Nayland Smith in his search for the missing formula of six fifty-four, a remedy for a strange epidemic effected by Dr. Petrie who has himself fallen a victim returns from London and learns from Paul Sterling of Fa Lo Swee's attempt to visit Petrie in the guise of his wife. He sends Sterling and Weymouth to the Villa Jasmine, Petrie's quarters where both are overcome by fumes and when Smith arrives he discovers Sterling gone. Smith and Weymouth pursuing a clue to the mystery learn that Petrie has died. Smith returning to the hospital in a cab is overcome by gas. He retains consciousness in a vast underground laboratory  

where he finds Sterling. Shortly after a panel in the glass wall opens  and on the threshold stands Dr. Fu Manchu...