An unknown thief steals an important naval treaty from the office of Mr. Percy Phelps, a Foreign Office clerk. Phelps's distress gives him a brain fever that lasts for nine weeks. As he begins to recover, he writes to his old schoolmate, Dr. Watson, begging him to invite Sherlock Holmes to investigate the matter and find the treaty. Otherwise, his honor and happiness are gone forever. Holmes takes up the case with zest, but gives Phelps no false hopes. The mystery is as dark and tangled as any he has known.