Fourth and last collection of detective fiction featuring Martin Hewitt, a famous private detective whose methods closely resemble those of Sherlock Holmes. The plot lines of all six linked sensation stories in this collection center on the mystery of the Red Triangle, a group of villains known only from the Red Triangle left stamped on the heads of their victims, and the actions of Martin Hewitt and his narrator, esteemed journalist Mr. Brett, in bringing the members of that group to justice. This work includes the following stories: „The affair of Samuel's diamonds", „The case of Mr. Jacob Mason", „The case of the Lever Key", „The case of the burnt barn", „The case of the Admiralty code" and „The adventure of Channel Marsh".