Godzilla faces his greatest threat... dirty hippies!!!! Okay, this is an impossible movie to take seriously, but the plot is that Hedora, an alien monster that feeds on pollution, is threatening Japan because when you are a giant monster, threatening Japan is what you do. At this point in the Godzilla series, the big G is a full on "good guy" monster and the series if geared towards kids. Except that you have so much death, people melting from acid, etc. Japanese kids were hard core back in the 1970's, apparently.
Anyway, you watch this movie and have to conclude everyone involved was high as a kite. The psychedelic images, the weird fish head party scene, the strange animated sequences... "I know, I know, we'll have Hedora take a hit off a smokestack like it's a giant bong, man!!!" "Akira, you're so high right now!!!"
I guess they were trying to make a statement, that pollution in 1970's Japan was really awful. Hedora, however, is not a great metaphor in the way Godzilla was for nuclear war in the 1950's.
And who can forget... Flying Godzilla!!! Yes, it's even sillier than it sounds.