A demonstration by unilateral nuclear disarmament protesters from the Campaign for Nuclear Disarmament (CND) in London is interrupted when one of the protesters is violently killed. British security forces learn that a terrorist group attached to CND has been planning a significant act of terrorism for the near future. The person killed during the protest demonstration was an undercover intelligence officer who had infiltrated the terrorists. The Commanding Officer of the Special Air Service, Colonel Hadley, suggests a new line of inquiry for the investigation.