A mysterious evildoer terrifies the French Riviera. That mysterious character, which is hidden under the personality of James Pearce, a diamond-maker, calls himself The Black X. This time, the Black X has noticed the rich jewels of a celebrated singer and, in the company of her henchmen, is preparing to carry out the great robbery.
The innkeeper's daughter is in love, but her mother has already decided that she is going to be married to another man.