The book centers August "Auggie" Pullman, a 10-year-old living in North Riverside Heights in Lower Manhattan. He has Treacher Collins syndrome, which has disfigured his face and required many surgeries and special care. Due to his condition, August has been homeschooled by his mother for several years; however, wanting him to experience the world, his parents enroll him into Beecher Prep, a private school, for the start of fifth grade. Auggie has an older sister, Olivia "Via" Pullman, who is entering her first year of high school.
Before the start of the school year, Auggie's mother takes him to meet the principal, Mr. Tushman, who has asked three other students — Jack Will, Charlotte, and Julian — to take him on a tour of the school. Auggie is treated unkindly by Julian, who acts respectfully in front of adults. On his first day of school, Auggie tries to avoid drawing attention to himself, but is bullied by Julian and his friends. Auggie is approached and befriended at lunch by a classmate named Summer, and is paired in most of his classes with Jack, whom he also considers a friend.
On Halloween, Auggie, dressed as Bleeding Scream, overhears Jack, who was expecting Auggie to dress up as Boba Fett, joining in with Julian and his friends in making fun of Auggie. Devastated, Auggie stays home for several days and isolates himself from his family, frustrating Via, who resents Auggie for the priority he receives over her from their parents. Returning to school, Auggie ignores Jack and confides in Summer about the incident with Jack. Jack presses Summer about Auggie being upset, who hints at the cause. When Jack realizes this, he is ashamed, and re-commits to his friendship with Auggie. This draws Julian's ire, and Jack and Julian have a fight. Jack reconciles with Auggie, but is ostracized from his popular classmates, as Julian's influence divides the students into factions over the conflict. Julian's mother writes to Tushman to voice her concerns over Auggie attending the school, citing that his appearance may be too much of a burden for the other students to handle.
Via confides to her mother that she does not want Auggie to attend her school play, as she has enjoyed the fresh start her new school has given her, free of the burden of being associated with Auggie and his condition. Auggie overhears and angrily sulks in his room, hoping his mother will come comfort him, but Via comes in instead to tell him that their dog Daisy is dying. She urges him to come out to say goodbye before Daisy is taken to the vet and euthanized, which he does.
Meanwhile, Via's best friend Miranda has started avoiding her, for reasons unknown to Via. Both audition for the lead in their school play, and Miranda gets the part, with Via as her understudy. On opening night, Miranda sees Via's family in the audience and feigns illness so that Via can take her place for the evening. After the show, the two reconcile.
At the end of Auggie's school year, the fifth grade class goes on a three-day trip to a nature reserve. Auggie is initially concerned about going because of Julian, but hears that Julian will not be attending.[5] On the last night of the trip, Auggie and Jack are walking alone in the woods when they are attacked by a group of older students from another school. Julian's friends happen to walk by, and they rush to the defense of their classmates, impressed by Auggie's boldness in standing up to the bullies. Auggie becomes accepted by his school peers.
At graduation, Auggie is awarded the Henry Ward Beecher Medal for his strength and character throughout the school year, while Julian's parents decide to send Julian to a different school the following year.
