When Granite Grant arrives at detective Blake Sexton's door on Baker Street, he has every intention of delivering news of the Black Quorum (the most powerful criminal organization of the century) and it's leader, the Hooded Terror. But he is garroted before Sexton's housekeeper, Mrs. Bardell, has time to answer the door. A clue on the body reveals the identity of the Hooded Terror to Sexton, who begins to pursue the Black Quorum, with the help of his faithful assistant Tinker and Mademoiselle Galley, a French secret service agent.