In the 1890's, the fun-loving and irresponsible Jasper Horn distinguishes himself by being the only man in his small town willing to operate a new electric trolley for his boss, Daniel McGowd.  Impressed by Jasper's bravery in the face of threatened mob violence, McGowd invites Jasper and his puritanical wife Jenny to spend a weekend at the McGowd country estate.  There Jasper, a notorious ladies man, works his charm on McGowd's young European wife Norma, who eagerly reciprocates his affections, and nine months later, Roger McGowd is born.

Ten years later, McGowd orders Jasper and his sickly son Andrew to his house and accuses Jasper of being Roger's father.  Fired from his motorman job, Jasper returns to Jenny with plans for a new life elsewhere, but Jenny, always suspicious of her husband, announces that Andrew and she are not going with him.  That night, however, Jasper sneaks out with Andrew and travels to Atlantic City, where he is befriended by Madame Talma, an alcoholic astrologer.  Jobless and nearly broke, Jasper prepares to return home when a dying Madame Talma wills him her astrology business and transforms him into "The Great Jasper."

Soon after, Jenny arrives and, while pleasantly surprised by Andrew's improved health, is shocked to discover Jasper's heathen, "for ladies only" occupation.  Once again rejecting Jasper, Jenny opens a boardwalk hot dog stand and takes Andrew with her.  Many years later, The Great Jasper is visited by Roger, now a playboy band leader, and his still beautiful widowed mother.  When Jenny, who has been softening toward Jasper, sees a drunken Roger and Jasper together, she surmises their relationship and vows to leave Atlantic City.  At the same time, Roger seduces Sylvia Bradfield, Andrew's fiancée, and wires Jasper that he is eloping with her.

A now sickly and repentant Jasper tries unsuccessfully to stop Roger from crippling Andrew's future, then collapses with grief.  After accepting Jenny's forgiveness and blessing, Jasper dies.