The enemy has received information listing the 4 cities where the U.S. missile control systems are located that would respond in the event of an enemy nuclear attack. This is a disasterous development because with the correct information the enemy will be able to take out those four cities with a first strike, and we will then be unable to retaliate.
The plan is to convince the enemy that they have received false information. Jim poses as an enemy contact, but his credentials are not perfect. The enemy notices, and concludes that he must be an American agent. Eventually, Jim provides information confirming the information.
On a second front, the enemy have sent Toland, an independent operator who has had substantial success, particularly in getting information from women, killing them after he has the information. He is going to try to verify the information. The IMF put him in contact with Mrs. Buchanan (Lee Grant in a special guest role), wife of a high official. He gets involved in a blackmail scheme led by Rollin and forces Mrs. Buchanan to turn over her husband's information to him -- a verification supplied by the IMF. After he has the information, he forces Mrs. Buchanan to take an overdose of sleeping pills. He leaves as she is dying, and the IMF and their doctor arrive just in time.
The plot all comes together in the embassy. The enemy having received verification from a known American agent conclude that the verification must therefore be false. Toland also verifies the information, and the enemy thus concludes that he must have sold out to the Americans. He is killed.
There is a major flaw in the plot. The idea is that the Americans have conspired to deliver to the enemy an incorrect list of the 4 cities where the missile defense control systems are located. Why would the U.S. ever consider doing that? To convince the enemy to launch an early strike on those 4 cities, after which we could then retaliate. If the enemy gave the matter even a minute's thought, they would have determined that the information was, in essence, self authenticating, as the Americans would not have faked it under any circumstances. In any event, they would certainly have looked very carefully at any thought that the information was not accurate.