A subplot concerns a love triangle between Fowler, an American CIA agent named Alden Pyle, and Phuong, a young Vietnamese woman.

The novel implicitly questions the foundations of growing American involvement in Vietnam in the 1950s, exploring the subject through links among its three main characters: Fowler, Pyle and Phuong.

The novel has received much attention due to its prediction of the outcome of the Vietnam War and subsequent American foreign policy since the 1950s.

He meets a young American idealist named Alden Pyle, a CIA agent working undercover.

Pyle lives his life and forms his opinions based on foreign policy books written by York Harding with no real experience in Southeast Asia matters.

When they first meet, the earnest Pyle asks Fowler to help him understand more about the country, but the older man's cynical realism does not sink in.

Fowler has a live-in lover, Phuong, who is only 20 years old and was previously a dancer at The Arc-en-Ciel (Rainbow) on Jaccareo Road, in Cholon.

Pyle had allied himself with General Thé, a renegade commander he was grooming to lead the "Third Force" described in Harding's book.

His father is a renowned professor of underwater erosion whose picture has appeared on the cover of Time magazine; his mother is well respected in their community.

Harding's theory is that neither Communism nor colonialism is the answer in foreign lands like Vietnam, but rather a "Third Force", usually a combination of traditions, works best.

US military counter-insurgency expert Edward Lansdale, who was stationed in Vietnam 1953–1957, is sometimes cited as a model for Pyle's character.

According to Greene, the inspiration for the character of Pyle was Leo Hochstetter, an American serving as public affairs director for the Economic Aid Mission in Indochina who was assumed by the French to "belong to the CIA", and lectured him on the "long drive back to Saigon on the necessity of finding a 'third force in Vietnam.

'”[2] Phuong, Fowler's lover at the beginning of the novel, is a beautiful young Vietnamese woman who stays with him for security and protection, and leaves him for the same reason.

He is a man anguished between doing his duty (pursuing Pyle's death and questioning Fowler) and doing what is best for the country (letting the matter be unsolved).

He and Fowler are oddly akin in some ways, both faintly cynical and weary of the world; hence their discussion of Blaise Pascal.

For example, it was criticised in The New Yorker for portraying Americans as murderers (largely based on one scene in which a bomb explodes in a crowd of people).

[citation needed] According to critic Philip Stratford, "American readers were incensed, perhaps not so much because of the biased portrait of obtuse and destructive American innocence and idealism in Alden Pyle, but because in this case it was drawn with such acid pleasure by a middle-class English snob like Thomas Fowler whom they were all too ready to identify with Greene himself".

The 1958 Hollywood film inverted the theme of the novel, turning it into an anti-communist story instead of a cautionary tale about American interventionism.