Johnny Casar (Mickey Rooney) runs away from a home for wayward boys, tired of being teased about being short and a poor athlete. He soon finds a pair of roller skates and is befriended by Bruno Crystal (Ralph Dumke), who allows him to wash dishes at his café, while a priest who runs the home, Father O'Hara (Pat O'Brien), secretly keeps an eye on him.
　　A traveling roller-skating team takes an interest in Johnny after he shows some aptitude. He clashes with Mack Miller (Glen Corbett), a cocky champion, and falls for Mary Reeves (Beverly Tyler), another top skater. Johnny ends up featured in grudge matches against Miller, where they take turns one-upping one another.
　　As his fame grows, Johnny becomes every bit as arrogant as Miller and more. Life takes a bad turn when he is diagnosed with polio. A long period of physical therapy follows, until a wheelchair-bound Johnny tries to get his life back on track.

　　基克爾（約翰·巴里摩爾 John Barrymore 飾）是一名知識非常淵博的科學家，他將畢生的經(jīng)歷都投注到了探索人性的善惡奧妙之中，并且最終制造出了一種可怕的藥劑，這種藥劑能夠令基克爾化身成為海德先生，基克爾將自己所有的惡意和壞念頭都投射到了海德的大腦里。白天，基克爾行醫(yī)救人樂善好施，待到夜幕降臨，海德先生便會現(xiàn)身，殺人害命，無惡不作。
　　基克爾有一位名叫伊芙（妮塔·納爾迪 Nita Naldi 飾）的未婚妻，兩人之間的感情非常的要好，然而，海德先生的存在讓基克爾不自覺的被名叫碧翠絲的放蕩妓女所吸引。分裂的人格讓基克爾感到萬分的痛苦，他想要消滅海德先生。