Thirty-eight years ago, Dr. Sarah Halifax earned worldwide fame by deciphering a radio transmission from an unknown intelligence reaching out from deep space. Now 87, Sarah receives news that, at long last, a new signal has been received, and her services are desperately needed once again.

An experiment has gone terribly wrong. Dr. Peter Hobson has created three electronic simulations of his own personality. One will test life after death; another, immortality. The third one is the control unit. But now all three have escaped from Hobson's computer into the worldwide electronic matrix. And one of them is a killer.