It's World War I, and all countries are at war. Germany is in search for Italy, Rome's grandson, and take him captive. Instead... he finds a box of tomatoes?
Oh, no, it's just useless Italy hiding. But there was one detail someone forgot to mention. Italy is annoying. So Germany couldn't be happier when he got rid of Italy at the end of war.
Now it is World War II, everything is going smoothly with Germany's invasion, when the worst thing possible happens. Italy becomes Germany's ally. And this is how our story begins.
Think of it like WWII, except every country is a pretty boy instead. A hilariously stereotypical pretty boy. America loves burgers, Russia is into the vodka, and China has pandas and a Wok.
It is centered mostly around Italy, who has both a North and South character. The "Hetalia" portion of the name is probably mocking Italy, as "hetare" means "useless."
(The master list and profiles, including extras and specials, can be found here.
It is highly recommended that new readers start with the 'Main Storyline Index' as they're from the original webcomic source, and therefore slightly different from the edited volumes contents.)