When a group of men and women throughout London begin exhibiting strangge psychotic behavior, Dr. John Seward, from the Dracula affair, is brought in on the case. Faced with a mystery beyond his realm of experiences, he turns to Sherlock HOlmes' trusted companion, Dr. Watson, for help. Together these two men of science must uncover the riddle behind a white powder drug with the ability to drive people mad before the entire population is infected...