Plot

Plot

After the Cuban Missile Crisis, the Soviet Union planted a number of long-term, deep-cover sleeper agents all over the United States, spies so thoroughly brainwashed that even they didn't know they were agents; they could only be activated by a special code phrase (a line from Robert Frost's poem "Stopping by Woods on a Snowy Evening" followed by their real given names). Their mission was to sabotage crucial parts of the civil and military infrastructure as a precursor to a possible US/USSR active conflict or war.
Over twenty years pass, and the Cold War gradually gives way to détente. Nikolai Dalchimsky (Donald Pleasence), a rogue KGB officer, defects to America, taking with him the Telefon Book, which contains the names, addresses and telephone numbers of all the agents. He starts activating them one by one. American counterintelligence is thrown into confusion when seemingly-ordinary citizens start blowing up what are, in some cases, outdated targets consisting of formerly top secret facilities that were declassified or abandoned years prior; the agents commit suicide right afterwards.
The KGB dares not tell its political leaders, much less the Americans, about its negligence in not deactivating the spy network. Major Grigori Bortsov (Charles Bronson), who is selected for his photographic memory, memorizes the contents of the only other copy of the Telefon Book and is sent to find and stop Nikolai Dalchimsky quickly and quietly, before the politicians on either side learn what is going on and either embarrass the KGB or start a full scale nuclear war. He is given the assistance of only a single agent planted in America, Barbara (Lee Remick).
Eventually, Bortsov discovers the method behind Dalchimsky's madness: he chooses the agents by the first letters of their hometowns, "writing" his own name in sabotage across America. Using this information, Bortsov is finally able to track Dalchimsky down and kill him.
However, there are a number of twists. Barbara has orders from the KGB to assassinate Bortsov once he succeeds in his mission, in order to get rid of a dangerous loose end. She is a double agent, but when she informs her American superiors, they also tell her to kill Bortsov; they want her to gain the confidence of the KGB. However, she has fallen in love with her would-be target. She informs Bortsov, and together they blackmail both sides into leaving them alone, holding the threat of the remaining Telefon agents over their heads.