Well, not much. But I'll try.

OK, as usual, a little backstory first. This manga was written and illustrated by Keiji Nakazawa. Nakazawa used his own childhood post-war experiences in a good deal of his work, including this one. It was originally serialized in -of all places- Weekly Shonen Jump. However, it was not very popular, and was canceled. It did manage to find publication in other formats, however, and quickly became legendary.
Now, for the story.
The story is quite simple. It revolves around the 'adventures' of Gen, a young boy living with his impoverished family in Japan. Things like bombing raids, food problems, shelter, and the like have become regular occurences for him.
That is, until the bomb is dropped.
For the rest of this story, you will see how not only Gen and his family, but others in his village, cope with the horrendous tragedy, of both the militarization of Japan, and the horrors of the A-bomb.
......I cannot really give away too much more. The power behind this work does not allow me to do so.
However, a word of warning. This thing contains stuff that young children should NOT be allowed to see.
Also....and there isn't any nice way to put this.....When you read this manga.....
You will weep.
You will weep like a little girl.
And you won't be able to stop, either. Because this work will not only break your heart, it will tear it in half, crush it, grind it into paste, send it through a concrete mixer, and roll it out on the pavement.
So, just a word to the wise, there.
Otherwise, I cannot reccomend this manga highly enough.
Oh....what else can I say?
PLEASE READ NOW!