SPOILER ALERT: My comments will give away important details of the story.
Plot: Three barefoot girls in bathing suits walk into an A & P grocery store, shocking the customers but drawing the admiration of the two young men working the cash registers. Eventually, the manager notices the girls and tells them that they should be "decently dressed" when they enter the store and that in the future, they will have to follow the store's policy and cover their shoulders.
As the girls are leaving, one of the cashiers, Sammy, tells the manager he quits. He does this partly to impress the girls ("quick enough for them to hear, hoping they'll stop and watch me, their unsuspected hero") and partly because he feels the manager took things too far and "didn't have to embarrass them."
The story ends with Sammy standing alone in the parking lot, the girls long gone. He says that his "stomach kind of fell as I felt how hard the world was going to be to me hereafter."
Narration: The story is told from the first person point of view of Sammy. From the opening line -- "In walks these three girls in nothing but bathing suits" -- Updike establishes Sammy's distinctively colloquial voice. Most of the story is told in the present tense, as if Sammy is talking.
Sammy's cynical observations about his customers (whom he often calls "sheep") can be humorous, such as when he comments that if one particular customer had been "born at the right time they would have burned her over in Salem." And it's an endearing detail when he describes folding his apron and dropping the bow tie on it, and then adds, "The bow tie is theirs, if you've ever wondered."
What else you should know: Some readers will find Sammy's sexist comments to be absolutely grating. The girls have entered the store seeking attention for their physical appearance, and Sammy comments on every detail. It's almost a caricature of objectification when he says, "You never know for sure how girls' minds work (do you really think it's a mind in there or just a little buzz like a bee in a glass jar?)[...]" The "you" to whom this question is addressed obviously couldn't be a teenage girl, and when I first read this story as a -- you guessed it -- teenage girl, I wondered why I was even bothering (except, of course, that a teacher had assigned it to me.)
But read on for some of the story's redeeming qualities.
What's important about the story: The tension arises not because the girls are in bathing suits, but because they're in bathing suits in a place where people don't wear bathing suits. They've crossed a line about what's socially acceptable.
"You know, it's one thing to have a girl in a bathing suit down on the beach, where what with the glare nobody can look at each other much anyway, and another thing in the cool of the A & P, under the fluorescent lights, against all those stacked packages, with her feet paddling along naked over our checkerboard green-and-cream rubber-tile floor."
Sammy obviously finds the girls physically alluring, but he's also attracted by their rebellion. He doesn't want to be like the "sheep" he makes such fun of, the customers who are befuddled when the girls enter the store.
There are clues that the girls' rebellion has its roots in economic privilege, a privilege not available to Sammy. The girls tell the manager that they entered the store only because one of their mothers asked them to pick up some herring snacks, an item that makes Sammy imagine a scene in which the "men were standing around in ice-cream coats and bow ties and the women were in sandals picking up herring snacks on toothpicks off a big glass plate." In contrast, when Sammy's parents "have somebody over they get lemonade and if it's a real racy affair Schlitz in tall glasses with "They'll Do It Every Time" cartoons stencilled on."
In the end, the class difference between Sammy and the girls means that his rebellion has far more serious ramifications than theirs does. By the end of the story, Sammy has lost his job and alienated his family. He feels "how hard the world [is] going to be" because not becoming a "sheep" won't be as easy as just walking away, and certainly won't be as easy for him as it will be for the girls, who inhabit a "place from which the crowd that runs the A & P must look pretty crummy."