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. Why is he being targeted as the recipient of these coded messages? All he can conjecture is that it might relate to his latest book, an unexpected bestseller about serial killers and the parallels between investigations into their crimes and certain mathematical theorems.
It is left to Seldom and a postgraduate mathematics student to work out the key to the series of symbols before the killer strikes again.