In America the biggest spy scandal for decades continues to unravel: already it threatens the security of Britain and the rest of NATO. Three members of the Walker family, and a friend, are accused of betraying to the Russians some of NATO's most sensitive and closely guarded secrets. For 20 years they had access to the vital details of submarine warfare - codes, communications and tactics - central to the nuclear deployments of the West. Tom Mangold reports on the gravity of the revelations for the USA and for Britain, and talks to some of the people involved in the case of the Family of Spies.