Two mathematicians must join forces to stop a serial killer in this spellbinding international bestseller It begins on a summer day in Oxford, when a young Argentine graduate student finds his landlady-an elderly woman who helped crack the Enigma Code during World War II -murdered in cold blood. Meanwhile, a renowned Oxford logician receives an anonymous note bearing a circle and the words "the first of a series." As the murders begin to pile up and more symbols are revealed, it is up to this unlikely pair to decipher the pattern before the killer strikes again.