Legendary jazz musician Gene Krupa faces highs and lows along the path of his life.

In 1927 Chicago, eighteen-year-old Gene Krupa lugs a set of drums into his parents' apartment, much to their chagrin. Gene's father, who wants his son to become a priest, condemns Gene's "noisy pounding" and childish behavior, then smashes the drums. Defying his father's wishes, Gene joins his friend, Eddie Sirota, in forming a band. Gene's drumming impresses young Ethel Maguire, who dreams of attending the Juilliard School of Music. Upon returning home from a party one night, Gene finds his family huddled in prayer and learns that his father is dying. After his father's death, a remorseful Gene gives up his dreams of drumming and enters the seminary to become a priest. At the end of his first year of training, Gene returns home for summer vacation and Eddie convinces him to play with the band for the summer. Eddie also tells Gene that he has fallen in love with Ethel and intends to marry her. One night, Gene's family pays an unexpected visit to the club in which he is performing. When his mother vehemently voices her disapproval, Gene informs her that he is not retuning to the seminary. Ethel witnesses the confrontation and urges Eddie and Gene to go to New York where there are more opportunities for jazz musicians. The friends all travel to New York together, and three months pass in which Eddie and Gene are unable to find employment. To support them, Ethel takes a job as a switchboard operator. Ethel, who has always admired Gene, confides that she has fallen in love with him, and he reciprocates her feelings. After Gene tells Eddie that he plans to marry Ethel, they are invited to an informal gathering of jazz musicians, including Tommy Dorsey and Red Nichols. When Gene brashly sits in with the band, they are impressed by his drumming. Later, Red hires Gene to perform in the pit band of a Broadway show. As Gene revels in his newly attained glamour and acclaim, he drifts apart from Ethel, who is busy with her work and classes. On the night of Ethel's birthday, Gene is seduced by temperamental songstress Dorissa Dinell, and as a result, misses Ethel's birthday dinner. To make it up to Ethel, Gene invites her to see him perform at a nightclub. When Dorissa arrives accompanied by a famous columnist and a powerful agent, however, Gene eagerly accepts their invitation to join them and asks Ethel to leave. Feeling neglected, Ethel decides to return to Chicago to see her ailing mother. In her absence, Gene becomes obsessed with being a star and rents a lavish, extravagant apartment to solidify his status. On the night of Ethel's return, Gene is too preoccupied with a party to meet her at the train station, and so sends Eddie instead. Ethel arrives to find Gene surrounded by adoring women, and when Dorissa makes a grand entrance, Ethel leaves in disgust. Gene follows Ethel to her hotel, where she informs him that she is ending their relationship. When a despondent Gene returns to the party alone, the predatory Dorissa offers him a marijuana cigarette to cheer him up. Soon Gene, drunk with fame, marijuana and alcohol, alienates Eddie, who leaves his longtime friend. After starting his own band, Gene embarks on a nationwide tour that ends in San Francisco, where he is arrested for possession of marijuana. Although Gene protests that the drugs were planted on him, Dorissa refuses to testify on his behalf, and as a result, he is sentenced to ninety days in jail. Upon returning to New York, Gene discovers that he has been blacklisted as a drug addict. Unable to find work, Gene takes a series of degrading out-of-town jobs, performing in dives and strip clubs, losing his confidence along the way. In Philadelphia, Ethel comes to see Gene and, after telling him that Tommy Dorsey is forming a band, encourages him to apply for a job. When Gene self-pityingly replies that Tommy would never hire him because he is unable to read music, Ethel encourages him to go to school and learn how. Some time later, a humbled Gene comes to see Eddie and make amends. Now able to read music, Gene asks Dorsey for a job. When Dorsey replies that he has already hired Davey Tough to be his drummer, Eddie suggests hiring Gene as a second drummer. At his comeback concert, Gene is heckled by the audience. Davey backs up the shaken Gene, who rallies and wins a standing ovation from the audience. At the end of the concert, the musicians invite Gene to a party, but he turns down their invitation to be with Ethel.