A cloud hangs over Jack as he begins fifth grade. Two boys from his class, Steve and Cliff, who began bullying him last spring, harass him again as soon as school starts. They take brownies and cookies from his lunch and call him "Mustard" because they think he's a coward. When Jack walks a different route to school to avoid them, they find him anyway. When he doesn't take a lunch but brings lunch money, they steal his money. When he rides his bike to school, they let the air out of the tires. Jack is miserable but he's ashamed to ask for help and doesn't want to tell anyone about his problem.

Lizabeth and C.J are Jack's best friends. C.J. has his own problems. He gets help in Chapter I reading, and Steve makes fun of him, calling him "Retardo." Lizabeth is spunky and won't put up with dirty tricks from Steve and Cliff, but they try to get even by calling her Miss Perfect. A new boy in school, Hans Ollig from Germany, speaks only a little English and is trying hard to learn to talk like the other kids. Jack is assigned to show him around the school.

Their fifth grade teacher, Mrs. Hollis, begins a "Melting Pot" unit in social studies. Each student will choose a country and prepare a report about the history of the country and what people who immigrated from that country have contributed to the United States. The student will share a food from the country and teach a few words of the language. C.J. chooses to study Italy, Lizabeth picks France, and the teacher assigns Jack to work with Hans on Switzerland.

Jack's mother arranges for Jack and Hans to meet with a new family in the neighborhood. The boys go to interview Mr. Schwartz who grew up in Switzerland. Jack is surprised to find that many people from Switzerland speak German, so Hans and Mr. Schwartz have lots to say to each other. The boys get a good start on their report even though Steve tries to ruin a map Hans has made. With white-out and a copy machine, the librarian helps save the day, and the boys end up with a great report.

Members of Jack's family are all baseball fans and tremendously excited that the Colorado Rockies are going to be in the World Series. Jack's uncle has given him an old glove from his high school days. When Jack takes the glove to school, it disappears. That same day, Steve's watch disappears and he's very worried because the watch belongs to his father. Steve accuses Hans of stealing the watch, which Hans denies. A search of the school does not turn up the glove or the watch.

While playing at the park, Jack sees that Steve has his glove. Steve refuses to give the glove back and tosses it into the back of a passing garbage truck. Jack and friends pursue the truck and recover the glove. As Jack and his buddies walk home from the park, they see that a police car is in front of Jack Mates's house. It becomes clear that Jack's father has been drinking and has hit both his wife and son. The cause of the fight may be the missing wristwatch which Steve took to school without his father's permission.

When Jack gets home, he tells his parents what has been going on. They want to help but he insists that he must take care of the problem himself. Next morning, Jack confronts Steve and tells him the bullying must end. Jack says he will go with Steve before school starts to Mrs. Hollis where Steve can confess he stole the glove. If not, Jack will tell what happened in front of the entire class. Steve reluctantly confesses. As the class cleans up the room getting ready to share presentations, Steve's watch is found.

Uncle Max comes flying in to visit for Jack's birthday and to take them to the World Series Game. With the bullies faced up to, Jack celebrates his birthday with a new glove from his Uncle Max, and with buddies and baseball.