The mood of the story is how the author makes the reader feel.
The mood of John Updike's "A & P" is that of wonder, at first, and later, of sadness and resignation.
Sammy is the name of the narrator of the story. He describes the day when three girls come to the grocery store where he works in their bathing suits. One of the girls he describes as queen-like: in the way she walks, carries herself and turns her head.
This may simply be his perception, but he is as taken with her as any boy his age is taken with a pretty girl: he follows her progress through the store, noticing even how her feet hit the floor as she walks. To him, this unnamed girl is simply amazing.
When the three girls make their way to the front of the store, Sammy is lucky enough to wait on them. After he rings up their purchase, the manager chastises the girls for entering the store in bathing suits instead of regular street clothes.
After the girls leave, Sammy feels he must speak up for them, defend them for some reason, even though they have already left. It is here that the mood begins to change. Sammy defends the girls on principle. The manager, a friend of the family explains the rule, but Sammy cannot and will not listen: he quits on the spot.
Even as he leaves, he knows that he will regret the move. He also knows, somehow in his heart, that things will never be the same for him. He has turned a corner that has taken him from his innocent youth, one step closer to being a man. The deed has been done, which makes him kind of sad, but he is resigned to it, almost as if he knows this is the natural way of things.