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.
基克尔（约翰·巴里摩尔 John Barrymore 饰）是一名知识非常渊博的科学家，他将毕生的经历都投注到了探索人性的善恶奥妙之中，并且最终制造出了一种可怕的药剂，这种药剂能够令基克尔化身成为海德先生，基克尔将自己所有的恶意和坏念头都投射到了海德的大脑里。白天，基克尔行医救人乐善好施，待到夜幕降临，海德先生便会现身，杀人害命，无恶不作。 基克尔有一位名叫伊芙（妮塔·纳尔迪 Nita Naldi 饰）的未婚妻，两人之间的感情非常的要好，然而，海德先生的存在让基克尔不自觉的被名叫碧翠丝的放荡妓女所吸引。分裂的人格让基克尔感到万分的痛苦，他想要消灭海德先生。
