Like hundreds of other students in Manhattan, August Pullman is starting fifth grade. But August is no ordinary 10-year-old. He's had 27 surgeries since birth, all on his face. August was born with a severe craniofacial abnormality. Despite his many surgeries, he doesn't — and will never — look normal. Children (and sometimes adults) who catch a glimpse of his face either stare or turn away quickly in shock and horror. This year will be August's first at a mainstream school. His mom thinks the experience will be good for him, but his dad disagrees. He says that sending August to school is like leading a lamb to the slaughter. August isn't sure what that means.
Before school begins, August's mom takes him to his new school, Beecher Prep, to meet the principal and get a tour from three of his new classmates — Jack, Julian and Charlotte. Jack and Charlotte are nice, but Julian asks August if he was burned in a fire. When school begins, Jack sits beside August in every class, and the two boys quickly become good friends. August also makes friends with a girl named Summer, the only student in the whole school brave enough to sit with him at lunch. And while only Julian is deliberately mean, August still has to cope with hundreds of secret stares and watch his peers elbow each other and whisper about him from behind cupped hands.
When August has a birthday party, he invites everyone in his class, but only six of them come. He also learns that nobody wants to touch him, and if someone does accidentally brush up against him, they run away quickly to wash their hands. Even the parents get involved — Julian's mom photoshops August out of the class photo and distributes copies to other parents.
Fortunately, one of August's favorite holidays, Halloween, is approaching. He plans to dress up like a Star Wars character, but at the last minute, he changes his mind and wears another costume instead — the Bleeding Scream. When he arrives at school, no one knows it's him under his mask, and he overhears Jack telling Julian that he only hangs out with August because the principal asked him to, and that if he looked like August, he would kill himself. August feels nauseous after hearing this and goes home sick. He tells Summer and his sister, Via, what happened but swears them to secrecy and refuses to talk to Jack for more than a month.
Meanwhile, Via is having troubles of her own. Via has always felt defined by August, and she was looking forward to attending a new high school where people would call her by her real name, Olivia, and where no one knew her family — except for her two best friends, Miranda and Ella. But on the first day of school, Via realizes that her "friends" have created a new image for themselves over the summer — one that doesn't include her. They ditch Via as soon as possible to pursue their quest for high school popularity. Because Miranda has always acted like a big sister to August, this creates tension between Via and August, especially when Miranda calls August to say hello but refuses to hang out with Via at school.
When Via learns about what Jack said about August, she consoles August and talks him into trick-or-treating with her.
Summer is also paying the price for being August's friend. One of the popular girls invites Summer to a Halloween party — but when she arrives, everyone wants to quiz her about why she hangs out with August. They tell her that if she didn't hang out with him, she would be popular, too — and Julian might even ask her to be his girlfriend. Summer leaves the party early but stays friends with August. When Jack asks her why August won't talk to him, she gives him a hint: "Bleeding Scream."
It's true that the principal asked Jack to befriend August at the beginning of the school year. At first, he was reluctant to agree. But after getting to know August personally, Jack realizes that August is the best friend he would have chosen for himself anyway. So when August stops hanging out with him, he wonders why. Although Jack enjoys his increased popularity, he doesn't really like Julian and his crowd. He doesn't fit in because their families are much wealthier than his, and his grades drop without August there to help him. In science class, the teacher assigns a project for the upcoming science fair, and Jack imagines his face screaming on the inside. Suddenly, he remembers Summer's hint (Bleeding Scream) and realizes that August is mad at him because of what he said on Halloween. After class, Julian calls August a freak. Jack punches Julian in the mouth and knocks out a tooth.
Since there are just a few days before the Christmas holidays, Jack is suspended until January. A flurry of emails are sent, and Julian's parents express their concern that perhaps August shouldn't have been admitted to Beecher Prep and that it was too much pressure to expect Jack and Julian to befriend August back in September. Mr. Tushman, the principal, and Jack's parents disagree with Julian's parents. Jack re-friends August on Facebook, and he apologizes for what he said on Halloween. After asking if Jack really punched Julian, August accepts his apology, and the two boys become better friends than ever.
In January, Charlotte tells Jack that the entire fifth-grade male population is at war. It's Jack and August versus Julian and his two sidekicks, Miles and Henry. Most of the boys are on Julian's side, but there are a handful of neutral boys. Jack learns for the first time what it's like for people to treat him as if he doesn't exist. Julian puts mean notes in Jack's and August's lockers, and they respond by putting fake love notes in his locker.
At August's house working on their science project, Jack and August meet Via's new boyfriend, Justin. They think his fiddle case looks like it might hold a machine gun, and they think it's funny that he plays in a Louisiana-style band when he's from Brooklyn. On his way home, Justin sees Julian and his friends making fun of Jack. Jack tells Justin about the war. After Jack leaves, Justin holds his fiddle case menacingly and tells Julian not to mess with Jack or he and his friends will be sorry.
The tricks Julian plays on Jack become meaner, and fifth-graders start becoming tired of the war. Even Amos, one of Julian's friends, stops Julian from emptying his pencil shavings into Jack's backpack. When Julian spreads a rumor that Jack has hired a hit man to get him, people start laughing at Julian behind his back. By springtime, only Miles and Henry are on Julian's side, and more of his classmates are being nice to Jack and August. They don't even tease August about the hearing aids he has started to wear.
But at home, Via and August are fighting because Via didn't want to tell August about the school play Justin and Miranda are starring in. (Via is Miranda's understudy.) When August finds out, Via wants to invite him, but her parents are trying to respect her feelings by not inviting him. Then their dog, Daisy, must be euthanized because she is old and sick. A few days later, Via brings home three tickets to the play, and nothing more is said.
When Miranda sees that Via's family is in attendance at the play, she pretends to be ill so Via can play her role instead. Miranda's parents aren't there to see her — they divorced the summer before Miranda started high school. When she went away to camp that summer, she pretended that August was her little brother instead of Via's. The lie made her popular, and she began hanging out with the cool crowd who smoked and sneaked through the woods at night to hang out with boys. When Miranda got back home, she felt embarrassed about her lie and didn't want Via to find out, so she started hanging out with Ella instead. After the play, Miranda sees August get lost in the crowd, and she brings him back to his parents. She and Via make up and become friends again.
At the end of the year, the entire fifth grade goes on a three-day nature retreat. At first, August is nervous about being away from home overnight, but when he learns that Julian opted out of the trip because he thought it was dorky, August is overjoyed. He has the time of his life until the second night of the trip, when the fifth-grade students watch an outdoor movie on the fairgrounds with students from several other schools. Midway through The Sound of Music, Jack and August have to use the restroom. Instead of waiting in the long lineup for the toilets, they go quietly in the woods, as do Amos, Miles and Henry.
On their way back to the movie, they run into a group of seventh-grade students from another school, who immediately start making fun of August. They push Jack to the ground and yank August's hoodie so he falls flat on his back. Amos, Miles and Henry stand up for Jack and August, pushing the seventh-graders out of the way so Jack and August can escape into the cornfields that surround the fairgrounds. August's sweatshirt is torn and his elbow is bleeding. When he realizes that his hearing aids are gone, August can't help but cry. But instead of making fun of him, the other boys pat him on the back and Amos lets him cry on his shoulder. August finally realizes what his dad meant when he talked about leading a lamb to the slaughter.
The boys walk back to camp together surrounding August like a guard, and news of their encounter spreads quickly among the fifth-graders. Suddenly everyone is concerned about August, even students he didn't know well. When the fifth-graders return home, August tells his mom about his misadventures. Then his dad and Via arrive home together, carrying a big, white box and tell August to open it. Inside is a little, black puppy looking up at him.
When August returns to school, the war is over. Because Julian missed the nature retreat, he is no longer as popular as before, and Miles and Henry become Amos' best friends, instead of Julian's. No one ignores August or Jack anymore, and August gets used to knuckle-punching jocks in the hallway and being called "little dude" by students he barely knows.
At the end-of-year graduation ceremony, August wins an award for being an exemplary student, not just academically but personally as well. Everyone cheers for him, and August finally feels like an ordinary kid — even though he knows no one else sees him that way. At the reception, his picture is taken with his friends and classmates, and nobody minds who is touching whom. When they walk home for cake and ice cream, August thanks his mom for making him go to school. She thanks him for being him — and tells him that he is a wonder.