When a blue Thunderbird pulls the trailer carrying the fattest boy in the world into Antler, Texas, in the summer of 1971, eighth-grader Toby has more on his mind than a sideshow. His mother left a few days earlier for a singing contest in Nashville, Tennessee, and his best friend’s (Cal’s) oldest brother, Wayne, has been gone even longer to fight in the Vietnam War.
But Cal is intrigued by the fattest boy. And Toby’s crush, Scarlett, has asked him to take her little sister to see the fattest boy because Scarlett doesn’t want to go herself. She’s rather hang out with her boyfriend Juan. So Cal, Toby and Scarlett’s little sister wait in line to see Zachary Beaver.
The boy inside the trailer is enormous but not much older than Toby and Cal. His answers to questions are short and disinterested, and their visit with him is over quickly, as Scarlett’s sister has wet her pants.
Mealtimes feel especially quiet without his chatty mom around. His father, who raises worms for the local bait shops, is not a man of many words. Wayne often writes letters home, and Toby is bothered that Cal never writes back.
The next day Cal and Toby go to the Bowl-a-Rama, the local bowling alley and diner that sits across from Zachary Beaver’s trailer. The boys notice that the blue Thunderbird that pulled the trailer into town is gone. Ferris, the owner of the Bowl-a-Rama, who had once intended to become a preacher, explains that the man drove away that morning, leaving Zachary behind.
The next day Toby’s father treats Toby and Cal to lunch at the Bowl-a-Rama, where Toby’s mother worked. Ferris and the lunchtime regulars all ask how his mother is fairing in Nashville, and his father grows quieter with each question. Eventually the conversation turns to Zachary, and the townspeople wonder what to do with the enormous boy who has been left without a guardian.
Three days later, when the Thunderbird still hasn’t returned, Cal and Toby leave a bag of groceries outside Zachary’s door. While they wait from the roof of the Bowl-a-Rama to see whether Zachary will emerge, Cal reads Toby the latest letter he’s received from Wayne in Vietnam.
Toby encourages Cal to write back, but Cal says he doesn’t need to, since his sister Kate writes to him every day. Leaving the Bowl-a-Rama, Cal drops the letter from Wayne. Toby picks it up, intent on returning it eventually. Zachary takes the groceries.
Another day passes, and Toby and Cal wonder who the man in the Thunderbird could be and how he’s connected to Zachary. They bring another bag of groceries to Zachary and again wait on the roof of the Bowl-a-Rama to make sure he gets it.
While waiting, a group of younger kids gather around the trailer, taunting and daring the fat boy to come outside. Incensed, Toby and Cal gather small rocks from the roof and begin pelting the other boys from above. A few of the younger boys scatter, but when Toby misses and sends a rock through the trailer window, they all run.
The next morning, the town sheriff arrives at Toby’s house. Toby is afraid that Zachary told the sheriff about the broken window, but the sheriff has only come to ask whether Toby and Cal would be willing to go with him to ask Zachary where his guardian went, since he doesn’t want to scare Zachary.
Toby, Cal and the sheriff learn that Zachary’s parents have passed away, and his guardian has gone in search of other acts to add to their sideshow. Zachary doesn’t know when he will return. While in the trailer, Cal finds a Bible in a gold box. Zachary says his mom gave it to him at his baptism. Once they leave, Cal tells Toby that the baptism information in Zachary’s Bible was blank, suggesting he hadn’t been baptized after all.
At dinner that night, Toby’s father announces his mother didn’t win the contest in Nashville, but she won’t be coming back right away. She may not come back at all. Toby starts to call his mom for an explanation, and then decides against it. Later, he reads over the letter from Wayne that he has yet to return to Cal. He writes a response to Wayne and signs it with Cal’s name.
The next day Toby delivers soil to Cal’s mom, per his dad’s request. Cal’s mom asks how Toby’s mother fared in the Nashville contest. Unwilling to admit his mother won’t be returning, Toby claims the contest was postponed due to a fire.
After a hard day of yard work, Toby visits his crush, Scarlett. Scarlett has recently broken up with her boyfriend Juan. Toby and Scarlett chat on the porch, and while Toby is smitten, he can see that Scarlett still has feelings for Juan.
Toby and Cal visit Zachary again. Zachary tells them of his travels to France, and when asked, Toby again blames a fire at the Grand Ole Opry for his mom’s delay. Cal asks about the blank baptism information in Zachary’s Bible. Zachary says the ceremony was never actually carried out.
Cal suggests that they help Zachary get baptized. Unsure of where to start, Toby asks Miss Myrtie Mae, whose lawn he mows, how one gets baptized. Miss Myrtie Mae tells him baptism — making a commitment to live a Christian life — shouldn’t be taken lightly. That’s why it should be performed by a preacher. She gives him John 3:16 as a verse to better explain baptism.
Convinced his mother will never come home, Toby steals his mother’s pearl necklace. He intends to give it to Scarlett. But there is a mixup, and Scarlett’s little sister ends up wearing the pearls.
Reverend Newton hears from Miss Myrtie Mae that Toby wants to get baptized. Toby explains that he’s only asking for a friend, and that he’s not the one who wants to get baptized. He asks specifically how that could happen. Reverend Newton explains that his friend would have to respond to an altar call at his church and then be scheduled for a public baptism in the new baptistery.
Toby chats with Cal’s mom. As they are talking, a pair of uniformed men arrive. They inform her that Wayne has been killed in the war.
The whole town mourns Wayne’s death, but Toby is especially affected by it. He shuts out everyone, including Cal, and insists on staying home from the wake. Instead of attending the funeral, he rides around town and finds himself visiting Zachary, looking for a distraction from the sadness.
Zachary describes foreign countries like Holland and Switzerland in detail. Eventually Cal arrives and confronts both of the other boys about their lies — Toby about his mom and Zachary about visiting other countries. Cal had seen the guidebooks in Zachary’s trailer.
Toby helps Ferris wait tables at the Bowl-a-Rama. The sheriff arrives for lunch and chats with Ferris, explaining that he’ll have to call social services since Zachary’s guardian still hasn’t returned. He’s learned that Zachary’s mother, also abnormally overweight, had died a few years ago.
Her funeral turned into a sideshow where everyone wanted to get a look at Zachary. Toby realizes this must be why Zachary was never baptized: He didn’t want the event to become about his size. Scarlett arrives and returns the necklace, saying it’s too nice for her to keep.
Juan waits for Toby at home, to ask what he could do to win Scarlett back. He explains that he wants to reconcile with Scarlett, but she keeps ignoring him. Toby surprises Juan, Scarlett and even himself when he convinces Scarlett to join him on a bike ride, and then rides to Juan’s house. He leaves the two alone to talk things over.
Toby’s father takes Toby fishing and finally opens up a little about their situation. He says he and Toby’s mother want different lives, so separating was for the best. He asks Toby not to blame his mom. He also encourages Toby to go with Cal to gather the rest of Wayne’s effects, including clothes and letters.
After the fishing trip, Toby finds Cal at Gossimer Lake. He apologizes for not attending the funeral. He also tells Cal about the letter he wrote to Wayne. Cal says he should have sent his own brother a letter.
The boys find a way to baptize Zachary without a large crowd gathering. They all go to the lake. They ask Kate to help drive Zachary to Gossimer Lake, Ferris to perform the baptism and another friend to help lift Zachary back out of the water. They sing the hymn “Amazing Grace.” The sheriff is waiting at the trailer to tell Zachary he’s made contact with his guardian, who will be returning for him in a few days.
Among Wayne’s possessions, Cal finds a letter Wayne wrote in response to Toby’s letter. Wayne discourages Cal from playing war this summer, as the war he’s fighting is too real. Zachary’s guardian returns briefly and leaves again, this time with Zachary in tow.