Godzilla has taken the form of a national icon within the story. The government no longer wants to kill him (as if) and children have taken a liking to him. Humans are a disgusting little bunch as they pollute Godzilla's world past the point of repair. From the sludge that we created comes Hedorah - a slimy creation of pollution. Some spores from a meteor mutated tadpoles in some rampant physiology experiment creating him. Oh yeah, and Godzilla learns how to fly.
Of all the Godzilla films, this is one of the more violent ones. In most of the Lizard's films, the death toll is only implied. You see a city get destroyed and you think to yourself "Wow, many people must have died" but your suspicions are never answered. Hedorah possesses an ability to mist an entire area with sulfuric acid-like vapor killing hundreds of people. He vaporizes and nullifies entire populations. This is quite a morbid piece of hokey eco-fun.
Godzilla is ultimately ineffective to Hedorah. Being a manifestation of sludge, Godzilla's fists go through him leaving his sneaky scales scarred and beaten. It's up to man to stop this creation using a bizarre science experiment charged by the powers of Atomic breath. This is considered the worst Godzilla film, but I don't see it. This film features amazing practical effects not limited to a giant mucky-looking tongue sliming down stairs and retreating, leaving a disgruntled undigested kitten.
Godzilla vs. Hedorah is a completely different formula for a film of its kind. It reminds me of a film translation of those pop-culture children's books that featured characters like He-Man teaching kids to look both ways to cross the street, albeit graphic and nihilistic towards kind souls. In this film you will be subjected to a sing-along environmental song, Godzilla flying through the air, gratuitous violence towards teenage hippies, and Godzilla acting incredibly human. This is one of my favorite Godzilla films I've seen so far.