A destitute Marthe Preston is in dire straits in Paris until gambler Richard Dorval comes to her aid. In gratitude, she agrees to a scheme of Dorval's to seduce and wed his rival, "Diamond Johnny" Silk, then help ruin Johnny's horse-racing business interests.
Marthe's inside information enables Dorval and an accomplice, bookie Joe Farley, to bribe Johnny's jockeys to deliberately lose races or to help them influence the odds. Johnny learns the truth and demands she leave. Martha has fallen in love with her husband, however, so she pretends to go along with a plot to poison Johnny's horse, double-crossing Dorval and rejoicing in Johnny's triumph.