Irin, Sherlock, and Lupin meet again in London. The three friends discover a strange chess problem in the Times newspaper signed by The Black Monk. It is written in an unknown code, and a peculiar glimmer shines in Sherlock's eyes. The next day, the city is shaken by the news of the murder of a wealthy merchant. A red rose was found in his office: the same flower that, twenty years ago, was the "signature" of a daring group of criminals. Has the Gang of the Purple Rose returned?