Farm boy Jack Cooper is drafted into the minor league Santa Rosa Rockets as a pitcher because of his rocket arm. He being drafted is despite he not having organized baseball experience at any level. He is nicknamed "Deuce" by his fellow players as the two finger signal is the catcher's sign to throw a curve ball, Coop's favorite and most killer pitch. But in the early going, Coop demonstrates that he chokes under pressure as throwing his famous curve ball usually results in an out-of-the-park home run. Kirby, the son of the team owner, purchases a chimpanzee as the team mascot, who the team members name Ed Sullivan. The coach, Chubb, "decides" that Ed will room with Coop, a move not without ulterior motives.
Ed ends up being an overgrown kid in mentality, which makes him a pain for Coop to manage, leading to one misadventure after another for the pair. Ed not only is a fan of baseball, but has a rocket arm of his own. As such, Chubb also decides to use Ed as a player at third base, a move which galvanizes the team on their march toward the pennant. But when something happens to Ed, Coop has to try to save him, his friend and roommate, which may jeopardize his major league prospects. Through it all, Coop begins a romance with his next door neighbor, single mom Lydia, whose precocious adolescent daughter Lizzie, who along with Ed, seems know what's better for Lydia and Coop than they do.