The hitherto untold story of the peacetime equivalent of ULTRA, an extraordinary cryptographic effort conducted in conditions of unprecedented secrecy over three decades which gave Western counter-intelligence experts a fascinating glimpse into how the KGB and GRU recruited and ran moles.

The US National Security Agency and its counterpart in Cheltenham, GCHQ, spent 37 years analysing cipher messages exchanged between Moscow, London and New York, before abandoning the project in 1977. In 1996 the White House authorised the release of the complete VENONA collection, which proves that the FBI and CIA had accumulated compelling evidence against the Rosenbergs, Alger Hiss, Klaus Fuchs, Donald Maclean and other Soviet spies. To protect the source, VENONA was never mentioned in any trial.

Study of the VENONA material reveals that there are nearly 300 unidentified Soviet agents in Britain and America, plus a smaller number of people (like the nuclear physicist Dr Theodore Hall) whose activities are described in compromising detail.

Nigel West's book is based on the only complete set of decrypts held in Britain outside Whitehall, supplemented by interviews with most of the principal players in the VENONA drama (NSA, GCHQ, FBI and MI5 officers).