In 1928, Jim Horst, a scout for the St. Louis Cardinals baseball team, comes to a small town in the Ozarks to watch pitcher Jerome Herman Dean. Dean, whose abundance of self-confidence verges on egotism, is sure that Jim wants him to start for the "Cards" immediately, and is surprised by Jim's insistence that he first play for the Houston Buffaloes, a team in the minor Texas league. Although he quickly demonstrates his extraordinary talents, Dean is teased about his country clothes and so goes to a large department store to buy new suits. There, Dean meets pretty credit officer Patricia Nash and courts her with great vigor. One day, Dean's beloved brother Paul, who is also a pitcher, attends an exhibition game between Houston and the Chicago White Sox. Although Dean is dismayed to see Pat there with another beau, he pitches an almost perfect game, allowing only one hit. During the game, the White Sox players taunt Dean by calling him "Dizzy," but the jovial Dean quickly adopts the nickname. Later that night, Dean asks Pat to elope, and although she is stunned by his proposal, she agrees to marry him the next day. Dean attempts to arrange a publicity stunt whereby he and Pat will marry at home plate during a game, but the no-nonsense Pat insists on a quiet ceremony. As the months pass, Dean, who is now called Dizzy by the press, becomes the Buffaloes' star pitcher and is told to report to the Cardinals in the spring. Dean is delighted, especially after Paul is awarded a spot with the Buffaloes. During his first major league game, Dean is nervous, but his talent and self-confidence prevail and help lead the team to victory. The sports reporters press Dean for anecdotes about his life, and he obliges by giving each of them an "exclusive," for which he reels off a different birthdate and place. The next spring, Paul, who is nicknamed "Daffy" by the press, joins Dean in St. Louis, and the irrepressible Dean brothers promote the team by acting as ushers, selling tickets in the box office and even cavorting with the marching band. Their antics get them into trouble, however, when they skip several games to go fishing and are each fined $100 by the team's manager, Frankie Frisch. Infuriated by the fine, Dean goes on strike and the obliging Paul follows, although Pat urges her husband to stop being so stubborn. After Pat's reprimand, Dean storms out of their apartment, and on the street, meets Johnny Kendall, a young, handicapped businessman who relies on crutches and a specially equipped car for transport. Dean helps Johnny with his errands, and Johnny's admiration of him and quiet acceptance of his handicap prompt Dean to end the strike. Dean and Paul then lead the Cardinals to the World Series, which they win. Later, during the next year's race for the pennant, Paul is injured by a line drive and is forced to retire. Dean is stunned by his brother's decision, as his entire life revolves around baseball, but soon faces a devastating injury himself when a line drive breaks one of his toes. Fretting during his enforced layoff, Dean returns to pitching too quickly, and during his first game back, is warned that he is risking serious injury to his pitching arm if he plays while he is still off balance due to the pain in his foot. The warning is borne out when a doctor tells Dean that bursitis and muscle strain have affected his arm, and soon the despondent Dean is sold by the Cardinals to the Chicago Cubs. Although he is no longer able to throw a fastball, Dean works hard to help the Cubs win the pennant. His arm strength continues to decline, however, and eventually Dean finds himself back in the Texas minor leagues. After a few months, even the Texas team lets Dean go, although he refuses to accept that his baseball career is over. Depressed and angry, Dean tries to forget his troubles by drinking and gambling and winds up losing large sums of money. Despite her love for her husband, Pat decides to leave him, and tells him that when he "grows up," she will return. Dean is devastated and asks Johnny, now a successful executive at his father's brewing company, for a job as a salesman. Johnny readily agrees yet arranges for his baseball-loving father to listen in as Dean comments on a game being broadcast over the radio. Johnny and Kendall, Sr. make Dean a baseball commentator on their radio station, and Dean becomes a great success, despite his thick Arkansas accent and often twisted English. Pat hears one of Dean's broadcasts and, bursting with pride, prepares to return home. Meanwhile, in St. Louis, an irate group of teachers oppose Dean, saying that his poor English is a bad influence on children. Dean is devastated by the charge and after much deliberation, decides to quit. During his final broadcast, Dean gives the children of St. Louis heartfelt instructions to pursue their education, then returns home, where Pat is waiting for him. After embracing his wife, Dean receives a call from Johnny, who refuses to accept his resignation, and then another call from Mrs. Martin, the head of the teacher's committee. Mrs. Martin admits that Dean's speech deeply moved the committee and tells him: "We'll keep teaching the children English and you keep on learning them baseball." Before Pat can again embrace her husband, however, a group of neighborhood kids come to the door and ask her if he can come out to play baseball.