a poison. If Ender can guess which one isn't poisoned, he may pass. He discovers that both cups are poisoned as the mouse sinks into the liquid, and he thinks the game is rigged. Graff and Anderson monitor him as he plays. Ender then thinks to have the mouse jump into the giant's eye, burrowing in there until the giant is dead. Anderson notes that nobody has ever tried anything like that.
Ender is transferred to the Salamander team where he must report to commanding officer Bonzo Madrid (Moises Arias). As he leaves, Alai tells Ender, "A salaam alaikum (peace be with you)". Ender arrives at the Salamanders' quarters where he meets Petra Arkanian (Hailee Steinfeld), the only girl on the team. Bonzo comes in and immediately dislikes Ender, calling him skinny and unfit. He orders Ender to basically stay out of his way.
Petra offers to help Ender practice in the battle room. The two go in and she shows off her capability with the stun gun. Ender proves less effective, though she demonstrates her own method to Ender, which helps him improve. When they return to their quarters, Bonzo is angry that Ender went to practice with Petra. Ender requests to speak with him in private, telling Bonzo that if he lets him practice, it will make him look generous and it will give Ender a chance to improve himself. The next day, Bonzo tells Ender in front of everyone that he has changed his mind about letting Ender practice and participate in battles with the team.
The Salamander team goes up against another team in the battle room, and Bonzo orders Ender to stay back. During the game, Petra and other team members are down, forcing Ender to jump in. With Petra's help, they dispatch the other team. As the two are practicing in combat later, Bonzo enters and socks Ender in the gut for making him look like a fool.
At night, Ender resumes playing the mind game. As the mouse, he leaves the giant's eye socket and wanders further until a bunch of leaves form to look like a Formic. The creature floats for a while until it transforms into Valentine. Ender is surprised but follows her as bombs fall onto the land. Valentine runs into the ruins of what appeared to be a Formic tower, and Ender continues following her. They enter the tower room and the mouse morphs into a digital Ender. To top off all the surprises, congratulating him for making it this far is none other than Peter. Graff asks Anderson how Peter could have made his way into this game, which she explains is because the game is connected to Ender's mind.
Graff promotes Ender to the defunct Dragon Army, giving him control of his own team of misfits that Ender can lead to victory. He meets his team members, which include Bean, Alai, and Bernard.
The Dragon Army is tasked with a battle against both teams Salamander and Leopard. One of Ender's team members, Fly Molo (Brandon Soo Hoo), jumps too high from his bed and twists his ankle. His replacement is Petra, ordered by Graff to join Ender's team, which Ender is quite pleased with. Ender organizes a strategy that has another Salamander member, Dink (Khylin Rhambo), hold onto Petra as he is stunned by the other teams while Petra single-handily dispatches everyone in view. Ender then ties Bean to a rope and has him float to scope out the other team members. He returns after being stunned repeatedly and briefs Ender on the the other teams' hiding spots. Ender has his whole team create a formation to get through the battle room, allowing most of the others to get stunned while Alai is protected. They make it to the other side of the room victoriously.
Ender is taking a shower when Bonzo and two of his boys come in. Bonzo orders the boys to watch the door, and Ender comments on Bonzo's attempt to fight a boy in the shower. When Bonzo isn't looking, he turns up the water's heat and sprays Bonzo in the face. He tries to run, but Bonzo grabs him. Ender nearly breaks his arm and lets go, not wanting to continue the fight. Bonzo charges toward Ender, forcing him to kick Bonzo backwards where he falls and hits his head hard on the floor. He twitches and becomes unconscious. Ender panics and cries for help. Bonzo is taken to a room for an operation, and he is comatose. Ender fears that he has killed him, but Graff tells him he did what was necessary, since everybody knows that Bonzo started the fight. Ender quits on the spot.
Back on Earth, Valentine comes home to find Graff in her home. He drives her to a location where Ender is hiding, still regretful over what he did to Bonzo, fearing that he will never wake up. Valentine runs to him and they embrace. They go on a boat ride across the lake, where Ender muses that he must understand his enemies before he can act against them. Valentine admits she was sent to convince Ender to return to space, though she manages to succeed by reminding him that he is fighting to preserve the human race. Ender returns to space with Graff, on the condition that he is able to maintain contact with his sister.
Ender is promoted to Command School and is taken by Graff to an IF base that was once a Formic post. In his new quarters, Ender wakes up to find an older man with a tattooed face (Ben Kingsley) sitting on the floor. Ender walks past him and the man grabs him by the foot, causing Ender to trip. Ender thinks this is some sort of test to fight back, and when the man subdues him, Ender tells him he's won. The man asks him how he can tell the enemy when he's won. Ender asks him if he has a name, and he replies "Mazer Rackham." Ender is speechless, seeing this legendary commander standing before him, very much alive. He asks Mazer about the videos he's seen which always show Mazer flying into the Formic queen ship. Mazer shows Ender the videos, which show him ejecting himself from his jet as it flies into the queen ship to destroy it. This causes all the other Formics to freeze in the air and then plummet to the ground. Ender observes Mazer flying through the clouds and detecting a pattern that allowed him to get past the Formics to defeat them. Mazer explains that the humans were able to come up with more than a few strategies in the fight, as they are able to think more than the Formics can.
As a new commander, Ender is taken to his team of soldiers - Petra, Bean, Alai, Dink, and Bernard. They are set up to prepare in a simulator room against a computerized army of Formics. On their first mission, the team defeats the Formics by firing from under a large block of ice to destroy the queen ship. Their second attempt is a failure when Ender's entire squadron is killed. Ender argues to Mazer and Graff that his team is tired and they need sleep, though Mazer fires back that sleep deprivation is necessary since a fight against the Formics could last days.
The team is all set for their final graduation battle. When the enemy's gate is down, Ender has Petra fire against the Formics, taking out most of their ships. Hundreds more Formic ships arrive, outnumbering Ender's squadron. He decides to sacrifice Bean and Dink's fighters to protect Petra, giving her a chance to fire a blast at the Formic planet to completely destroy it. The planet is slowly charred to a crisp and the screen is disabled for a bit. Ender's peers and team members cheer him on for his victory. "HOW ABOUT THAT?!" he proudly cries to Graff, Mazer, and the other adults. They begin to applaud Ender until the images on the screen returns, giving a clear view of the Formic planet now a volcanic wasteland. Graff comes out to congratulate Ender, but he asks why these images are on the screen. Graff admits that this was never a game, and that he was really commandeering his fleet to attack the Formic fleet. Ender leaves the room tearfully, knowing he has effectively killed off an entire species. Graff reminds him that the Formics would have done the same to mankind, but Ender says they only came to Earth to form a colony, and it was the humans who initiated the war. Instead of being remembered as a hero, he thinks he would be remembered as a killer. Two adults come in and sedate Ender, taking him back to his quarters. Petra and the others stay by his side as he sleeps.
When Ender wakes up, he leaves the base and steps outside, followed by Petra bringing oxygen tanks for them. Ender spots the ruined tower that he saw in his dreams and in the mind game. He tells Petra that he is going in there, alone. Upon entering the tower, Ender finds a sac containing an unborn Formic. A queen steps out from the shadows and comes before Ender. He allows her to touch his face with her claw, which she retracts when Ender doesn't react. He realizes that the unborn Formic is a queen and that the queen he sees right now is dying. He tells her that he will find a new home for it.
Ender writes one last message to Valentine, saying he's been promoted to admiral, and he is going across the universe, vowing to keep his promise to the Formics.
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