A new radio-telescope picks up from the constellation of Andromeda, two hundred light-years away, a complex series of signals which prove to be a program for a giant computer. Someone in outer space is trying to communicate, using a supremely clever yet entirely logical method.

When the necessary computer is built and begins to relay the information it receives from Andromeda, the project assumes a vital importance: politically, militarily and commercially. For scientists find themselves possessing knowledge previously unknown to man, knowledge of such a nature that the security of human life itself is threatened.