With the French fighting the unwinnable war against communism in Vietnam the establishment of the third force was inevitable. At the time no one really knew who was backing this third force, everyone only knew who was leading it, General Thé. To find out this information Britain sent in reporter Thomas Fowler who was told to write article on the progress of the situation, but got deterred as he met a woman who he fell immediately in love with, named Phuong. The problem with their relationship was that there would be no future for the two, as Fowler had a wife back home who was not going to grant him a divorce. As the situation unfolds an American, Alden Pyle, is sent to Vietnam to work for the Economic Aid Mission, but is really undercover as the movie drops hints towards what he is actually there for.
Fowler at this point is lacking on his amount of articles written and fears he will soon get called back to Britain, so he decides to focus on one article that will win over his superiors to let him stay in Vietnam longer. When Fowler is working on his article, Phuong slowly realizes that she cannot have a future with Fowler and her relationship with Pyle starts to develop. Pyle promises her that she can move back with him in Boston after the situation in Vietnam calms down and he is allowed to return home. This is promising to Phuong because Pyle has no wife or family back home. However, she also does not know who Pyle really is. He never really reveals this to Phuong, but is careless as he leaves multiple clues with Fowler.
The first of the many clues comes when Fowler is doing outside research for his article, but Pyle tags along to do some recon as well. Fowler does not realize what he is doing, only thinking that he is trying to learn more about what Fowler does. The second major clue is when Fowler is trying to get an interview with General Thé but the guards would not let him in.