On a balmy summer's day in Oxford an old lady who once helped decipher the Enigma Code is killed. After receiving a cryptic anonymous note containing only the address and the symbol of a circle, Arthur Seldom, a leading mathematician, arrives to find the body. Then follow more murders - an elderly man on a life-support machine is found dead with needle marks in this throat; the percussionist of an orchestra at a concert at Blenheim Palace dies before the audience's very eyes - seemingly unconnected except for notes appearing in the maths department, for the attention of Seldom.