Boston, 1911.Lloyd Osbourne, wrapping up a book tour, is contracted to locate a medieval scimitar and is soon met with apparently insurmountable obstacles. After traveling to England, he meets with the American consul there, who has his own interest in the weapon. Then he is threatened with an odd note, signed, "Humann Assasinn." A woman is brutally kidnapped and he finds himself up against a thousand-year-old Islamic terrorist organization, the Khawarij, who brings the unfortunate woman to their home ground in Zanzibar. Lloyd teams up with a young Harvard historian, Samuel Eliot Morrison, who offers keen insights into the case. An array of real characters come on the scene, including lawyers, occultists, the America's Cup yachtsman Sir Thomas Lipton, and Salme, the Princess of Zanzibar. It was her father, the Sultan of Zanzibar, who had given the scimitar, or the Sayf, as it was known, to the legendary American naval hero, Captain John "Mad Jack" Percival.

Osbourne must draw on his early sword-fighting skills, as well as his deductive reasoning, to defeat the Khawarij, rescue the captive woman, and restore the scimitar to its rightful owner. But first he must establish who that owner might be.