To the outside world, the boss presents the perfectly normal image of a capable, hard-working businessman. In actual fact, however, he's a shell of a man, scarred forever by the tragic death of his son in World War I.
When his friend Mr. Woodifield pays a visit to his office, the boss seems perfectly normal. But when Mr. Woodifield mentions his daughters' recent visit to his son's war grave, it triggers the boss's deep trauma over the death of his own son.
Somehow or other, the boss manages to keep it together while Mr. Woodifield is in his office. But as soon as his friend leaves, he is plunged into despair and feels the overwhelming urge to cry. However, he resists the urge but only by keeping himself occupied. This he does by torturing a fly to death until he can no longer remember what he was sad about.
With his son no longer alive, the boss feels that his life is essentially meaningless. He must somehow give his life meaning, perhaps by throwing himself into his business, but that's easier said than done. The boss needs something in his life that will help him come to terms with his son's death and deal with his loss. For now, torturing a fly to death will suffice, but in the future, he will need to find something else to do.