Henry Rowengartner (Nicholas), a 12-year-old Little Leaguer, has dreams of playing in the major leagues. One day, Henry breaks his arm trying to catch a fly ball and has to wrap it in a cast. Once the arm is healed the doctor removes the cast and discovers Henry's tendons have healed "a little too tight", thus enabling Henry to cock his arm back and fire it forward with incredible force. In celebration of the cast's removal, Henry's mom presents him and two friends with tickets to the day's Chicago Cubs game. At the game, Henry and his friends catch a home run hit by the visiting team. In keeping with Wrigley Field tradition, they decide to throw the ball back onto the field. Henry takes the ball and, because of the way the tendons healed, launches it from the outfield bleachers all the way to the catcher standing at home plate (which is, as according to the radio announcer (John Candy), about 435 feet). The crowd, players on the field, and even the members in the owners box stare in astonishment at the throw.
The Cubs, in need of a "miracle" to save the poor-performing team, sign Henry to a contract, making him the youngest-ever major leaguer. Henry's mother's boyfriend, Jack (Bruce Altman), whom Henry dislikes, signs on as Henry's manager. When Henry first enters the locker room he is awestruck to be in the presence of some of his heroes, including his idol Chet "Rocket" Steadman (Gary Busey), an irascible pitcher in the twilight of his career.