The twenty-ninth installment in the Magic Tree House series, a children's fantasy adventure series in which siblings Jack and Annie travel through time using a magical tree house, opens on a Saturday morning in Frog Creek, Pennsylvania. Jack sits on his front porch, dreading baseball tryouts at the local rec center. At last season's tryouts, he fell while swinging the bat and everyone laughed. His sister Annie joins him and admits she embarrassed herself, too, by throwing the ball to the wrong team. Despite their family's shared love of baseball, both children are ready to skip tryouts altogether.
Before they can decide, a baseball drops mysteriously from the sky and rolls across their yard. Annie suspects it is a sign from Morgan le Fay, a magical librarian from the legendary realm of Camelot who owns the tree house and periodically sends the children on missions. They race into the Frog Creek woods and find the magic tree house waiting for them. Inside, they discover two gray baseball caps, a book titled
History of Baseball open to a photograph of Ebbets Field, a baseball stadium in Brooklyn, New York, dated April 15, 1947, and a red leather bookmark bearing a rhyming message from Morgan. The message instructs them to travel to Ebbets Field, explains that the magic caps will give them skill and make others see them as the roles they pretend to fill, and directs them to give "the ball with the name" to "the one who knows best / The rules of the game" (9) after they hear the final score. Jack examines the baseball but finds no name on it. Annie names it "Fly Ball." Jack points to the photograph and wishes them to Brooklyn, and the tree house spins them away.
They arrive in 1947 Brooklyn wearing magically provided baseball uniforms with the word "batboy" stitched on the jerseys. Annie notes that her cap's magic will make everyone see her as a boy, since batgirls did not exist in 1947. When they put the caps on, both children feel a surge of confidence and baseball knowledge. They land in a city park and ask directions from children playing marbles. Two of the smallest children, six-year-old twins named Olive and Otis, chase after them and offer a shortcut to the stadium. Along the way, Jack spots a newspaper headline announcing Opening Day: the Brooklyn Dodgers versus the Boston Braves. He realizes they will be working at a major-league game.
The twins lead them to Ebbets Field's entrance, where a security guard recognizes them as the late batboys and rushes them inside. Annie asks if the twins can come in for free, but the guard refuses. Olive and Otis explain they always watch from Bedford Avenue by peeking under a gate. Inside the ballpark, Annie points out a Black Dodgers player signing autographs and drawing attention from reporters and photographers. Jack does not recognize him but guesses he must be a star.
The clubhouse manager arrives and puts them to work preparing for the visiting Boston Braves. Guided by the knowledge their magic caps provide, Jack and Annie hang uniforms, clean cleats and helmets, haul bats to the dugout, and stock it with supplies. They finish with five minutes to spare. When the national anthem begins and players remove their caps, Jack and Annie retreat to a shadowy corner of the dugout, keeping their magic caps on to avoid being exposed. Once the game starts, Annie takes charge of collecting bats dropped by players at home plate, while Jack manages the baseballs, retrieving foul balls from his stool near the diamond. The wind gusts repeatedly, and Jack grips his cap each time to keep it secure.
During a Dodgers batting turn, Jack is stationed in center field. He hears increasingly hostile shouts from the stands and realizes the crowd is directing racist abuse at the Black Dodgers player at bat, the same man Annie noticed earlier and the only Black player in the game. The player ignores the jeers, holds his head high, and walks calmly back to the dugout after being called out. Jack wants to say something kind to him but cannot.
Moments later, a foul ball rolls into the outfield. As Jack bends to grab it, the wind blows his magic cap off. A kid who jumped the wall snatches the cap and disappears. Without the cap, Jack's skill vanishes; his throw nearly hits the first-base umpire. Jack flees to the clubhouse, where he finds Annie filling water jugs. She places her own cap on his head just before the security guard and clubhouse manager enter looking for a trespassing child. Jack, calm under Annie's cap, denies seeing anyone. But the manager spots Annie without her cap and identifies her as a girl. Jack tosses the cap back to Annie, briefly restoring her disguise, but the guard snatches it away. Confused and angry, both men march Jack and Annie out of the stadium and order them never to return.
Outside, Jack feels defeated, believing Morgan's plan has failed. Annie reminds him they still need to hear the final score and give away Fly Ball. They round the corner and bump into Olive and Otis, who had been watching from under the gate and witnessed the transformations. The twins offer to take them to hear the rest of the game and lead them through neighborhood alleys to the Granny House, a small brick home where the twins' two grandmothers, 101-year-old great-great-grandmother, and two great-aunts all live together.
Inside the kitchen, four women sit around a large radio, intensely following the game. They cheer for "Jackie," player number 42. Jack listens as the announcer calls plays. A double play, in which a single play records two outs, disappoints the women. Later, a bunt, a soft tap of the ball along the baseline, advances runners. In the bottom of the seventh inning, with the Braves leading 3 to 2, number 42 bunts, advances to second base, and scores on a hit by his teammate. The women burst into tears of joy. Jack finally realizes that number 42 is Jackie Robinson, the first African American to play major-league baseball, and understands why this is a big day for baseball. One of the women tells Jack and Annie they are the first white people ever to visit the home and welcomes them warmly. The final score is announced: Dodgers 5, Braves 3. Jackie Robinson has finished his first major-league game.
With the final score heard, Jack knows it is time to give away Fly Ball. He offers the baseball to the twins, but they decline. The twins lead Jack and Annie into the room of Granny Beck, their great-great-grandmother, who was born into slavery. Granny Beck asks whether the crowd cheered for Jackie Robinson. Jack tells her honestly that most did, but some yelled mean things. She asks what Jackie did, and Annie describes how he held his head high and walked back to the dugout without a word. Granny Beck observes that Robinson kept his dignity and "walked toward the good" (66), advising the children to always keep their eyes on the good and maintain their self-respect. Writing magically appears on the baseball, spelling out "Jackie Robinson." Annie places the autographed ball in Granny Beck's hands, fulfilling Morgan's instruction to give the ball with the name to the one who knows best the rules of the game. The "rules" turn out to be not just baseball rules but rules of dignity and perseverance.
Jack and Annie say goodbye, return to the park, and climb the rope ladder to the tree house. Annie wishes them home, and the tree house spins them back to Frog Creek. In the tree house, Jack reads aloud from the baseball history book about Jackie Robinson: He was named Rookie of the Year in 1947, became a World Series champion in 1955, and helped inspire the civil rights movement. Jack acknowledges that Morgan did not send them to Brooklyn to become great baseball players. Annie articulates the real lesson: Morgan sent them to learn how to be brave and keep going. Jack agrees, calling those "the rules of the game." They discuss backup plans if they do not make the team, embracing the idea that participation matters more than perfection. Then Jack and Annie run home through the sunlight, ready to try out.