The Fly in the Ointment explores the shaky and dysfunctional relationship between a college lecturer, Harold, and his father. The plot develops when Harold goes to visit his father with the purpose of helping him financially with whatever resources he is able to help. The problem comes mainly because the old man is a shrewed, calculating, and manipulative. He also just lost his business, which has depressed him and, knowing how important money is for his father, Harold has voluntarily come to try to make his father less miserable.

The old man's "double-face" is described in the directions.

[He] had two faces… soft warm and innocent daub of innocent sealing wax…[then] shrewd, scared and hard.

The father is glad to switch moods and modes at will because the only way that he knows how to express his emotions is through the humiliation and emasculation of his son. When he realizes that the son is bringing in money, that soft/warm face almost seems welcoming to the son. However, whenever the father felt as if he was lesser than the son for not having any money, he would insult Harold's career, his life choices, and his current conditions. This is when the father, in the same, one Act in which the play develops switches from having "big smiles", to "banging his fist as a judge in an auction house". This is the mixture of hot and cold reception that the father knows to bestow upon his son; this, in turn, makes Harold feel scared, cold, and sorry for his father.

In reality, the father's split personality comes from the frustration of having once been successful and self-made, only to come crumbling down and needing handouts. The topic of money has always been imperative in the life of the old man and he is obviously a character that is strong and ambitious. The fact that the son is content with a lower-end lecturer position also rubs off wrongly on the father, who sees it as a sign of weakness. So much is the animosity between the son and the father that Harold can almost predict the reactions that some specific questions will cause. Therefore, the main motivation in the uneasy and weird behavior of the father is the desperate hope for circumstances to be different, and for things to get better.