The mood of "The Fly" is humorous and somewhat light at first, as the story presents Woodifield as a bumbling old fool. Everyone who knows Woodifield, it seems, sees him as a rather ridiculous person, and this invites readers to laugh at him. The story even compares him to a "baby peer[ing] out of its pram" in the very first sentence—a humorous image that instantly makes the story feel very lighthearted. Even the boss's behavior adds to the story's amusing and playful feel, since his vanity about his swanky office is a bit overblown.
However, the story becomes increasingly dark and emotional as the boss's bravado transforms into grief when Woodifield mentions his dead son. Suddenly, the boss can hardly stand to listen to Woodifield's vapid remarks, thus altering the overall mood of the story. When Woodifield references the boss's son, a "quiver" passes through the boss's eyelids, and this signals the shift from the story's lighthearted atmosphere to something more serious and emotionally troubling. Whereas Woodifield was previously all too happy to relax in his office and listen to Woodifield prattle on about meaningless things, he can now hardly hold himself together long enough to see his old friend out of the office. Once he's alone, he ends up effectively torturing a fly in order to keep himself from dwelling on the grief of losing his son. What began as a funny scene poking fun at two older men, then, shifts into something much heavier, as it becomes clear that the story's initially playful mood simply masked an undercurrent of sadness and agony.
