For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
August 6, 1945 was a "normal" day in Hiroshima, where "normal" means residents living in constant fear/expectation of an American attack. Fun.
Because a warning siren had just gone off early that morning (and was followed by the all-clear), people thought they would get a quick rest from their pretty much constant fear of being bombed. However, they were wrong—fifteen minutes after the all-clear sounded, the atomic bomb was dropped. Hersey follows six individuals and recounts where they were/what they were doing at that moment.
Act II
Naturally, the bomb caused chaos; people were killed, cut, burned, and sick, and there were fires spreading all over the city. In the days and weeks that followed, many who survived the attack came down with radiation poisoning. The six people at the center of the story struggled to pick up their lives where they left off, with varying success.
Act III
Hersey details the aftermath of the bombing for the same six people, both in the years immediately following the event and 40 years beyond. Although some struggled more than others (for example, a few had long-term physical/health problems, while others did not), they all ended up relatively comfortable in the end, each working through the meaning of the event and its impact in their own ways. Dang. Life finds a way, eh?