It is the latter which provides the subject for the conversation. Psychological complications aside, it is a standard narrative of colonial life in Africa, with a description of a tornado, a native revolt, and incidents at the club. A... and Franck discuss it animatedly, while sipping the mixture of cognac and soda served by the mistress of the house in three glasses.
The main character of the book is a customs official. This character is not an official but a high-ranking employee of an old commercial company. This company's business is going badly, rapidly turning shady. This company's business is going extremely well. The chief character - one learns - is dishonest. He is honest, he is trying to re-establish a situation compromised by his predecessor, who dies in an automobile accident. But he had no predecessor, for the company was only recently formed; and it was not an accident. Besides, it happens to be a ship (a big white ship) and not a car at all.
Franck, at this point, begins to tell an anecdote about a truck of his with engine trouble. A..., as politeness demands, asks for details to prove the attention she is paying to her guest, who soon stands up and takes his leave, in order to return to his own plantation, a little farther east.
The husband uses the conversation of the novel between his wife A... and Franck to comment on his marital situation - The chief character, being A..., is dishonest - affair with Franck. The company perhaps referring to his marriage and the predecessor being Franck. A car wreck is described only once in the book before these final pages, which either suggests that it actually happened (unlike the repeated description of a small number of actions which suggests that they are revisited memories) or that it was a wild fantasy of the husband's, quickly dispelled and not thought of again. But surely jealousy would cause him to revisit this fantasy of getting rid of Franck - possibly even A... too. There is a peculiar unlabored shortness and certainty to 'and it was not an accident' in light of the weight of the rest of the text that implies that it is something that did occur and further still, that is was the husband's interference with the car that caused it.
The more and more I re-read these final pages, the more I'm of the opinion that the husband had killed both A... and Franck and the events of this story are his ceaseless recollections of events in the house, spurred on by his guilt.