The Purple Cipher.

Jeanne Baldwin, Leonard Staunton, Alan Fitzhugh and Jack Baldwin are guided on a tour of underground dens in Chinatown by Hop Lee. During their trip, Wang Foo abducts Jeanne, and she and her companions are rescued through the intervention of private detective Frank Condon. Three months later, Fitzhugh is missing, supposedly murdered by the Chinese. Jack Baldwin is threatened, poisoned and his body disappears. Next, a threat against Jeanne is received by Staunton. To save her life, he is to meet the blackmailers at a bay in the harbor. Enlisting the aid of a submarine, Staunton captures the three conspirators, who turn out to be the missing victims in the case, led by Condon. The three had planned the escapade to extort money from Staunton. The mystery thus resolved, Staunton and Jeanne are married. A lost film.