The Greenwell mystery

When young Ian Campbell disappeared it was suspected that he had sold to a foreign government an invaluable process for the synthetic manufacture of gasoline. Lois Brendon, Ian's fiancee, refused to believe it. Her faith led her to risk her life in his vindication. The matter was put into the hands of Scotland Yard and Chief Inspector MacDonald began operations. Against him were ranged the subtlest conspirators of two foreign powers, and the case led him through as absorbingly tangled an international intrigue as has ever been spun in mystery fiction.