At Wingate High School, studious, socially inept Vernon Kinswood is teased by fellow seniors Arthur "Beau" Beaumont and Sanford "Fofo Bidnut" Wilson. After school, the boys race their jalopies to their hangout called the "Shamrock Club," an organization devoted to their interests in speed boats and their quest for a mythical dream girl named Bernardine from Sneaky Falls, Illinois. When Sanford, a clumsy, would-be Casanova, declares that he intends to take a date to see bongo king Jack Costanzo perform at the Black Cat Club, the boys decide to call the information operator once again and ask for the fictional Bernardine's phone number. After young, gullible operator 22 answers their inquiry, Sanford decides to go to the telephone office to ask her for a date. There, he meets operator 22, Jean Cantrick, who simply smiles when the seasoned operators expose the boys's ruse by explaining that Bernardine does not exist. Determined, Sanford invites Jean to join him that night, and then follows her until she accepts his invitation. Triumphant, Sanford phones Beau to tell them he has found Bernardine, and suggests checking her out at the Black Cat Club. After Beau confirms that Jean is indeed, Bernardine, Sanford invites her to watch him race his boat. Sanford's newfound bliss is shattered when his mother receives a letter from school notifying her that her son may fail to graduate unless he passes all his final exams. When Mrs. Wilson considers marrying her pompous suitor, J. Fullerton Weldy, to provide discipline for her son, Sanford pleads with his mother to reconsider and promises to devote himself to his studies for the next two weeks. After Sanford tells Beau that he needs a final "first-class fling" with Jean before sequestering himself with his books, the suave, smooth-talking Beau offers to let him drive his older brother Lee's red Thunderbird. Lee, an Air Force lieutenant stationed in Alaska, arrives home on leave just as Sanford is about to drive off in the Thunderbird and immediately reclaims his car. Relinquishing his dreams of a taking Jean to a fancy nightclub in a sporty car, Sanford settles for going on a picnic in his old jalopy. When Sanford clumsily spills coke on Jean's dress and then tries to maul her, she runs off down the road. Soon after, Lee drives by and offers her a ride. Later that night, Sanford calls to apologize and when Jean readily forgives him, he assumes that the attraction is mutual. To occupy Jean while he is studying for exams, Sanford, unaware that Lee is smitten with Jean, asks Beau to convince his older brother to date her for the next two weeks. Mrs. Wilson arranges for Vernon to tutor Sanford, but one night, fed up with studying, Sanford drops Vernon off at the library while he goes to the clubhouse. When Griner, one of the members, reports that he saw Lee romancing Jean in his Thunderbird, Sanford decides to sell his boat and buy a new car, and Beau tricks the thrifty Vernon into buying it. As exams commence, Beau and the boys escort the frazzled Sanford to school. Sanford passes his tests, but pride in his accomplishment turns to feelings of betrayal when his mother refuses to end her relationship with Weldy. Beau is delighted when Lee is recalled to duty, but his joy turns to dismay when Lee announces his plans to marry Jean before leaving. After Beau breaks the news to Sanford, Sanford drives off in a fury, runs his car off the road and then decides to enlist in the military. When Sanford informs his mother of his decision, she laments that she has failed to understand her son, who has suddenly become an adult. At Christmastime, Sanford comes home for a visit and sullenly refuses to see his old friends. When he enters his room, he finds Beau waiting to surprise him, and after the other boys join in a rendition of the song "Bernardine," Sanford smiles and hugs Beau.