Larry "Coop" Cooper works as a peanut vendor at Whacker Stadium, home of the Bisons, a baseball team, on which Coop used to play. Coop, a widower, is also a devoted father to his nine-year-old son Christy, to whom he has imparted his vast knowledge of baseball. The Bisons are the worst team in the league, much to the dismay of their owner, Fred F. Whacker, and Coop blames manager Billy Lorant for mishandling the players, such as third baseman Pete Haines and outfielder Bobo Noonan. One day, the fiery Coop argues with concessions manager J. R. Johnson and loses his job. After Coop gets drunk by trading his baseball glove for liquor, Christy decides to get his job back and sneaks into the stadium to confront Johnson. The unforgiving Johnson chases Christy away, and while he is running, he meets secretary Marion Foley. When Christy confides in Marion that he wants to be a batboy, she presents him to Whacker, who is so impressed by his baseball knowledge that he hires him and orders Johnson to take Coop back. Lorant is displeased with the undersized batboy, but Pete, who is Marion's fiancé, helps him adjust to his new job. Frustrated by Pete's batting slump, Coop gives Christy advice to pass along to him, and in the next game, Pete gets three hits. Pete is thrilled by his success, but tells Marion that he will quit the team at the end of the season to accept a steady job. Later, Christy gives Pete more of Coop's advice, and his improved batting helps the Bisons to win, which stuns Whacker. Pete's hitting streak prompts him to turn down the other job, and Marion reproaches him for basing their future on what a child tells him. Later, Coop says goodbye to Christy as the boy embarks on a team road trip, and urges him not to reveal who is behind the advice Christy has been giving to Pete. One night, Christy goes over Coop's notes with rookie Johnny Grant, and soon his performance improves dramatically. Lorant takes credit for the Bisons' successes when Whacker questions him, while the players, learning that Christy has been helping Pete and Johnny, all begin seeking his help. When the team returns home, Coop congratulates Christy on his work, although soon after, Lorant deduces that Coop is behind Christy's helpfulness. The spiteful Lorant tells Christy that his father, rather than being the star player he claims, was a tempermental loudmouth who ended his career in the minor leagues. Lorant fires Christy, but when the Bisons start losing without him, they appeal to Whacker to reinstate the boy. Whacker offers Christy the job of managing the team, prompting Lorant to quit when Whacker asks him to "front" for Christy. Marion convinces Whacker that having a nine-year-old manager would be a sensational publicity stunt, and soon the sports world is focused on the boy manager. The jeers of the other players are quashed by the Bisons' improved play, and Christy realizes that Bobo's poor catching is due to myopia. The Bisons' winning streak soon has them in position to enter the playoffs, although Pete's depression over the end of his relationship with Marion has resulted in a batting slump. Marion tells Christy that, at thirty-five, Pete is getting too old to play, and asks him to help Pete to become more realistic about his future. The Bisons' progress is briefly interrupted when Christy is arrested by a truant officer, but his devotion to the game persuades a judge to allow him to return to his job. Soon the Bisons are in the running for the pennant, and Pete realizes that his slowness could prevent the team from reaching the World Series. Pete removes himself from the lineup, although Christy persuades him to serve as a coach for the rest of the season, and Marion gratefully embraces Pete when he gives her the news. During the next game, Christy falls ill and the doctor diagnoses him with pneumonia. With Christy hospitalized, player Hunchy Harrison takes over as manager, but the Bisons lose their next three games. Desperate to help, Christy reveals to the players that Coop has been their real leader all along, and they unanimously vote to make him manager. Despite his fear of failure, Christy accepts the job, and during a crucial game with the Yankees, his unusual strategy enables the Bisons to win the pennant. With his self-confidence restored, Coop celebrates Christy's recovery by making him the third base coach, while Pete becomes the first base coach. The Bisons then begin the World Series against the Brooklyn Dodgers, and Marion and Whacker watch with delight as Coop and Christy exchange smiles.
