It was the summer of 1984 when the then Prime Minister Margaret Thatcher declared to a gathering of the faithful that "We had to fight an enemy without in the Falklands," and that now they were to tackle "the enemy within, which is much more difficult to fight, and more dangerous to liberty."

The Enemy Within is not a book about the 1984-5 Miners' Strike. Guardian journalist Milne's task here is altogether darker than that and begins in earnest many years after the end of the dispute, when the dire predictions of NUM leader Arthur Scargill had all but come to pass. On 5 March 1990 the Daily Mirror launched a broadside against the leaders of the union, backed by that evening's Cook Report on ITV, charging that not only had monies been received during the year long dispute from Colonel Gaddafi scant months after a British policewoman had been shot in London, but that the cash had subsequently been used not to ease the hardship of strikers but to pay off the mortgages of officials.

The story was nonsense. Much of the first section of Milne's book exhaustively seeks the truth of the Libyan money -- and that sent by Soviet miners -- and the debts of the alleged embezzlers. (Scargill and deputy Peter Heathfield didn't even have mortgages). It takes a long time to disprove the lies, certainly more than the meagre newspages it took to establish them, but it is important that Milne present the what if he is ever to reach the why and, crucially, the who.

Towards the end of 1984, later admitted by Government ministers and even Thatcher herself, the NUM were close to winning. And this, of course, would hardly have been the first time. Heath had been brought down in the seventies by similar means, and this Prime Minister was not about to let something similar happen. This was revenge as much as war.

The meat of Milne's book is built on tracing the methods by which the machinery of the secret state was brought into play to help defeat a legitimate political action. Stella Rimington, now head of MI5, cut her teeth heading up the section directly responsible for policing the dispute. Phones were tapped, GCHQ deployed, buildings bugged, bundles of supposed Libyan cash faked; the full Cold War handbook. But the heart of Milne's story is darker still -- that the man who largely substantiated the allegations so lauded by the Mirror was in fact an MI5 plant. Roger Windsor, living in France and refusing to talk to Milne, was at the time the highest ranking non-elected official in the NUM and yet is revealed as a double-dealing security service agent positioned to destabilise the dispute, and, after, act as chief witness in the prosecution of Arthur Scargill.

The sheer size and complexity (admirably handled) of this tale takes some following as Milne journeys far from the homes of miners striking to safeguard their futures and across the world to Russia and even inside the tent of Colonel Gaddafi himself; a tale noticeably absent from the same front pages that were so eager to carry the original allegations. But it is also a story, cutting as it does deep into the very fabric of concepts like freedom, democracy and accountability, that needs telling again and again. What it leaves untold, of course, is what is left, that which even the tenacious Milne has yet to uncover. Be afraid. Be very afraid.