Sherlock Holmes investigates the murder of two Americans whose deaths have some mysterious connection to sinister Socialist groups gathering power in both Britain and America.
The novel is split into two quite separate halves. The first is titled "Being a Reprint from the Reminiscences of John Watson, M.D., Late of the Army Medical Department". This part is told in first person by Sherlock Holmes' friend Dr Watson, and describes his introduction in 1881 to Holmes through a mutual friend and the first mystery in which he followed Holmes' investigations. The mystery revolves around a corpse found at a derelict house in Brixton, London with the word "RACHE" scrawled in blood on the wall beside the body.